Research Themes

Shelf seas cover only 7 - 8% (26 x 106 km2) of the global oceans area and contribute to 0.5% of the global oceans volume, but they are among the most active biological and geochemical areas in the world, due to considerable input of terrestrial and oceanic organic matter and nutrients, and the close coupling of benthic and pelagic systems (efficient cycling of matter). While first results in sandy sediments revealed that biogeochemical processes can be as active as in organic rich muddy sediments, the foci of our current studies were on 1) correlation of bacterial acitivity and biodiversity, 2) small scale variability of biogeochemical processes and 3) bentho-pelagic coupling in subtidal sediments. During our field campaigns we combine in situ measurements of geochemical zonations and benthic fluxes (EU 7th FP Sensenet) with microbial diversity analyses (LTER Sylt).

Extreme environments are defined by one or more environmental characteristics close to the limits known for life in its various forms (ESF report 2007). But sometimes the greatest challenge to life are extreme temporal fluctuations or short-term impacts, In this regard, the main focus of our study is to identify and quantify biogeochemical processes and their link to microbial diversity at different types of ecosystems with one or more extreme characteristics: cold seeps (EU 7th FP HERMIONE; Excellence Cluster MARUM), acidic vents (EU 7th FP ECO2), hydrothermal vents, wood and whale falls (ESF EuroCores CHEMECO), oxygen depleted ecosystems (EU 7th FP HYPOX) as well polar habitats (HAUSGARTEN LTER). To improve our knowledge of the functioning of these ecosystems but also how they react on environmental changes, studies on the dynamics of fluxes and biogeochemical processes are combined with biodiversity studies. Case studies include: In the Arctic deep sea, temporarily severe nutrient limitations affect biological and biogeochemical functions. At seep systems, gas seepage is a major driver for the biogeochemical activity and thus the interaction between chemosynthetic communities and the abiotic environment. High CO2 emission from hydrothermal vents can cause extreme pH gradients affecting the distribution of benthic organisms. Hypoxic conditions in quatic ecosystems increase in number and duration and are accompanied with changes in biodiversity and ecosystem functions. Special types of reduced hot spot ecosystem in the deep sea are sunken woods where we investigate the microbial ecology developing anoxic habitats and how they attract chemosynthetic organisms.

Hydrocarbon seeps form where tectonic or gravitational forces advect free gas, methane-rich porewater and/or muds upward into the sulfate-penetrated surface sediments. This high availability of chemical energy leads to a natural enrichment of methanotrophs and high rates of methane oxidation (aerobic methane oxidation - MOx and anaerobic oxidation of methane – AOM). The anaerobic oxidation of methane (AOM) by archaeal methanotrophs (ANME) functions as a major sink in oceanic methane geochemistry, and is a key biogeochemial process in the anoxic seabed. In situ biogeochemical and microbiological observations of natural seabed communities and in vitro enrichments contribute to the understanding of the ecology and physiology of these “uncultivables”. Our studies focus on the quantification of hydrocarbon seepage associated with subsurface hydrate and oil deposits (Excellence Cluster MARUM). Furthermore, we assess the diversity and function of benthic communities controlling hydrocarbon fluxes at cold seeps (EU 7th FP HERMIONE). Long-term observations improve our understanding of temporal dynamics at seep systems with respect to physico-chemical, geological as well as biological processes and their correlations (EU 7th FP ESONET DM LOOME).

The ocean is affected by global change in multiple ways: fisheries and other forms of resource exploitation, land use, warming, ice melt, eutrophication (especially with regard to nitrogen), acidification and introduction of alien invasive species are the most important impacts. It is obvious that any of these factors alone or in combination will have an impact on ocean ecosystems, but it is not trivial to monitor changes in ecosystem biodiversity or function (e.g. productivity, remineralization, bioturbation) due to the technical and logistical challenges of long term observation in the sea. Furthermore, the role of microbes in global change effects and feedback mechanisms is not known.

Microbes occupy distinct niches on animal surfaces, exudates and tissues forming symbiotic, commensalistic or pathogenic relationships with their hosts. Using high-resolution molecular tools and biogeochemical methods, we have found that deep-water biodiversity hotspots on continental margins like cold-water coral reefs and associated sponge accumulations also provide distinct microbial habitats (ESF EuroCores EURODIVERSITY MiCROSYSTEMS; EU 7th FP HERMIONE). In investigations on corals and their reef environment, special focus is given to the question in how far cold-water corals act as “ecosystem engineers” for microbial communities by shaping their diversity via habitat formation or release of organic matter. Sponges are known to host very high amounts of associated microbes, and we have investigated the impact of microbial processes on the metabolic capacities of the sponge animal as a whole, with an emphasis on anaerobic processes of the nitrogen and sulfur cycle.

Marine environments are characterized by a complex interplay of physical, biological, geochemical, and geological processes. The sediment wate rinterface is one of the most important transition zones for solute exchange; it is often characterized by a thin oxic horizon with steep O2 gradients and an extensive spatial and temporal heterogeneity. Faunal activity, patchy distribution of settling organic matter, fluid flow and benthic primary producers may also introduce a mosaic-like pattern of microzones. A fundamental understanding of the biogeochemical processes in sediments therefore requires a quantitative assessment of in situ rates of benthic processes. A variety of in situ instruments (e.g. Microprofiler, BenthicChamber, Eddy correlation and INSINC) have thus been developed and further improved, in close collaborations with the microsenor group and the electronical and mechanical workshops.We can operate these instruments on different underwater platforms (e.g. ROVs, submersibles, benthic crawlers and free-falling frames) (EU 7th FP EUROFLEETS). The technological improvements have lead to smaller modules, which are easier to handle, less power consuming and easier to connect to underwater platforms (e.g. by Ethernet connection). To investigate long-term variations at cold seeps a deep-sea observatory has been designed and deployed at the Hakoon Mosby Mud Volcano (EU 6th FP ESONET). We are currently preparing an array of instruments for the monitoring of oxygen depletion in suboxic and hypoxic aquatic environments (EU 7th FP HYPOX), and train graduate students and post docs in the use and adaptation of various biogeochemical sensors (EU ITN SenseNet).

The exploration of microbial diversity in marine habitats relies on the description of the biogeographic patterns of variation in community structure, abundance and functions at different spatial, temporal and taxonomic scales. The current challenge is two-fold: to develop new high-throughput, reproducible molecular methods so as to deal with numerous samples, but also to develop the necessary statistical and numerical tools to analyze large datasets of biological and contextual parameters. Changes in microbial community structure are examined by using a combination of cutting-edge, highly reproducible molecular ecology techniques such as Terminal Restriction Fragment Length Polymorphism (T-RFLP), Automated Ribosomal Intergenic Spacer Analysis (ARISA) and 454 massive tag sequencing (Census of Marine Life project ICOMM).