Stockholm university

Rachel Ann FosterProfessor

About me

After earning a B.S. in Biology with a focus on Marine and Freshwater Biology from the University of New Hampshire, USA, Dr. Rachel A. Foster spent several years exploring the California coast prior to volunteering on her first oceanographic research cruise to the Western Tropical North Atlantic.  It was an experience that set her on track to applying and continuing her studies in Marine Science. She later received a MSc in Marine and Environmental Biology (1996-2000) and a PhD in Coastal Oceanography (2000-2004) from Stony Brook University, USA.

After her PhD work, Dr. Foster spent several years as a postdoctoral researcher at the University of California, Santa Cruz (UCSC; 2004-2010) working on microbial ecology of diazotrophic cyanobacteria. In 2010, Dr. Foster joined the Biogeochemistry Department of the Max Planck Institute for Marine Microbiology in Bremen, Germany as a Lead Scientist and the facility manager for the nanoSIMS 50L. In 2013, Foster was sponsored by the Department of Ecology, Environment and Plant Sciences of Stockholm University and later selected as a Wallenberg Academy Fellow to lead research on planktonic symbioses.

Dr. Foster’s research focuses on the distribution, activity and diversity of marine phytoplankton and their overall roles in biogeochemical cycles and ecosystem function. To study the in situ activity and interactions (i.e. nutrient exchanges) between marine microorganisms Foster’s lab uses combinations of microscopic techniques with targeted molecular biological and isotope tracer assays.


Link to google scholar profile



My lab focuses on planktonic (open ocean) populations, with a strong emphasis on autotrophic populations (phytoplankton) important to carbon (C) and nitrogen (N) cycling.  I have been largely interested in free-living and symbiotic cyanobacteria populations, which are capable of fixing di-nitrogen gas (N2). In large expanses of the open ocean N is considered the primary limiting nutrient, and thus N2-fixing (diazotrophic) populations play an important role in providing fixed N to the surrounding community. Given that the N and C budgets are tightly coupled, diazotrophs are also critical to primary production.

I have three general themes in my current research: two focus on my interest in diatom-cyanobacteria symbioses and free-living cyanobacteria, and the third concentrates on modeling how microbial populations and activities influence N and C cycling in the upper water column. To do this research, we develop new tools and techniques for studying single cells then we use our results to scale to the water column.

What is the intricate relationship between host and symbiont?

There are at least 4 different diatom genera that associate with different strains of heterocystous cyanobacteria symbionts. Thus determining the commonalities and differences between these partnerships in terms of host and symbiont diversity, distribution and activity is of interest. Second, the relationships are considered highly specific, yet the driver of selectivity is unknown. In addition, we are trying to identify what other nutrients (i.e. C, P, Fe, viatmins) are exchanged, how coordinated the partners are for nutrient acquisition and growth, and finally the extent of ‘host control’ and potential benefit to the symbiont. To date, we have been largely focused on the symbiont, and therefore exploring the host eukaryotic diversity is of great interest.  Most diatoms live and dominate in the coastal habitats; likewise heterocystous cyanobacteria also tend to proliferate in brackish and benthic habitats. The symbiotic diatoms are considered therefore a rare partnership, which allows success in the open ocean and could potentially represent a model for co-evolution.

What is the role of transposons in N2 fixation and phenotype plasticity of unicellular N2 fixing cyanobacteria?

The current paradigm of the global marine N cycle considers the activities by unicellular N2-fixing (diazotrophs) cyanobacteria an important source of new N due to their high abundances and broad distributions.  Genome comparisons of several isolated strains and comparisons to environmental sequences have identified unprecedented genome conservation, while the same strains display high phenotype plasticity.  The plasticity in phenotype is thought to be mediated by high numbers of mobile elements called transposons. Determining the role of transposons in phenotype plasticity, i.e. colony formation, is a primary aim, as is isolating new strains from new habitats, including the Indian Ocean. Currently isolated strains are limited to 2 ocean basins (4 Atlantic, 6 Pacific) and most have been in isolation in excess of 14 years.

How does ‘lebensformen’ influence community structure and nutrient cycling?

In the last few years, I have been fascinated by a microbes ‘lebensformen’ or life style and how it might influence activity, genetic content, distribution, and the surrounding community. Cells that live in close association have a higher propensity for genetic (i.e. transposases) and nutrient exchanges. Currently, we employ methodologies for determining the bulk activities, however we know less about cell-to-cell variation. We also have gaps in our understanding of how microbes contribute to the dissolved pools. (i.e. N, C). The lifestyle (colonial, symbiotic, free-living) and cell integrity (morphology, diameter, activity) will dictate the sinking rate for a given population and it’s contribution to the surround. Recently I have begun to use mathematical modeling approaches to determine the sinking rates of the symbiotic and free-living cyanobacterial populations relative to their diffusion of newly fixed N and C. Ultimately, my goal is to build physiological and nutrient exchange models for the open ocean diazotrophs in order understand how microbial communities are influencing nutrient inventories and potentially responding to global climate changes.

Research projects