Shedding light on dark ocean’s detrital particles

There is a major mismatch between the organic matter supply and the organic matter demand of the heterotrophic organisms inhabiting the deep-sea. The consumption of organic matter is much higher than the organic matter flux in to the deep-ocean and this mismatch increasing with increasing depth reaching one order of magnitude at 1000 m depth. Resolving this discrepancy between organic matter supply and consumption is the goal of DEPOCA. We have evidence, based on earlier work, that there is a major stock of organic particles in the deep-sea which is not sinking through the oceanic water column but is neutrally buoyant and hence, transported horizontally through the water column. While the source of these buoyant particles is unknown, we do have evidence that these particles are heavily colonized by bacteria and fungi utilizing this material. The project DEPOCA aims at deciphering the source and fate of these particles in the deep waters of the North Atlantic. During a research expedition, sinking and neutrally buoyant particles will be collected and the age and source of these particles determined. Also, the microbial community composition and the activity of the microbes will be assessed. We expect that these neutrally buoyant particles, which escape routine measurements will resolve the enigma of the missing organic matter in the deep ocean.

The project revealed that in the open ocean deep-sea marine snow is inhabited by heterotrophic microbes, particularly, bacteria and fungi. While bacteria associated with marine snow accounted for less than 1% of the total bacterial abundance, they contributed more than 10% to total bacterial biomass production and 28% to total community respiration. This indicates that in the waters of the North Atlantic below 200 m depth, particle associated microbes are disproportionally active remineralizers of organic matter. Due to their high respiratory activity, particle associated bacteria exhibit a lower growth efficiency than the free-living microbial counterpart. The microbial community composition on marine snow is also strikingly different from that in the ambient water, indicating a specialized community develops on these sinking particles. In contrast non-sinking buoyant particles are resemble a bacterial community composition which is very similar to that of the ambient water collected at the same depth as these non-sinking particles. Single-cell respiration rates determined by redox-sensor-green, a fluorescent dye, indicated that hydrostatic pressure does not affect respiration of deep-sea bacteria. Hence, the respiration rate is insensitive to pressure changes. Overall, the project revealed that bacteria associated with detrital particles in the deep waters play an important role on remineralizing organic matter and thus, control to a large extent the sequestration of organic carbon in the ocean's interior.