MOCA_Microbial Oceanography of ChemolithoAutotrophic planktonic Communities

Simulation models predict that the oxygen content of the global ocean will decrease by 25% until the end of the century due to an increased stratification of the oceanic surface waters and a rise in temperature. This loss in oxygen will inevitably lead to an expansion of hypoxic and anoxic areas in the global ocean with major consequences for the oceanic carbon and nitrogen cycling. In this proposal, we assess the functional diversity of chemolithoautotrophic prokaryotic communities in two contrasting marine environments, the deep-water masses of the North Atlantic along a latitudinal gradient and around the redoxcline in the central Baltic Sea. Both environments have been shown previously to harbor highly active chemolithoautotrophic prokaryotic communities with dark carbon dioxide fixation rates approaching surface water phytoplankton activity. Specific focus is put on the functional diversity of prokaryotes in the carbon and nitrogen cycling in both systems, including the sulfur cycle in the central Baltic. Biogeochemical rate measurements are tightly linked to functional gene analyses using among other approaches metagenomics and metatranscriptomics. Information obtained from these analyses will guide the development of primers for QPCR to determine the abundance of genes indicative for geochemically relevant processes in the water column of the two systems. Incubation experiments using stable and radio-isotopes in combination with molecular techniques such as SIP-RNA analyses, single-cell analyses using Raman-FISH, NanoSIMS and MICRO-FISH will allow insights into the dynamics of the functional diversity of chemolithotrophic microbial communities in suboxic and anoxic marine planktonic systems. Field studies will be complemented by laboratory model systems with isolated key players in order to understand the adaptive capacity and performance of chemolithoautotrophs in response to different environmental conditions. The combination of these approaches will provide the base for a significant advancement in our understanding of planktonic chemolithoautotrophy in the dark ocean.

The conventional view of life in the deep sea is that they depend on the consumption of sinking organic particles produced by phytoplankton in the sunlit surface waters of the ocean. The vast majority of the consumption of organic matter in the deep sea is due to the so-called heterotrophic microbes. However, some microbes are not utilizing organic matter as a carbon and energy source but utilize carbon dioxide as carbon source and reduced inorganic substances as energy source. The amount of carbon dioxide fixed in the deep oceanic water column is largely unknown. In the frame of the international research project, the magnitude of the so-called chemolithoautotrophic production of microbes has been studied in the Atlantic and the organisms responsible for that determined. The main outcome of the project was that the carbon dioxide fixation in the deep oceanic water column of the Atlantic is of the same magnitude as the heterotrophic microbial production. This novel source of organic matter synthesis in the deep ocean has not been taken into account in global carbon budgets and hence, has the potential to explain the high biomass of the deep ocean biota. These findings represent new ground in microbial oceanography.