Volker BrüchertAssociate professor of Biogeochemistry
I am Associate Professor for Geochemistry in the Department of Geological Sciences at Stockholm University and co-leader of the research area "Biogeochemical Cycles and Climate in the Bolin Centre for Climate Research. My research focuses on understanding the regulation of the major biogeochemical cycles in aquatic environments through the action of macro- and microorganisms and their response and adaptation to climate change and eutrophication effects.
In the department I teach both ground and advanced courses in geochemistry. I have been active in the design of new courses for our geological sciences curriculum covering both undergraduate and Master’s studies.
I am the Study Director of the Masterprogram in Geological Sciences and member of the department board and the university section committee for Earth and Environmental Sciences.
I teach in the following courses:
Basics of Geochemistry (from 2018)
Geochemistry in the field and laboratory (from 2018)
Master and PhD level
Biogeochemistry (coordinator and teacher)
Isotope Geology (teacher)
Global Biogeochemical Cycles (teacher)
Applied Environmental Geochemistry (teacher)
I am a marine biogeochemist with a background in geochemistry and microbiology. My research interests are the coupled biogeochemical cycles of carbon, nitrogen, sulfur, and phosphorus. In my group we work in coastal and offshore sedimentary environments and the water column of the Baltic Sea, the Barents Sea, and the Arctic shelf seas. Research questions focus on benthic and pelagic nitrogen cycling processes and carbon mineralization of terrestrial and marine organic matter. We also investigate sea-air exchange of methane and nitrous oxide.
My research approach draws on a broad array of analytical instrumentation that makes use of stable isotope mass spectrometry, liquid scintillation-based radioisotope analysis, gas chromatography, and liquid chromatography, as well as microelectrode and optode-based instrumentation. These anaytical techniques are used in a mixture of field- and laboratory-based work with an even balance between field data acquisition and experimental work. We conduct isotope labeling experiments with radiotracers (mostly S-35 sulfate, P-33, and 3H in (CH4)-based experiments) and stable isotope tracers (C-13, N-15, S-34 and 18O), as well as high-resolution O2, pH, and H2S microelectrode analyses. In addition, we are developing non-invasive seafloor sampling instruments for seafloor in-situ measurements to quantify the exchange of important biogeochemicals between the seafloor and the overlying the overlying water.
Through collaborations at the Stockholm SciLifelab we combine our biogeochemical research with molecular biological work.
Current integrated large research activities:
SWERUS-C3: Carbon, climate, cryosphere interactions in the Eastern Siberian Arctic Ocean
Focus: Sediment and water column methanogenesis and methane oxidation, carbon mineralization rates and pathways of carbon mineralization
Focus: Nitrogen cycling processes in sediment and water of the Baltic Sea; coupled iron-silica-phosphorus-carbon cycles in sediment; decelopment of non-invasive benthic lander systems for long-term biogeochemical in-situ measurements at the seafloor; greenhouse gas exchange in eutrophied near-shore environments; carbon and nitrogen cycling associated with cyanobacterial aggregates
Methane and N2O emissions from near-shore environments of the Baltic Sea
A selection from Stockholm University publication database
Activity and community structures of sulfate-reducing microorganisms in polar, temperate and tropical marine sediments
2016. Alberto Robador (et al.). The ISME Journal 10 (4), 796-809Article
Temperature has a fundamental impact on the metabolic rates of microorganisms and strongly influences microbial ecology and biogeochemical cycling in the environment. In this study, we examined the catabolic temperature response of natural communities of sulfate-reducing microorganisms (SRM) in polar, temperate and tropical marine sediments. In short-term sediment incubation experiments with S-35-sulfate, we demonstrated how the cardinal temperatures for sulfate reduction correlate with mean annual sediment temperatures, indicating specific thermal adaptations of the dominant SRM in each of the investigated ecosystems. The community structure of putative SRM in the sediments, as revealed by pyrosequencing of bacterial 16S rRNA gene amplicons and phylogenetic assignment to known SRM taxa, consistently correlated with in situ temperatures, but not with sediment organic carbon concentrations or C:N ratios of organic matter. Additionally, several species-level SRM phylotypes of the class Deltaproteobacteria tended to co-occur at sites with similar mean annual temperatures, regardless of geographic distance. The observed temperature adaptations of SRM imply that environmental temperature is a major controlling variable for physiological selection and ecological and evolutionary differentiation of microbial communities.
Denitrification and DNRA at the Baltic Sea oxic-anoxic interface
2016. Stefano Bonaglia (et al.). Limnology and Oceanography 61 (5), 1900-1915Article
The dependence of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) on different electron donors was tested in the nitrate-containing layer immediately below the oxic-anoxic interface (OAI) at three stations in the central anoxic basins of the Baltic Sea. Additionally, pathways and rates of fixed nitrogen transformation were investigated with N-15 incubation techniques without addition of donors. Denitrification and anammox were always detected, but denitrification rates were higher than anammox rates. DNRA occurred at two sites and rates were two orders of magnitude lower than denitrification rates. Separate additions of dissolved organic carbon and sulfide stimulated rates without time lag indicating that both organotrophic and lithotrophic bacterial populations were simultaneously active and that they could carry out denitrification or DNRA. Manganese addition stimulated denitrification and DNRA at one station, but it is not clear whether this was due to a direct or indirect effect. Ammonium oxidation to nitrite was detected on one occasion. During denitrification, the production of nitrous oxide (N2O) was as important as dinitrogen (N-2) production. A high ratio of N2O to N-2 production at one site may be due to copper limitation, which inhibits the last denitrification step. These data demonstrate the coexistence of a range of oxidative and reductive nitrogen cycling processes at the Baltic OAI and suggest that the dominant electron donor supporting denitrification and DNRA is organic matter. Organotrophic denitrification is more important for nitrogen budgets than previously thought, but the large temporal variability in rates calls for long-term seasonal studies.
