Final Report Summary - MEDEA (Microbial Ecology of the DEep Atlantic pelagic realm) In MEDEA, the enigma of the mismatch between organic carbon supply and demand by heterotrophic microbes was addressed. Available measurements indicate that the organic carbon supply is about one order of magnitude lower than the microbial demand. In MEDEA, it has been hypothesized that this mismatch is due to overestimating deep sea microbial metabolism by the conventionally performed metabolic rate measurements under surface pressure conditions. fundamental mismatch between organic carbon flux into the deep ocean and the organic carbon demand by heterotrophic microbes in the dark ocean's realm. As the organic carbon demand is commonly measured under atmospheric pressure conditions, it has been hypothesized in MEDEA that this mismatch is due to lower metabolic activity of deep sea microbes under in situ pressure conditions. The main goal of MEDEA was, therefore, to determine the influence of hydrostatic pressure on microbial activity in the deep sea and the role of detrital particles for microbes in the deep sea. Using high pressure sampling and incubation systems, we measured in situ heterotrophic microbial activity and compared these measurements with those made under surface pressure conditions during three research cruises in the Atlantic and Pacific. We found that the difference in heterotrophic microbial activity under these two hydrostatic pressure conditions increases linearly with depth. At 2000 m depth, microbial activity under in situ pressure conditions is only about 20% of the microbial activity in the corresponding sample measured under surface pressure conditions. Also, we found that there is a large stock of neutrally buoyant marine-snow type particles in the deep ocean. These neutrally buoyant particles do not decrease in abundance with depth, in striking contrast to the sinking particles flux. These neutrally buoyant particles are harbored by microbes at abundances 3-4 orders of magnitude higher than in the ambient water. Moreover, we found that these particles are inhabited by a large number of fungi. The biomass of fungi on these particles is similar to that of prokaryotes. The most significant outcome of MEDEA is that with increasing hydrostatic pressure microbial activity decreases linearly. This means that less organic carbon is used by microbes in the dark ocean than hitherto assumed based on the metabolic measurements performed at surface pressure conditions. Based on these measurements, it appears that the mismatch between organic carbon supply and demand in the deep ocean could be resolved. Our findings also indicate that most deep sea microbes are not adapted to the pressure conditions, i.e. are not piezophilic microbes. The large number of neutrally buoyant particles in the deep ocean which are not collected by sediment traps, commonly used to determine organic flux throughout the water column, provides additional substrate for heterotrophic microbes. This type of particles are not included in carbon budgets of the deep sea. The finding that these particles are heavily colonized by fungi is novel. Future work will focus on the metabolism of these fungi.There is an increasing demand in knowledge on deep sea ecology and biogeochemistry, particularly to address the need for a better understanding of these ecosystems as a base for a sustainable exploitation, such as for deep sea mining. Taken together, MEDEA revealed that the vast majority of deep-sea microbes are not adapted to the in situ pressure conditions and are significantly retarded in their activity. For the dark ocean's carbon budget the outcome of MEDEA indicates that dark ocean's microbial activity is substantially lower than hitherto assumed. The microbial activity rates measured in MEDEA indicate that the dark ocean's carbon supply and demand are basically in balance thus, a long standing enigma has been solved.
