Research Projects

Research in the Biogeochemistry Department focuses on microbiological and geochemical processes that control bioactive element cycling in the marine environment. We employ geochemical, microbiological, modeling, molecular and single-cell techniques to study the environmental regulation of these processes, and their effects on the global biogeochemical cycles.

Our goal is to provide fundamental insights into microbially-mediated processes in the Ocean that ultimately affect ocean chemistry, biology and climate, and will provide vital input for models used to predict potential future changes resulting from human activities.

Autofluorescence of Dolichospermum
Image showing autofluorescence of Dolichospermum (in blue) and Aphanizomenon (filamentous, in red) from the Baltic Sea recorded with the SECOM platform (Delmic) mounted on a secondary electron microscope (photo: MPI-MM).

Single-cell environmental microbiology

The vast majority of environmental bacteria remain uncultured. Our knowledge of their physiology and metabolic capacities therefore strongly depends on methods capable of analyzing single cells. Reliable identification of target populations in environmental samples remains a key prerequisite for analyses by modern microscopic and spectrometric techniques. We develop labeling and imaging methods that would allow for correlative imaging using light-, ion-, laser- and electron microscopy in order to gain new insights into the contributions of microorganisms to the biogeochemical element cycling.

On deck incubations
Incubators for nitrogen fixation experiments on board R/V Sonne during a cruise to the South Pacific Gyre.

N2 fixation 

Microbial fixation of N2 gas into biologically available ammonium is the main source of nitrogen in the open Ocean. N2 fixation is only catalyzed by a small but diverse group of prokaryotic microorganisms. In the open ocean, the majority of N2 fixation is attributed to larger colonial and heterocystous cyanobacteria populations. However, there are also other, unicellular, N-fixing microorganisms and it is thought that the activity of these other N2 fixing populations may help balance the N budget; however we know little about their distribution and activity.

Sand on a beach
Sand on a beach

Sandy sediments

Permeable sediments dominate the seafloor of continental shelves and yet they are amongst the most understudied habitats within our oceans. Permeable sediments act as a biocatalyst for organic matter mineralization and nitrogen turnover, both of which are driven by advective porewater flow. With the help of a new benthic lander and flow-through reactors we measure oxygen uptake and nitrogen loss in both intertidal and subtidal sediments of the Wadden Sea and German Bight.

CTD Rosette
A CTD Rosette sampler is being lowered into water during a cruise of the R/V Discovery to the Eastern Tropical North Pacific oxygen minimum zone (photo: L.A. Bristow).

Nitrogen loss from oxygen minimum zones

Oxygen minimum zones (OMZ) are expanding globally, and at present account for up to 40 % of oceanic nitrogen loss. For decades, oceanic nitrogen loss has been attributed entirely to heterotrophic denitrification. By using a combination of molecular techniques and incubation experiments we have shown that, instead, anaerobic ammonium oxidation (anammox) is the main nitrogen loss process in the Namibian, Peruvian and Arabian Sea OMZ. However, suboxic conditions of the OMZ allow for both anaerobic and aerobic processes to co-occur and despite their obvious importance, the magnitude and interactions of these processes in OMZ are not well understood.

Diatom and bacterial aggregate
An aggregate of circular and pennate diatoms and various bacteria from the oxycline of a freshwater lake Cadagno (photo: MPI-MM).

Marine snow and sinking particles

Aggregate sedimentation and remineralization control the vertical flux of organic matter, and directly affect elemental cycling and food-web structures. These processes constitute an important variable in the Ocean’s response to climate change. Sinking marine aggregates are the main vehicles removing organic carbon from the sea surface to the deep sea and are hot-spots of microbially-mediated bioactive element cycling. Remineralization of sinking POM also leads to the formation of oceanic oxygen minimum zones (OMZ).

Filamentous methane-oxidizing bacteria
Fluorescence image of methane-oxidizing bacteria (in green) and various other bacteria (in blue) from a freshwater lake (photo: MPI-MM).

Methane oxidation

Despite covering most of the Earth’s surface, methane emissions from the oceans are comparably small. On the contrary, terrestrial aquatic reservoirs are major sources of methane, with freshwater lakes contributing up to 16%. Methanotrophic microorganisms play a key role in reducing methane fluxes in oxic as well as in anoxic aquatic systems. Both bacteria and archaea posses the capacity to oxidize methane. It is recognized that in marine environments anaerobic methane oxidation processes act as a very efficient methane filter. On the contrary, the full variety of processes responsible for methane removal in lacustrine environments is not understood, nor are the key microbial players known.

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