Aerobic and anaerobic nitrogen transformation processes in N2-fixing cyanobacterial aggregates
2015. Klawonn Isabell (et al.). The ISME Journal 9 (1), 1456-1466Article
Colonies of N2-fixing cyanobacteria are key players in supplying new nitrogen to the ocean, but the biological fate of this fixed nitrogen remains poorly constrained. Here, we report on aerobic and anaerobic microbial nitrogen transformation processes that co-occur within millimetre-sized cyanobacterial aggregates (Nodularia spumigena) collected in aerated surface waters in the Baltic Sea. Microelectrode profiles showed steep oxygen gradients inside the aggregates and the potential for nitrous oxide production in the aggregates’ anoxic centres. 15N-isotope labelling experiments and nutrient analyses revealed that N2 fixation, ammonification, nitrification, nitrate reduction to ammonium, denitrification and possibly anaerobic ammonium oxidation (anammox) can co-occur within these consortia. Thus, N. spumigena aggregates are potential sites of nitrogen gain, recycling and loss. Rates of nitrate reduction to ammonium and N2 were limited by low internal nitrification rates and low concentrations of nitrate in the ambient water. Presumably, patterns of N-transformation processes similar to those observed in this study arise also in other phytoplankton colonies, marine snow and fecal pellets. Anoxic microniches, as a pre-condition for anaerobic nitrogen transformations, may occur within large aggregates (1 mm) even when suspended in fully oxygenated waters, whereas anoxia in small aggregates (<1 to 0.1 mm) may only arise in low-oxygenated waters (25 μM). We propose that the net effect of aggregates on nitrogen loss is negligible in NO3−-depleted, fully oxygenated (surface) waters. In NO3−-enriched (>1.5 μM), O2-depleted water layers, for example, in the chemocline of the Baltic Sea or the oceanic mesopelagic zone, aggregates may promote N-recycling and -loss processes.
Contrasting regimes for organic matter degradation in the East Siberian Sea and the Laptev Sea assessed through microbial incubations and molecular markers
2015. Emma S. Karlsson (et al.). Marine Chemistry 170, 11-22Article
Compositional studies of organic matter on the East Siberian Arctic Shelf (ESAS) suggest that different terrestrial carbon pools have different propensities for transport and/or degradation. The current study combined laboratory-based microbial degradation experiments with earlier published degradation-diagnostic composition of several classes of terrestrial biomarkers on the same sediments to investigate differences and driving forces of terrestrial organic matter (TerrOM) degradation in two biogeochemically-contrasting regimes of the ESAS. The incubation-based anaerobic degradation rates were consistently higher (by average factor of 6) in the East Siberian Sea Kolyma Paleoriver Channel (ESS-KPC) (15 mu mol CO2 g OC-1 day(-1)) compared to the Laptev Sea Buor-Khaya Bay (LS-BKB) (2.4 mu mol CO2 g OC-1 day(-1)). The reported molecular markers show similarities between the terrestrial carbon pools in the two systems, but impose contrasting degradation regimes in combination with the incubation results. For the LS-BKB, there was a strong relationship between the degradation rates and the three lignin phenol-based degradation proxies (r(2) = 0.93-0.96, p < 0.01, linear regression) and two wax lipid-based degradation proxies (r(2) = 0.71 and 0.66, p < 0.05, linear regression). In contrast, for the ESS-KPC system, there was no relationship between incubation-based degradation rates and molecular marker-based degradation status of TerrOM. A principal component analysis indicated that short-chain fatty acids and dicarboxylic acids from CuO oxidation are mainly of terrestrial origin in the LS-BKB, but mainly of marine origin in the ESS-KPC. Hence, the microbial degradation in the western (LS-BKB) system appears to be fueled by TerrOM whereas the eastern (ESS-KPC) system degradation appears to be driven by MarOM. By combining molecular fingerprinting of TerrOM degradation state with laboratory-based degradation studies on the same ESAS sediments, a picture evolves of two distinctly different modes of TerrOM degradation in different parts of the ESAS system.
Meiofauna increases bacterial denitrification in marine sediments
2014. Stefano Bonaglia (et al.). Nature Communications 5, 5133Article
Denitrification is a critical process that can alleviate the effects of excessive nitrogen availability in aquatic ecosystems subject to eutrophication. An important part of denitrification occurs in benthic systems where bioturbation by meiofauna (invertebrates <1mm) and its effect on element cycling are still not well understood. Here we study the quantitative impact of meiofauna populations of different abundance and diversity, in the presence and absence of macrofauna, on nitrate reduction, carbon mineralization and methane fluxes. In sediments with abundant and diverse meiofauna, denitrification is double that in sediments with low meiofauna, suggesting that meiofauna bioturbation has a stimulating effect on nitrifying and denitrifying bacteria. However, high meiofauna densities in the presence of bivalves do not stimulate denitrification, while dissimilatory nitrate reduction to ammonium rate and methane efflux are significantly enhanced. We demonstrate that the ecological interactions between meio-, macrofauna and bacteria are important in regulating nitrogen cycling in soft-sediment ecosystems.
2014. Kam W. Tang (et al.). Limnology and Oceanography 59 (1), 275-284Article
The widely reported paradox of methane oversaturation in oxygenated water challenges the prevailing paradigm that microbial methanogenesis only occurs under anoxic conditions. Using a combination of field sampling, incubation experiments, and modeling, we show that the recurring mid-water methane peak in Lake Stechlin, northeast Germany, was not dependent on methane input from the littoral zone or bottom sediment or on the presence of known micro-anoxic zones. The methane peak repeatedly overlapped with oxygen oversaturation in the seasonal thermocline. Incubation experiments and isotope analysis indicated active methane production, which was likely linked to photosynthesis and/or nitrogen fixation within the oxygenated water, whereas lessening of methane oxidation by light allowed accumulation of methane in the oxygen-rich upper layer. Estimated methane efflux from the surface water was up to 5 mmol m(-2) d(-1). Mid-water methane oversaturation was also observed in nine other lakes that collectively showed a strongly negative gradient of methane concentration within 0-20% dissolved oxygen (DO) in the bottom water, and a positive gradient within >= 20% DO in the upper water column. Further investigation into the responsible organisms and biochemical pathways will help improve our understanding of the global methane cycle.
Seasonal oxygen, nitrogen and phosphorus benthic cycling along an impacted Baltic Sea estuary: regulation and spatial patterns
2014. Stefano Bonaglia (et al.). Biogeochemistry 119 (1-3), 139-160Article
The regulatory roles of temperature, eutrophication and oxygen availability on benthic nitrogen (N) cycling and the stoichiometry of regenerated nitrogen and phosphorus (P) were explored along a Baltic Sea estuary affected by treated sewage discharge. Rates of sediment denitrification, anammox, dissimilatory nitrate reduction to ammonium (DNRA), nutrient exchange, oxygen (O2) uptake and penetration were measured seasonally. Sediments not affected by the nutrient plume released by the sewage treatment plant (STP) showed a strong seasonality in rates of O2 uptake and coupled nitrification-denitrification, with anammox never accounting for more than 20% of the total dinitrogen (N2) production. N cycling in sediments close to the STP was highly dependent on oxygen availability, which masked temperature-related effects. These sediments switched from low N loss and high ammonium (NH4+) efflux under hypoxic conditions in the fall, to a major N loss system in the winter when the sediment surface was oxidized. In the fall DNRA outcompeted denitrification as the main nitrate (NO3-) reduction pathway, resulting in N recycling and potential spreading of eutrophication. A comparison with historical records of nutrient discharge and denitrification indicated that the total N loss in the estuary has been tightly coupled to the total amount of nutrient discharge from the STP. Changes in dissolved inorganic nitrogen (DIN) released from the STP agreed well with variations in sedimentary N2 removal. This indicates that denitrification and anammox efficiently counterbalance N loading in the estuary across the range of historical and present-day anthropogenic nutrient discharge. Overall low N/P ratios of the regenerated nutrient fluxes impose strong N limitation for the pelagic system and generate a high potential for nuisance cyanobacterial blooms.
