- The microbial assemblages, are in the basis of the food chain and thus influence all ecosystem services of the Baltic Sea. The microbes in this study are important for evaluating the environmental status and water quality of the Baltic Sea, because they contribute to biomass, nutrient cycling, and biodiversity, says Dandan Izabel-Shen, one of the leading authors of the study.

Séréna Albert, PhD student at DEEP, was on the boat out for sampling sediment cores for the experiment. The Sampling was conducted on 4th September 2017 from Hållsviken, in the northern Baltic Sea proper. Photo: Séréna Albert.

Particularly the abundance of filamentous cyanobacteria is likely to increase with global warming and excessive nutrients in the aquatic environments, more than the diatoms. The bloom of cyanobacteria may start early of the year and dominate the bloom-forming phytoplankton throughout the year.

Cyanobacteria may be a bit “worse” food sources for benthic animals; since they are poor sources of fatty acids and amino acids compared to diatoms.

However, from the perspective of microbes (bacterial assemblages), as mentioned early, they can decompose and break down organic matter released from cyanobacteria and diatoms, in the overlying water. They regenerate nutrients for other bacteria to take up and produce living biomass and thereby the food sources for microbe consumers (e.g., protists, nematodes) to feed on.

The sketch tells an overview of interaction between phytoplankton and heterotrophic bacteria: During and after the bloom, some phytoplankton are being dispersed via water column as sinking particulate organic matters (POM) and settle down on the sediment surface. Once settle-down, these living phytoplankton-dominated POM per se are carbon-amino acids rich foods sources for benthic organisms. at the same time, the form of dissolved organic carbon released from them can be readily taken up by bottom water and sediment-dwelling heterotrophic bacteria to produce living biomass. Photo: Dandan Izabel-Shen.

Cyanobacteria and bacterial associates appear to have efficient nutrient transformation, and thereby the transfer of substrates benefit the growth of those bacteria. In our study, a higher proportion of bacteria associated with cyanobacteria was found than those associated with diatom. 

Séréna was implementing the experiment in the Askö Marine Research Station. The Baltic Sea has been subjected to eutrophication progressively, which has been projected to favour the predominance of cyanobacteria over diatoms in the water column and thus in the composition of bloom-forming phytoplankton. Thus, we designed a microcosm experiment in which diatom and cyanobacterial slurries were mixed in varying proportions and added into each sediment core to simulate different quality of particulate organic matter. Photo taken by Séréna Albert.

We hypothesized that organic matter released from diatoms includes complex and high molecular weight substrates which require more energy from microbe to degrade; however less complex organic carbon molecules (e.g., glucose), are made available when cyanobacteria per se reuse and degrade extracellular organic carbon. Those simple carbon can be readily assimilated by a great number of bacteria. Furthermore, changes in bacterial community composition has the potential for cascading effects on food web yield. For instance, the bacterial assemblages shifted to the communities with high utilization of organic matter, which would lead to the consumption of oxygen and cause partially oxic to anoxic sediments.  Despite adaption of benthic organisms to low oxygen environments, oxygen deficient in the sediment could affect the behavior of organisms with low sensitivity to hypoxia; consequently, inadequate energy flow passes through the food web chain.

The scientific article “Quality of phytoplankton deposition structures bacterial communities at the water-sediment interface” was published in the journal Molecular Ecology.