In most seafloor habitats, microorganisms make up more than 90% of the total benthic biomass. A tablespoon of ocean floor sediment contains a billion (10⁹) of bacterial and archaeal cells, and even some kilometer below the seafloor, more than 10000 cells are found in a mililitre of subsurface sediment. Hence, it was estimated that seafloor bacteria make up a third of Earth’s biomass, but little is known about their identity and functioning. The ocean seabed is a diverse environment that ranges from the desert-like deep-sea floor to the rich oases present at seeps and vents. Most seafloor microorganisms live under extreme energy limitation with apparent mean generation times of months to up to thousands of years. But where chemical energy like hydrogen, methane, hydrogen sulfide, and iron is delivered to the seafloor, rich and diverse microbial communities thrive which act as primary producers in chemosynthetic food webs. Compared to the water column, the ocean seafloor provides a huge area of solid surfaces, heterogeneous pore spaces, and a high concentration of detrital organic matter per volume. Benthic microbial communities are partially immobilized so that distinct energy-rich gradients can form. Transport processes such as diffusion, advection and mixing are normally limited by sediment porosity and compaction to a few centimetres per year. The sequence of depletion of microbial electron acceptors is generally from oxygen and nitrate, via oxidized manganese and iron minerals, to sulfate and ultimately to bicarbonate. Anoxic ocean floor - where sulfate and CO₂ are the main electron acceptors - is found in upwelling areas, oxygen minimum zones at continental margins, as well as at hydrothermal vents and cold seeps. The most oligotrophic seafloor where oxygen penetrates metres into the seafloor is found in the central ocean gyres, but also in the Eastern Mediterranean and in the ice-covered deep Arctic.

The goal of our research is to understand structure and change of microbial ecosystems, the formation of niches for microbial populations and the environmental dynamics and their consequences on the occurrence, biodiversity and distribution of microbial populations. This includes research on the physical and chemical characteristics of microbial habitats, such as transport processes between marine sediments and the water column, and energy fluxes across oxic/anoxic chemoclines, and from solid to solute and gaseous phases. Together with the observation of physico-chemical characteristics we analyze their ecological consequences, and develop models for these processes. Our research is accompanied by advances in marine technologies to resolve the spatial and temporal dynamics of microbial habitats in situ as well as in experimental set-ups. Two mysteries are fascinating us: first, how benthic communities satisfy their metabolic needs and second, which factors account for the high biodiversity of deep sea benthic communities, especially in the small size classes of organisms. Taking into account the low turnover of deep-sea benthic microbial communities, which results from limited energy supplies and large standing stocks of carbon, the environmental factors that drive microbial diversity at the ocean floor and the spatial and temporal scales on which they operate remain fascinating questions that are highly pertinent to microbial evolution.

Further information could be found on the MPI-MM webpages.

Contact: A. Boetius, A. Ramette