Sulfidization of lacustrine glacial clay upon Holocene marine transgression (Arkona Basin, Baltic Sea)
2014. Lars Holmkvist (et al.). Geochimica et Cosmochimica Acta 142, 75-94Article
Towards the end of the last deglaciation more than 13,500 years ago the southern Baltic Sea was a freshwater lake, the Baltic Ice Lake, for several thousand years during which iron-rich, organic-poor clay was deposited. The modern brackish-marine stage started about 8600 years ago with the deposition of organic-rich mud, which is today characterized by high rates of sulfate reduction and high concentrations of free sulfide. We studied the iron-sulfur diagenesis in gravity cores from the Arkona Basin, SW Baltic Sea, to track the progressing sulfidization front in the buried Ice Lake sediment. The geochemical zonation was unusual as the sulfate concentration dropped steeply by two thirds below which it increased again due to a deep sulfate reservoir. The reservoir had been established during the early Holocene marine period as sulfate and other seawater ions diffused down into the lake sediment for several thousand years. Sulfur isotope analyses confirmed its origin as seawater sulfate, while its oxygen isotope composition indicated a microbially catalyzed equilibration with ambient interstitial water, decoupled from net sulfate reduction. Today, hydrogen sulfide diffuses from the marine mud down into the lake sediment where a black band with high magnetic susceptibility and high iron monosulfide, greigite and elemental sulfur content shows progressing sulfidization of the large pool of solid-phase reactive iron. Dissolved iron from the deep Ice Lake sediment diffuses up to the sulfide front and provides a small supplement to the solid Fe(III) pool as a sulfide sink. Pyrite formation at the sulfidization front may involve surface-bound zero-valent sulfur while, above the front, polysulfides are in equilibrium with the system hydrogen sulfide - polysulfide - rhombic sulfur and may not be important for further pyrite formation. The Holocene iron-sulfur diagenesis observed in the Arkona Basin represents an important transitional state for post-glacial transgressions with organic-rich marine sediment overlying lacustrine clay, such as in other areas of the Baltic Sea or in the Black Sea.
BENTHIC BOUNDARY LAYER NUTRIENT AND OXYGEN BIOGEOCHEMISTRY IN A EUTROPHIED BALTIC SEA ESTUARY
2013. Volker Brüchert, Stefano Bonaglia, Barbara Deutsch.Conference
We present dissolved nutrient and oxygen concentrations determined with a benthic boundary layer profiling system for a set of stations along a eutrophication gradient in a Baltic Sea estuary. The sampling system yields vertically highly resolved CTD, oxygen, and nutrient profiles of the lowermost 80 cm of water overlying the sediment. Continuous oxygen and CTD measurements over 8 – 24 hours at fixed depths above the sediment surface provided information on the temporal variability of nutrients and the physical structure within the benthic boundary layer. These data indicate multiple short-term episodes of vertical mixing and stable stratification within the boundary layer that can lead to short-term fluctuations in bottom water oxygen of more than 100 µM. This high degree of temporal variability needs to be taken into account for benthic flux calculations that assume vertically mixed benthic boundary layers.
EFFECT OF MEIOFAUNA ON BENTHIC ELEMENT CYCLING IN A BALTIC SEA COASTAL AREA
2013. Stefano Bonaglia (et al.).Conference
We have studied the role of meiofaunal communities for nutrient cycling and organic matter mineralization in coastal sediments of the Baltic Sea. Although meiofauna is orders of magnitude more abundant than macrofauna and has commonly a much more diverse community structure, its importance for sediment biogeochemical pathways is poorly understood because of objective experimental difficulties when manipulating meiofauna communities due to small body sizes (0.04 to 1 mm) and inherent fragility. We used a density extraction method to separate intact and living metazoans from sediment and tested the effect of low meiofauna and high meiofauna abundances in the presence and absence of macrofauna for exchange rates of nutrients, O2, DIC, N2, and CH4. High abundances of meiofauna stimulated O2 uptake and the net N2 efflux by 16% and 34%, respectively, but did not change oxygen penetration depths significantly. By contrast, macrofauna increased oxygen penetration depths by 21% and stimulated methane emissions by a factor of 8. These results demonstrate the importance of meiofauna in the regulation of aerobic and anaerobic microbial processes and benthic fluxes in marine sediments.
Effect of reoxygenation and Marenzelleria spp. bioturbation on Baltic Sea sediment metabolism
2013. Stefano Bonaglia (et al.). Marine Ecology Progress Series 482, 43-55Article
Nutrient reduction and the improvement of bottom water oxygen concentrations are thought to be key factors in the recovery of eutrophic aquatic ecosystems. The effects of reoxygenation and bioturbation of natural hypoxic sediments in the Baltic Sea were studied using a mesocosm experiment. Anoxic sediment box cores were collected from 100 m depth in Kanholmsfjärden (Stockholm Archipelago) and maintained in flow-through mesocosms with 3 treatments: (1) hypoxic: supplied with hypoxic water; (2) normoxic: supplied with oxic water; and (3) Marenzelleria: supplied with oxic water and the polychaete Marenzelleria spp. (2000 ind. m–2). After a 7 wk long conditioning period, net fluxes of dissolved O2, CH4, Fe2+, Mn2+, NH4+, NO2-, NO3-, PO43- and H4SiO4, and rates of nitrate ammonification (DNRA), denitrification and anammox were determined. Phosphate was taken up by the sediment in all treatments, and the uptake was highest in the normoxic treatment with Marenzelleria. Normoxic conditions stimulated the denitrification rate by a factor of 5. Denitrification efficiency was highest under normoxia (50%), intermediate in bioturbated sediments (16%), and very low in hypoxic sediments (4%). The shift from hypoxic to normoxic conditions resulted in a significantly higher retention of NH4+, H4SiO4 and Mn2+ in the sediment, but the bioturbation by Marenzelleria reversed this effect. Results from our study suggest that bioturbation by Marenzelleria stimulates the exchange of solutes between sediment and bottom water through irrigation and enhances bacterial sulfate reduction in the burrow walls. The latter may have a toxic effect on nitrifying bacteria, which, in turn, suppresses denitrification rates.
