Marine microorganisms control biogeochemical exchange processes between the surface ocean and the atmosphere as well as between the surface ocean and the deep sea. To investigate the microbial cycling of dissolved and particulate organic matter our studies address processes of production and exudation, uptake and respiration, transformation as well as aggregation and export of particles.
Research approaches related to the marine carbon cycle include compound and size specific analyses of organic matter combined with microbial rate measurements, incubation experiments, culture studies with single species/strains under well-controlled laboratory conditions, field experimentation, field surveys across natural gradients of key environmental parameters to substitute space for time, and numerical modelling of ecosystem and biogeochemical processes. Our field studies take us from the ice-covered polar region to the warm tropical seas.
In the sunlit layer of the ocean, microscopic algae form the base of the marine food web. Some of the carbon they contain sink into the ocean’s interior (>100 m) fueling dark ocean and benthic communities. This biologically mediated process is known as the biological carbon pump. The biological carbon pump is important for the earth system because it tempers some of the anthropogenically induced increase of CO2 concentration in the atmosphere. However, the factors driving the biological carbon have yet to be clearly identified.
The biological pump is an ecosystem service that influences atmospheric CO2 concentration by transporting dissolved and particulate biogenic elements (C:N:P) into the ocean’s interior. Without, the biological carbon pump (BCP), the atmospheric CO2 concentration would be twice as high as the current concentration. Current assessments of the global strength of the BCP vary widely (5-20 GtC yr-1). Both, uncertainties associated to its temporal/spatial dynamic and to its mechanistic representation in numerical models prevent any better estimate. Consequently, making predictions on the BCP with respect to warming, increasing stratification, acidification and anoxia, is presently equivocal.
Our approach is dual: (1) we use lab-controlled experiments to provide mechanistic understanding on processes involved in the BCP. This involves understanding the changes in phytoplankton physiology that promote secretion of sticky compounds (transparent exopolymers) that promote particle formation as well as the heterotrophic pathways of particulate organic matter degradation resulting from climate induced changes in CO2, O2 concentration or organic matter quality. We also investigate the effect of increasing anthropogenic pollutant such as microplastic on processes linked to the biological carbon pump (2) we use field study to test and parameterize our findings on the biological carbon pump cycling earned in vitro using innovative tools like sediment traps and particle degradation bioassays. Also, field studies provide unique opportunity to assess the biological carbon pump in a quantitative manner. Results are then used to better resolve the biological carbon pump in numerical models and further integrated in large IPCC like simulations of the earth system. Our approach uses innovative technologies like the surface tethered sediment traps and various particle incubation systems developed in collaboration with the GEOMAR Technical and Logistic center (TLZ).
Current projects:
Heterotrophic bacteria are the main producers of CO2 in the ocean, thereby counteracting the biological drawdown of CO2 by primary production. The transfer of organic carbon to the deep ocean and the subsequent long term removal of CO2 from the atmosphere are strongly attenuated by heterotrophic recycling processes that regenerate inorganic nutrients and CO2 in the microbial loop. Bacterial activity in the ocean is controlled by multiple abiotic and biotic environmental factors. Major constraints on bacterial growth and activity are temperature and the availability of organic matter and inorganic nutrients. Effects of climate change on heterotrophic processes driven by bacterial activity in the ocean are still largely unknown. Studies conducted in the last decade have shown a high potential of warming and acidification to enhance heterotrophic bacterial activity. Tipping the balance of autotrophic carbon fixation and heterotrophic recycling would have a high potential to change biogenic carbon fluxes in the ocean. Our studies investigate the impact of warming, acidification and deoxygenation on the bacterial turnover of organic matter and aim to identify processes that are potentially sensitive to upcoming changes in the marine environment.
Projects:
The euphotic zone is the principal site of dissolved organic matter (DOM) production in the open ocean. Various mechanisms are responsible for DOM net accumulation in the ocean, such as phytoplankton release, grazer mediated release and excretion, release via viral or bacterial cell lysis, particle remineralization, release from prokaryotes. In water column, DOM undergoes numerous biotic and abiotic structural changes, resulting in the formation of various organic compounds. Therefore, oceanic DOM is a complex mixture of molecules that are produced by biotic and abiotic processes in the ocean.
