Microbial activity in the ocean´s interior

The proposal addresses one of the most fundamental, albeit unsolved problems in microbial oceanography and marine biogeochemistry, i.e., the apparent discrepancy between the supply of organic carbon and its demand to maintain the measured heterotrophic microbial activity in the dark ocean. One of the biggest uncertainties is that essentially all the metabolic activity measurements on deep-water microbes, the main consumers of organic carbon, have been made on decompressed samples incubated under surface pressure conditions. We tackle this problem along a latitudinal transect in the North Atlantic and 1) compare biomass production, ectoenzymatic activity and respiration of heterotrophic prokaryotic plankton and autotrophic carbon fixation measured under in situ pressure conditions with that under decompressed conditions (as commonly performed) on samples collected in the meso-, bathy-, and abyssopelagic realm. Furthermore, we will determine 2) the interspecific differences in the sensitivity of specific prokaryotic groups to pressure changes and 3), the contribution of piezophilic prokaryotes on the total dark ocean prokaryotic community. This project has been made possible by major preparatory efforts put into the design and fabrication of a high- pressure sampling and incubation system (HPS) made out of titanium and consisting in total of 175 50-ml chambers and eight 200-ml chambers. These chambers are mounted in racks and these, in turn, into regular CTD frames. The HPS are fired at depth like regular Niskin bottles. Upon arriving on board of the vessel, these chambers can be removed from their outer housing and placed in temperature-controlled incubators. Extensive testing has been done over the past four years and now, these HPS are fully operational and absolutely free of contamination. The large number of available chambers allows for sufficient replicates and controls and cost- efficient work at sea. Thus, we have the tools available to evaluate the error introduced if deep-water auto- and heterotrophic activity are measured under decompressed conditions instead of under in situ pressure conditions. Combined with single-cell analyses using radiolabeled and stable isotopes in combination with fluorescence in situ hybridization and halogen in situ hybridization applying microautoradiography and NanoSIMS, respectively, we will be able to resolve the enigma on the magnitude of the actual microbial activity in the ocean`s interior. The outcome of the project will not only provide substantial new insights into the pressure sensitivity of deep-water prokaryotic communities but will also help to refine our current ocean carbon flux models and allow for a major advancement towards a mechanistic understanding on the ecology of deep-water microbial communities.

The role of hydrostatic pressure on deep-sea microbes was investigated in this project. The main finding of MICRO-ACT was that the hydrostatic pressure inherent to the deep ocean water column has only a minor effect on the overall microbial activity. The metabolic activity of deep-sea heterotrophic microbes measured under the actual pressure conditions at the depth of sampling amounts to about 80% of the metabolic activity measured under decompressed conditions. Hence, the hydrostatic pressure plays only a minor role in the metabolic activity of deep-sea organisms. These results were unexpected as it is known for more than half of a century that there are some pressure-loving deep-sea microbes which can only been grown under high pressure conditions. Apparently, these pressure-loving microbes constitute only a minor fraction of the deep-sea microbial community. To perform these experiments, high-pressure sampling and incubation devices were fabricated and after extensive testing, water was collected down to 7000 m depth, representing approximately 700 bar. Metabolic rate measurements were performed under in situ pressure conditions and compared with decompressed samples. The measurements were done during one 4-week research cruise in the Mediterranean Sea and on two research expeditions (4-weeks each) in the North Atlantic. With these measurements, we could convincingly show that deep-sea pressure conditions only marginally affect the majority of the deep-sea microbes. Since microbes mediate the vast part of the metabolism and biogeochemical cycles of the ocean and consequently, also in the deep sea, these findings imply that hydrostatic pressure can be largely ignored when performing metabolic rate measurements as the main part of the deep-sea microbial community is insensitive to hydrostatic pressure, at least down to 7000 m depth.