The Impact of Sediment and Carbon Fluxes on the Biogeochemistry of Methane and Sulfur in Littoral Baltic Sea Sediments (Himmerfjarden, Sweden)
2013. Nguyen Manh Thang (et al.). Estuaries and Coasts 36 (1), 98-115Article
Three sediment stations in Himmerfjarden estuary (Baltic Sea, Sweden) were sampled in May 2009 and June 2010 to test how low salinity (5-7 aEuro degrees), high primary productivity partially induced by nutrient input from an upstream waste water treatment plant, and high overall sedimentation rates impact the sedimentary cycling of methane and sulfur. Rates of sediment accumulation determined using Pb-210(excess) and Cs-137 were very high (0.65-0.95 cm year(-1)), as were the corresponding rates of organic matter accumulation (8.9-9.5 mol C m(-2) year(-1)) at all three sites. Dissolved sulfate penetrated < 20 cm below the sediment surface. Although measured rates of bicarbonate methanogenesis integrated over 1 m depth were low (0.96-1.09 mol m(-2) year(-1)), methane concentrations increased to > 2 mmol L-1 below the sulfate-methane transition. A steep gradient of methane through the entire sulfate zone led to upward (diffusive and bio-irrigative) fluxes of 0.32 to 0.78 mol m(-2) year(-1) methane to the sediment-water interface. Areal rates of sulfate reduction (1.46-1.92 mol m(-2) year(-1)) integrated over the upper 0-14 cm of sediment appeared to be limited by the restricted diffusive supply of sulfate, low bio-irrigation (alpha = 2.8-3.1 year(-1)), and limited residence time of the sedimentary organic carbon in the sulfate zone. A large fraction of reduced sulfur as pyrite and organic-bound sulfur was buried and thus escaped reoxidation in the surface sediment. The presence of ferrous iron in the pore water (with concentrations up to 110 mu M) suggests that iron reduction plays an important role in surface sediments, as well as in sediment layers deep below the sulfate-methane transition. We conclude that high rates of sediment accumulation and shallow sulfate penetration are the master variables for biogeochemistry of methane and sulfur cycling; in particular, they may significantly allow for release of methane into the water column in the Himmerfjarden estuary.
The Role of Microorganisms in the diversity and distribution of siderophores in Podzolic Forest Soil
2013. Engy Ahmed (et al.). Mineralogical magazine 77 (2), 161--208(48)Article
Iron is a key component of the chemical architecture of the biosphere. Due to the low bioavailability of iron in the environment, microorganisms have developed specific uptake strategies. The most important one is the production of siderophores, which are operationally defined as low-molecular-mass biogenic Fe (III)-binding compounds which may greatly increase bioavailability of Fe . One of the primary biogeochemical functions of siderophores is therefore to increase Fe bioavailability by promoting the dissolution of iron-bearing minerals . This study aims to understand the role of microorganisms in the chemical diversity and distribution of siderophores in podzol soil and how this diversity can contribute to the bioavailability of Fe in forest soil.Soil samples were collected from an experimental site in the area of Bispgården in central Sweden (63°07′N, 16°70′E) from the O (organic), E (eluvial), B1 (upper illuvial), and C (mineral) horizons. Concentration and chemical composition of dissolved and adsorbed siderophores in the soil samples were determined using colorimetric assays and high-performance liquid chromatography.The highest siderophore concentrations were found in the O layer and thereafter decreased by depth. Concentrations of dissolved hydroxamate, catecholate and carboxylate siderophores were up to 84, 17 and 0.2 nmol/ g soil, respectively. In contrast, concentrations of adsorbed hydroxamates, catecholates and carboxylates were only up to 1.8, 3 and 0.2 nmol/ g soil, respectively.Siderophore-producing microorganisms were isolated from the same soil samples. Viable fungi, bacteria and actinomycete counts ranged from 7 to 300, from 300 to 1800, and from 0 to 5 cfu/gm, respectively. The highest counts were found in the O and E layers. Only the E layer contained the three types of siderophore-producing microorganisms investigated in this study. Siderophores were extracted from culture filtrates of the isolated microorganisms when grown under iron-limited conditions. These extracts varied considerably in siderophore composition. Fungal isolates produced up to 183 μM of hydroxamates, especially those isolated from the O layer, whereas bacteria and actinomycete isolated from the O and E layers of the soil produced high amounts of carboxylate, catecholate and hydroxamate siderophores. Actinomycete produced up to 93 μM of hydroxamates and 47 μM of catecholates, while bacteria produced up to 34 μM of carboxylates and up to 14 μM of catecholates.The depth variability in concentration and chemical composition and the good correlation between abundance of siderophore-producing microorganisms and siderophore soil concentrations strongly suggest that these siderophore-producing microorganisms play an important role in the mobilization of iron in the podzol soil that may be important in iron availability to plants in forest environment.
 Clay et al. (1981) Biochemistry 20, 2432-2436.  Duckworth et al. (2009) ChemGeol 260, 149-158.
Seasonal benthic nutrient cycling in a Baltic sea estuary
2012. Stefano Bonaglia (et al.).Conference
Decades of urban, industrial, and agricultural discharge of nitrogen and phosphorus to the Baltic Sea have contributed to the spreading of water column hypoxia and annual widespread cyanobacteria blooms. Central to mitigating Baltic Sea eutrophication is to resolve how much reduction strategies of external N and P loading are offset by internal loading of the Baltic through nutrient recycling from the sediment. We investigated the seasonal variation of benthic nitrogen and phosphorus cycling in an estuary of the Baltic impacted by decades of sewage discharge. Sediment nutrient fluxes, denitrification, Anammox, DNRA, potential nitrification, and total and diffusive oxygen uptake (TOU/DOU) were quantified with 15N-tracer methods and microsensor profiling. Data indicate benthic net efflux of ammonium and phosphorus during the summer months, decreasing N2 loss with increasing organic matter content, and benthic N/P regeneration with a ratio of 3 to 7 compared to the sewage discharge N/P of ≈ 25, and a significant contribution (6 to 25%) of Anammox to N2 loss. On average benthic denitrification and Anammox may reduce the N load to the estuary by up to 54%.