DOM serves as important energy and nutrient source for marine heterotrophic communities. Readily bioavailable DOM is consumed within hours to days. The DOM fraction that is resistant to microbial degradation may stay in the ocean interior for months to millennia, therefore serving an important sink for atmospheric CO2. An important sink of DOM is the formation of organic gels, i.e. transparent exopolymer particles (TEP) and coomassie-stainable particles (CSP). DOM can self-assemble and form porous microgels that can reversibly exchange material with DOM and particulate organic carbon (POM), or further aggregate into macrogels, thereby constituting an important link between the DOM and POM pools. We study DOM distribution, composition, cycling and it’s partitioning to POM in climate relevant areas. As DOM is a complex mixture of organic compounds, we use several approaches to learn about DOM quality.
Chemically characterizable DOM compounds, such as carbohydrates (DCHO) and amino acids (DHAA) may serve up to 30% of dissolved organic carbon in the ocean. DCHO and DHAA represent the most labile/semilabile fraction of DOM in the ocean and their residence time is relatively short (minutes to months) in the water column. DCHO and DHAA measurements give important information on bioavailability of DOM at open ocean sites and on microbial DOM consumption during incubation experiments. The fraction of total DOM that absorbs light (230-700nm), i.e. chromophoric DOM (CDOM), and the fraction of CDOM that is able to fluoresce, i.e. fluorescent DOM (FDOM), give additional information about DOM cycling. CDOM spectral properties and FDOM components may be used for studying changes in the relative molecular weight of DOM and also to discriminate between different organic matter pools, including as proteinaceous DOM as well as humic-like substances.
Projects:
From the surface, the ocean might appear uniform and smooth, but just as there are fronts and vortices in the atmosphere - the high and low pressure fields in meteorological maps that determine sunny or rainy weather - there are fronts and vortices in the ocean. These so-called mesoscale eddies are ubiquitous phenomena in coastal regions and at current fronts with horizontal scales on the order of 100 km and timescales on the order of a month. In coastal upwelling systems, mesoscale eddies are formed through the interplay of ocean currents and winds and they significantly affect the physical, biogeochemical and biological properties of the ocean. For example, mesoscale eddies are important vehicles for water mass transport from Eastern Boundary Upwelling Systems, which are some of the most productive marine ecosystems, to the open oligotrophic ocean. These water masses are rich in carbon and nutrients affecting both primary production and export fluxes, which has consequences for the distribution of oxygen through locally variable remineralization processes. Since oxygen concentration is one of the major controls for the distribution of larger pelagic organisms, such as zooplankton as well as meso- and epipelagic fish, mesoscale activities in and around coastal upwelling systems also have socio-economic impacts. It is further hypothesized that climate change will alter the characteristics and statistics of oceanic eddies with probably profound effects on the dynamics and functions of these systems.
With a truly collaborative effort, we investigated the role of mesoscale eddies for the lateral transport of biogeochemical properties and its coupling to the biological carbon pump in the Canary Current System as part of the REEBUS (Role of Eddies in the Carbon Pump of Eastern Boundary Upwelling Systems – Demonstration Case Canary Current System) project. Our particular goals were (1) to determine the influence of mesoscale eddies on upper ocean organic carbon distribution, (2) to understand the influences of eddy dynamics on microbial productivity and organic matter turn-over and (3) to assess the role of EBUS in the lateral supply of organic matter to the central Atlantic Ocean.
Within the project we were able to describe and quantify the influence of cyclonic eddies that form in coastal upwelling regions on the carbon cycle. We could demonstrate that eddies enhance the transport of fresh organic carbon from the coast to the open ocean and potentially support net heterotrophy in the open ocean. Our results further suggest that mesoscale eddies are hotspots for organic carbon export and contribute significantly to carbon sequestration in the ocean.
