Response of Epipelagic Marine Microorganisms to High Hydrostatic Pressure: “Marine Snow” Experiments in a Rotating Pressure Tank

Evidence is accumulating that sinking phytoplankton aggregates carry relatively fresh particulate organic matter to great water depths sustaining benthic communities in the deep sea. We hypothesized that organic matter degradation through microbial respiration is inhibited by the increase in hydrostatic pressure during the descent of phytoplankton aggregates. Real-time and end-point oxygen measurements in rotating pressure tanks were therefore used to measure dark respiration rates in sinking diatom-bacteria aggregates. The key microbial organisms that form these aggregates, i.e., an axenic strain of the diatom Skeletonema marinoi and diatom-free surface seawater, were investigated also separately. At pressure levels of 100-500 bar (corresponding to water depths of 1000-5000 m), respiration rates were inhibited by 10-50% in the aggregates, the diatom strain, and the diatom-free seawater. At pressure levels of 500-1000 bar, however, pressure effects on respiration differed between the three sample types: (1) Respiration stopped completely and without delay in diatom-free seawater, implying the complete inhibition of bacterial degradation of organic matter. (2) By contrast, respiration continued for a few hours in the aggregates and the diatom strain, before respiration also stopped completely in these sample types. We conclude that hydrostatic pressure inhibits the respiratory activity of diverse epipelagic marine microorganisms that are not adapted to high-pressure conditions. The entailing reduction of microbial organic matter degradation likely contributes to the deposition of relatively fresh organic matter in the deep sea and in hadal trenches.