Temperature characteristics of bacterial sulfate reduction in continental shelf and slope sediments
2012. Joanna E. Sawicka, B. B. Jorgensen, Volker Bruchert. Biogeosciences 9 (8), 3425-3435Article
The temperature responses of sulfate-reducing microbial communities were used as community temperature characteristics for their in situ temperature adaptation, their origin, and dispersal in the deep sea. Sediments were collected from a suite of coastal, continental shelf, and slope sediments from the southwest and southeast Atlantic and permanently cold Arctic fjords from water depths ranging from the intertidal zone to 4327 m. In situ temperatures ranged from 8 A degrees C on the shelf to -1 A degrees C in the Arctic. Temperature characteristics of the active sulfate-reducing community were determined in short-term incubations with S-35-sulfate in a temperature gradient block spanning a temperature range from 0 to 40 A degrees C. An optimum temperature (T-opt) between 27 A degrees C and 30 A degrees C for the South Atlantic shelf sediments and for the intertidal flat sediment from Svalbard was indicative of a psychrotolerant/mesophilic sulfate-reducing community, whereas T-opt < 20 A degrees C in South Atlantic slope and Arctic shelf sediments suggested a predominantly psychrophilic community. High sulfate reduction rates (20-50%) at in situ temperatures compared to those at T-opt further support this interpretation and point to the importance of the ambient temperature regime for regulating the short-term temperature response of sulfate-reducing communities. A number of cold (< 4 A degrees C) continental slope sediments showed broad temperature optima reaching as high as 30 A degrees C, suggesting the additional presence of apparently mesophilic sulfate-reducing bacteria. Since the temperature characteristics of these mesophiles do not fit with the permanently cold deep-sea environment, we suggest that these mesophilic microorganisms are of allochthonous origin and transported to this site. It is likely that they were deposited along with the mass-flow movement of warmer shelf-derived sediment. These data therefore suggest that temperature response profiles of bacterial carbon mineralization processes can be used as community temperature characteristics, and that mixing of bacterial communities originating from diverse locations carrying different temperature characteristics needs to be taken into account to explain temperature response profiles of bacterial carbon mineralization processes in sediments.
Dietary success of a 'new' key fish in an overfished ecosystem
2011. M. G. van der Bank (et al.). Marine Ecology Progress Series 428, 219-233Article
The bearded goby Sufflogobius bibarbatus has become a key component of the pelagic food web off Namibia following the crash in pelagic fish populations during the 1970s, and its biomass is increasing despite significant predation pressure and apparent life-history constraints. The integrated feeding of the bearded goby was studied from samples collected during April 2008, using stable isotope ratios (delta(13)C, delta(15)N, delta(34)S) and fatty acids, to resolve conflict amongst previous dietary studies based on gut-content analysis and to understand how diet could influence its success within the region. Isotopes of carbon and nitrogen suggest that the now abundant jellyfish could contribute up to 74% of the diet, and delta(34)S signatures indicate that the diatom- and bacteria-rich sulphidic sediments on the central shelf may contribute around 15% to the diet. Fatty acid analyses provided support for sulphur bacterial and jellyfish-feeding amongst gobies, and further suggest that small gobies fed more on zooplankton while large gobies fed more on sedimented diatoms. Both data sets suggest that ontogenetic changes in diet were linked to changes in habitat: pelagic when small, more demersal when large. The study highlights the value of using multiple tracers in trophic studies and indicates that the dietary flexibility of the bearded goby, in conjunction with its behaviour and physiology, likely contributes to its success within the northern Benguela ecosystem.
Measurement and interpretation of solute concentration gradients in the benthic boundary layer
2011. Moritz Holtappels (et al.). Limnology and Oceanography 9, 1-13Article
The coastal ocean is characterized by high exchange rates of organic matter, oxygen, and nutrients between the sediment and the water column. The solutes that are exchanged between the sediment and the overlying water column are transported across the benthic boundary layer (BBL) by means of turbulent diffusion. Thus, solute concentration gradients in the BBL contain valuable information about the respective fluxes. In this study, we present the instrumentation and sampling strategies to measure oxygen and nutrient concentration gradients in the BBL. We provide the theoretical background and the calculation procedure to derive ratios of nutrient and oxygen fluxes from these concentration gradients. The noninvasive approach is illustrated at two sampling sites in the western Baltic Sea where nutrient and oxygen concentration gradients of up to 5 and 30 mu M m(-1), respectively, were measured. Nutrient and oxygen flux ratios were used to establish a nitrogen flux balance between sediment and water column indicating that 20% and 50% of the mineralized nitrogen left the sediment in form of N(2) (station A and B, respectively). The results are supported by sediment incubation experiments of intact sediment cores, measuring denitrification rates, and oxygen uptake. The presented flux ratio approach is applicable without knowledge of turbulent diffusivities in the BBL and is, therefore, unaffected by non-steady-state current velocities and diffusivities.
Effects of freeze-thaw cycles on anaerobic microbial processes in an Arctic intertidal mud flat
2010. Joanna E. Sawicka (et al.). ISME Journal 4 (4), 585-594Article
Insight into the effects of repeated freezing and thawing on microbial processes in sediments and soils is important for understanding sediment carbon cycling at high latitudes acutely affected by global warming. Microbial responses to repeated freeze-thaw conditions were studied in three complementary experiments using arctic sediment collected from an intertidal flat that is exposed to seasonal freeze-thaw conditions (Ymerbukta, Svalbard, Arctic Ocean). The sediment was subjected to oscillating freeze-thaw incubations, either gradual, from -5 to 4 degrees C, or abrupt, from -20 to 10 degrees C. Concentrations of low-molecular weight carboxylic acids (volatile fatty acids) were measured and sulfate reduction was assessed by measuring S-35 sulfate reduction rates (SRRs). Gradual freeze-thaw incubation decreased microbial activity in the frozen state to 0.25 % of initial levels at 4 degrees C, but activity resumed rapidly reaching >60 % of initial activity in the thawed state. Exposure of sediments to successive large temperature changes (-20 versus 10 degrees C) decreased SRR by 80% of the initial activity, suggesting that a fraction of the bacterial community recovered rapidly from extreme temperature fluctuations. This is supported by 16S rRNA gene-based denaturing gradient gel electrophoresis profiles that revealed persistence of the dominant microbial taxa under repeated freeze-thaw cycles. The fast recovery of the SRRs suggests that carbon mineralization in thawing arctic sediment can resume without delay or substantial growth of microbial populations.
Geochemical processes and chemosynthetic primary production in different thiotrophicmats of the Ha°kon Mosby Mud Volcano (Barents Sea)
2010. Anna Lichtschlag (et al.). Limnology and Oceanography 55 (2), 931-949Article
Abstract: We have investigated if in a cold seep methane or sulfide is used for chemosynthetic primary production and if significant amounts of the sulfide produced by anaerobic oxidation of methane are oxidized geochemically and hence are not available for chemosynthetic production. Geochemically controlled redox reactions and biological turnover were compared in different habitats of the Håkon Mosby Mud Volcano. The center of the mud volcano is characterized by the highest fluid flow, and most primary production by the microbial community depends on oxidation of methane. The small amount of sulfide produced is oxidized geochemically with oxygen or is precipitated with dissolved iron. In the medium flow peripheral Beggiatoa habitat sulfide is largely oxidized biologically. The oxygen and nitrate supply is high enough that Beggiatoa can oxidize the sulfide completely, and chemical sulfide oxidation or precipitation is not important. An internally stored nitrate reservoir with average concentrations of 110 mmol L-1 enables the Beggiatoa to oxidize sulfide anaerobically. The pH profile indicates sequential sulfide oxidation with elemental sulfur as an intermediate. Gray thiotrophic mats associated with perturbed sediments showed a high heterogeneity in sulfate turnover and high sulfide fluxes, balanced by the opposing oxygen and nitrate fluxes so that biological oxidation dominates over geochemical sulfide removal processes. The three habitats indicate substantial small-scale variability in carbon fixation pathways, either through direct biological use of methane or through indirect carbon fixation of methane-derived carbon dioxide by chemolithotrophic sulfide oxidation.
Kinetic oxygen isotope effects during dissimilatory sulfate reduction
2010. Alexandra V. Turchyn (et al.). Geochimica et Cosmochimica Acta 74 (7), 2011-2024Article
Kinetic isotope effects related to the breaking of chemical bonds drive Sulfur isotope fraction at ion during dissimilatory sit I fate reduction (DSR), whereas oxygen isotope fractional ion during DSR is dominated by exchange between intercellular sulfur intermediates and water. We use a simplified biochemical model for DSR to explore how a kinetic oxygen isotope effect may be expressed. We then explore these relationships in light of evolving sulfur and Oxygen isotope compositions (delta S-34(SO4) and delta O-18(SO4)) during batch culture growth of twelve strains of surface-reducing bacteria. Cultured under conditions to optimize growth and with identical delta O-18(H2O) and initial delta O-18(SO4), all strains show 34 S enrichment, whereas only six strains show significant O-18 enrichment. The remaining six show no (or minimal) change in delta O-18(SO4) over the growth of the bacteria. We use these experimental and theoretical results to address three questions: (i) which Sulfur intermediates exchange oxygen isotopes with water, (ii) what is the kinetic oxygen isotope effect related to the reduction of adenosine phosphosulfate (APS) to sulfite (SO32-), (iii) does a kinetic oxygen isotope effect impact the apparent oxygen isotope equilibrium values? We conclude that oxygen isotope exchange between water and a sulfur intermediate likely occurs downstream of APS and that our data constrain the kinetic oxygen isotope fractionation for the reduction of APS to sulfite to be smaller than 47 parts per thousand. This small oxygen isotope effect impacts the apparent oxygen isotope equilibrium as controlled by the extent to which APS reduction is rate-limiting.
Microbial sequestration of phosphorus in anoxic upwelling sediments
2010. Tobias Goldhammer (et al.). Nature geoscience 3 (8), 557-561Article
Phosphorus is an essential nutrient for life. In the ocean, phosphorus burial regulates marine primary production(1,2). Phosphorus is removed from the ocean by sedimentation of organic matter, and the subsequent conversion of organic phosphorus to phosphate minerals such as apatite, and ultimately phosphorite deposits(3,4). Bacteria are thought to mediate these processes(5), but the mechanism of sequestration has remained unclear. Here, we present results from laboratory incubations in which we labelled organic-rich sediments from the Benguela upwelling system, Namibia, with a P-33-radiotracer, and tracked the fate of the phosphorus. We show that under both anoxic and oxic conditions, large sulphide-oxidizing bacteria accumulate P-33 in their cells, and catalyse the nearly instantaneous conversion of phosphate to apatite. Apatite formation was greatest under anoxic conditions. Nutrient analyses of Namibian upwelling waters and sediments suggest that the rate of phosphate-to-apatite conversion beneath anoxic bottom waters exceeds the rate of phosphorus release during organic matter mineralization in the upper sediment layers. We suggest that bacterial apatite formation is a significant phosphorus sink under anoxic bottom-water conditions. Expanding oxygen minimum zones are projected in simulations of future climate change(6), potentially increasing sequestration of marine phosphate, and restricting marine productivity.
Organic Carbon Degradation in Anoxic Organic-Rich Shelf Sediments
2010. Elsabe M. Julies (et al.). Geomicrobiology Journal 27 (4), 303-314Article
Identifying and explaining bottlenecks in organic carbon mineralization and the persistence of organic matter in marine sediments remain challenging. This study aims to illuminate the process of carbon flow between microorganisms involved in the sedimentary microbial food chain in anoxic, organic-rich sediments of the central Namibian upwelling system, using biogeochemical rate measurements and abundances of Bacteroidetes, Gammaproteobacteria, and sulfate-reducing bacteria at two sampling stations. Sulfate reduction rates decreased by three orders of magnitude in the top 20 cm at one sampling station (280 nmol cm-3 d-1 - 0.1 nmol cm-3 d-1) and by a factor of 7 at the second station (65 nmol cm-3 d-1 - 9.6 nmol cm-3 d-1). However, rates of enzymatic hydrolysis decreased by less than a factor of three at both sampling stations for the polysaccharides laminarin (23 nmol cm-3 d-1- 8 nmol cm-3 d-1 and 22 nmol cm-3 d-1- 10 nmol cm-3 d-1) and pullulan (11 nmol cm-3 d-1- 4 nmol cm-3 d-1 and 8 nmol cm-3 d-1- 6 nmol cm-3 d-1). Increasing imbalance between carbon turnover by hydrolysis and terminal oxidation with depth, the steep decrease in cell specific activity of sulfate reducing bacteria with depth, low concentrations of volatile fatty acids (less than 15 M), and persistence of dissolved organic carbon, suggest decreasing bioavailability and substrate limitation with depth.
Temperature induced decoupling of enzymatic hydrolysis and carbon remineralization in long-term incubations of Arctic and temperate sediments
2010. Alberto Robador (et al.). Geochimica et Cosmochimica Acta 74 (8), 2316-2326Article
Extracellular enzymatic hydrolysis of high-molecular weight organic matter is the initial step in sedimentary organic carbon degradation and is often regarded as the rate-limiting step. Temperature effects on enzyme activities may therefore exert an indirect control on carbon mineralization. We explored the temperature sensitivity of enzymatic hydrolysis and its connection to subsequent steps in anoxic organic carbon degradation in long-term incubations of sediments from the Arctic and the North Sea. These sediments were incubated under anaerobic conditions for 24 months at temperatures of 0, 10, and 20 degrees C. The short-term temperature response of the active microbial community was tested in temperature gradient block incubations. The temperature optimum of extracellular enzymatic hydrolysis, as measured with a polysaccharide (chondroitin sulfate), differed between Arctic and temperate habitats by about 8-13 degrees C in fresh sediments and in sediments incubated for 24 months. In both Arctic and temperate sediments, the temperature response of chondroitin sulfate hydrolysis was initially similar to that of sulfate reduction. After 24 months, however, hydrolysis outpaced sulfate reduction rates, as demonstrated by increased concentrations of dissolved organic carbon (DOC) and total dissolved carbohydrates. This effect was stronger at higher incubation temperatures, particularly in the Arctic sediments. In all experiments, concentrations of volatile fatty acids (VFA) were low, indicating tight coupling between VFA production and consumption. Together, these data indicate that long-term incubation at elevated temperatures led to increased decoupling of hydrolytic DOC production relative to fermentation. Temperature increases in marine sedimentary environments may thus significantly affect the downstream carbon mineralization and lead to the increased formation of refractory DOC.
Trophic Structure and Community Stability in an Overfished Ecosystem
2010. Anne C. Utne-Palm (et al.). Science 329 (5989), 333-336Article
Since the collapse of the pelagic fisheries off southwest Africa in the late 1960s, jellyfish biomass has increased and the structure of the Benguelan fish community has shifted, making the bearded goby (Sufflogobius bibarbatus) the new predominant prey species. Despite increased predation pressure and a harsh environment, the gobies are thriving. Here we show that physiological adaptations and antipredator and foraging behaviors underpin the success of these fish. In particular, body-tissue isotope signatures reveal that gobies consume jellyfish and sulphidic diatomaceous mud, transferring ""dead-end"" resources back into the food chain.
A constant flux of diverse thermophilic bacteria into the cold Arctic seabed
2009. Casey Hubert (et al.). Science 325 (18), 1541-1544Article
Microorganisms have been repeatedly discovered in environments that do not support their metabolic activity. Identifying and quantifying these misplaced organisms can reveal dispersal mechanisms that shape natural microbial diversity. Using endospore germination experiments, we estimated a stable supply of thermophilic bacteria into permanently cold Arctic marine sediment at a rate exceeding 108 spores per square meter per year. These metabolically and phylogenetically diverse Firmicutes show no detectable activity at cold in situ temperatures but rapidly mineralize organic matter by hydrolysis, fermentation, and sulfate reduction upon induction at 50°C. The closest relatives to these bacteria come from warm subsurface petroleum reservoir and ocean crust ecosystems, suggesting that seabed fluid flow from these environments is delivering thermophiles to the cold ocean. These transport pathways may broadly influence microbial community composition in the marine environment.
An integrated sulfur isotope model for Namibian shelf sediments
2009. Andrew Dale (et al.). Geochimica et Cosmochimica Acta 73 (7), 1924-1944Article
In this study the sulfur cycle in the organic-rich mud belt underlying the highly productive upwelling waters of the Namibian shelf is quantified using a 1D reaction-transport model. The model calculates vertical concentration and reaction rate profiles in the top 500 cm of sediment which are compared to a comprehensive dataset which includes carbon, sulfur, nitrogen and iron compounds as well as sulfate reduction (SR) rates and stable sulfur isotopes (32S, 34S). The sulfur dynamics in the well-mixed surface sediments are strongly influenced by the activity of the large sulfur bacteria Thiomargaritanamibiensis which oxidize sulfide (H2S) to sulfate () using sea water nitrate () as the terminal electron acceptor. Microbial sulfide oxidation (SOx) is highly efficient, and the model predicts intense cycling between and H2S driven by coupled SR and SOx at rates exceeding 6.0 mol S m−2 y−1. More than 96% of the SR is supported by SOx, and only 2–3% of the pool diffuses directly into the sediment from the sea water. A fraction of the produced by Thiomargarita is drawn down deeper into the sediment where it is used to oxidize methane anaerobically, thus preventing high methane concentrations close to the sediment surface. Only a small fraction of total H2S production is trapped as sedimentary sulfide, mainly pyrite (FeS2) and organic sulfur (Sorg) (∼0.3 wt.%), with a sulfur burial efficiency which is amongst the lowest values reported for marine sediments (<1%). Yet, despite intense SR, FeS2 and Sorg show an isotope composition of ∼5 ‰ at 500 cm depth. These heavy values were simulated by assuming that a fraction of the solid phase sulfur exchanges isotopes with the dissolved sulfide pool. An enrichment in H2S of 34S towards the sediment-water interface suggests that Thiomargarita preferentially remove H232S from the pore water. A fractionation of 20–30‰ was estimated for SOx (εSOx) with the model, along with a maximum fractionation for SR (εSR–max) of 100‰. These values are far higher than previous laboratory-based estimates for these processes. Mass balance calculations indicate negligible disproportionation of autochthonous elemental sulfur; an explanation routinely cited in the literature to account for the large fractionations in SR. Instead, the model indicates that repeated multi-stepped sulfide oxidation and intracellular disproportionation by Thiomargarita could, in principle, allow the measured isotope data to be simulated using much lower fractionations for εSOx (5‰) and εSR (78‰).
Detoxification of sulphidic African shelf waters by blooming chemolithotrophs
2009. Gaute Lavik (et al.). Nature 457 (7229), 581-584Article
Hydrogen sulphide and methane emissions on the central Namibian shelf
2009. Volker Brüchert, Bronwen Currie, Kathleen R Peard. Progress in Oceanography 83 (1-4), 169-179Article
Hydrogen sulphide occurs frequently in the waters of the inner shelf coastal upwelling area off central Namibia. The area affected coincides with hatching grounds of commercially important pelagic fish, whose recruitment may be severely affected by recurring toxic sulphidic episodes. Both episodic biogenic methane gas-driven advective and molecular diffusive flux of hydrogen sulphide have been implicated as transport mechanisms from the underlying organic-matter-rich diatomaceous mud. To test hypotheses on the controls of hydrogen sulphide transport from the sediments on the inner Namibian shelf, water column and sediment data were acquired from four stations between 27 and 72 m water depth over a 3 year long period. On 14 cruises, temperature, salinity, dissolved oxygen, nitrate, methane, and total dissolved sulphide were determined from water column samples, and pore water dissolved methane, total dissolved sulphide, biomass of benthic sulphide-oxidising bacteria Beggiatoa and Thiomargarita, and bacterial sulphate reduction rates were determined from sediment cores. Superimposed on a trend of synchronous changes in water column oxygen and nutrient concentrations controlled by regional hydrographic conditions were asynchronous small-scale variations at the in-shore stations that attest to localized controls on water column chemistry. Small temporal variations in sulphate reduction rates determined with 35S-labeled sulphate do not support the interpretation that variable emissions of sulphide and methane from sediments are driven by temporal changes in the degradation rates of freshly deposited organic matter. The large temporal changes in the concentrations of hydrogen sulphide and the co-occurrence of pore water sulphate and methane support an interpretation of episodic advection of methane and hydrogen sulphide from deeper sediment depths – possibly due to gas bubble transport. Effective fluxes of hydrogen sulphide and methane to the water column, and methane and sulphide concentrations in the bottom waters were decoupled, likely due to the activity of sulphide-oxidising bacteria. While the causal mechanism for the episodic fluctuations in methane and dissolved sulphide concentrations remains unclear, this data set points to the importance of alternating advective and diffusive transport of methane and hydrogen sulphide to the water column.
Sulfide-oxidizing bacteria mediate authigenic apatite formation in phosphorite-containing sediments of the Namibian upwelling system – evidence from 33P-labeling experiments
2009. Volker Brüchert (et al.).Conference
Temperature sensitivity of extracellular enzymatic hydrolysis in Arctic and temperate marine sediments
2009. Alberto Robador (et al.). Abstracts ASLO Aquatic Science Meeting2009, 224Conference
The impact of temperature change on the activity and community composition of sulfate-reducing abcteria in arctic versus temperate marine sediments
2009. Alberto Robador, Volker Brüchert, Bo Barker Jørgensen. Environmental Microbiology 11 (7), 1692-1703Article
Arctic regions may be particularly sensitive to climate warming and, consequently, rates of carbon mineralization in warming marine sediment may also be affected. Using long-term (24 months) incubation experiments at 0°C, 10°C and 20°C, the temperature response of metabolic activity and community composition of sulfate-reducing bacteria were studied in the permanently cold sediment of north-western Svalbard (Arctic Ocean) and compared with a temperate habitat with seasonally varying temperature (German Bight, North Sea). Short-term <sup>35</sup>S-sulfate tracer incubations in a temperature-gradient block (between −3.5°C and +40°C) were used to assess variations in sulfate reduction rates during the course of the experiment. Warming of arctic sediment resulted in a gradual increase of the temperature optima ( T<sub>opt</sub>) for sulfate reduction suggesting a positive selection of psychrotolerant/mesophilic sulfate-reducing bacteria (SRB). However, high rates at in situ temperatures compared with maximum rates showed the predominance of psychrophilic SRB even at high incubation temperatures. Changing apparent activation energies ( E<sub>a</sub>) showed that increasing temperatures had an initial negative impact on sulfate reduction that was weaker after prolonged incubations, which could imply an acclimatization response rather than a selection process of the SRB community. The microbial community composition was analysed by targeting the 16S ribosomal RNA using catalysed reporter deposition fluorescence in situ hybridization (CARD-FISH). The results showed the decline of specific groups of SRB and confirmed a strong impact of increasing temperatures on the microbial community composition of arctic sediment. Conversely, in seasonally changing sediment sulfate reduction rates and sulfate-reducing bacterial abundance changed little in response to changing temperature.
Microbial Mn(IV) and Fe(III) reduction in northern Barents Sea sediments under different conditions of ice cover and organic carbon deposition
2008. Maren Nickel (et al.). Deep-Sea Research II 55, 2390-2398Article
Survival of Arctic bacteria under freezing and thawing conditions
2008. Joanna Sawicka, Volker Brüchert.Conference
Temperature sensitivity of bacterial carbohydrate hydrolysis in Arctic and temperate marine sediments
2008. Volker Brüchert, Andrew Steen, Carol Arnosti.Conference
Carbon mineralization in Laptev and East Siberian sea shelf and slope sediment
2018. Volker Brüchert (et al.). Biogeosciences 15 (2), 471-490Article
The Siberian Arctic Sea shelf and slope is a key region for the degradation of terrestrial organic material transported from the organic carbon-rich permafrost regions of Siberia. We report on sediment carbon mineralization rates based on O2 microelectrode profiling, intact sediment core incubations, 35 S-sulfate tracer experiments, porewater dissolved inorganic carbon (DIC), δ13 CDIC, and iron, manganese, and ammonium concentrations from 20 shelf and slope stations. This data set provides a spatial overview of sediment carbon mineralization rates and pathways over large parts of the outer Laptev and East Siberian Arctic shelf and slope, and allowed us to assess degradation rates and efficiency of carbon burial in these sediments. Rates of oxygen uptake and iron and manganese reduction were comparable to temperate shelf and slope environments, but bacterial sulfate reduction rates were comparatively low. In the topmost 20 to 50 cm of sediment, aerobic carbon mineralization dominated degradation and comprised on average 82% of the depthintegrated carbon mineralization. Oxygen uptake rates and 35 S-sulfate reduction rates were higher in the eastern East Siberian Sea shelf compared to the Laptev Sea shelf. DIC/NH4 + ratios in porewaters and the stable carbon isotope composition of remineralized DIC indicated that the degraded organic matter on the Siberian shelf and slope was a mixture of marine and terrestrial organic matter. Based on dual end member calculations, the terrestrial organic carbon contribution varied between 32% and 36%, with a higher contribution in the Laptev Sea than in the East Siberian Sea. Extrapolation of the measured degradation rates using isotope end member apportionment over the outer shelf of the Laptev and East Siberian Sea suggests that about 16 Tg C per year are respired in the outer shelf sea floor sediment. Of the organic matter buried below the oxygen penetration depth, between 0.6 and 1.3 Tg C per year are degraded by anaerobic processes, with a terrestrial organic carbon contribution ranging between 0.3 and 0.5 Tg per year.
Turbulence simultaneously stimulates small-and large-scale CO2 sequestration by chain-forming diatoms in the sea
2018. Johanna Bergkvist (et al.). Nature Communications 9Article
Chain-forming diatoms are key CO2-fixing organisms in the ocean. Under turbulent conditions they form fast-sinking aggregates that are exported from the upper sunlit ocean to the ocean interior. A decade-old paradigm states that primary production in chain-forming diatoms is stimulated by turbulence. Yet, direct measurements of cell-specific primary production in individual field populations of chain-forming diatoms are poorly documented. Here we measured cell-specific carbon, nitrate and ammonium assimilation in two field populations of chain-forming diatoms (Skeletonema and Chaetoceros) at low-nutrient concentrations under still conditions and turbulent shear using secondary ion mass spectrometry combined with stable isotopic tracers and compared our data with those predicted by mass transfer theory. Turbulent shear significantly increases cell-specific C assimilation compared to still conditions in the cells/chains that also form fast-sinking, aggregates rich in carbon and ammonium. Thus, turbulence simultaneously stimulates small-scale biological CO2 assimilation and large-scale biogeochemical C and N cycles in the ocean.
Show all publications by Volker Brüchert at Stockholm University