It is generally anticipated that particulate organic carbon (POC) for most part is degraded by attached microorganisms during the descent of “marine snow” aggregates toward the deep sea. There is, however, increasing evidence that fresh aggregates can reach great depth and sustain relatively high biological activity in the deep sea. Using a novel high-pressure setup, we tested the hypothesis that increasing levels of hydrostatic pressure inhibit POC degradation in aggregates rapidly sinking to the ocean interior. Respiration activity, a proxy for POC degradation, was measured directly and continuously at up to 100 MPa (corresponding to 10 km water depth) in a rotating pressure tank that keeps the aggregates in a sinking mode. Model diatom-bacteria aggregates, cultures of the aggregate-forming diatom Skeletonema marinoi, and seawater microbial communities devoid of diatoms showed incomplete and complete inhibition of respiration activity when exposed to pressure levels of 10–50 and 60–100 MPa, respectively. This implies reduced POC degradation and hence enhanced POC export to hadal trenches through fast-sinking, pressure-exposed aggregates. Notably, continuous respiration measurements at ≥50 MPa revealed curved instead of linear oxygen time series whenever S. marinoi was present, which was not captured by discrete respiration measurements. These curvatures correspond to alternating phases of high and low respiration activity likely connected to pressure effects on unidentified metabolic processes in S. marinoi.
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We would like to thank Louise Reinbach Hansen, Anni Glud, Erik Laursen, Morten Elsner Kieler, Kenneth Knudsen, Per Martensen, Martina Alisch, and Johannes Ritter for technical help. Anja Kamp is acknowledged for microscopic axenia checks of the cultures and Morten Larsen for his help with establishing the high‐pressure setup. This study was financially supported by the European Union's Horizon 2020 Research and Innovation Program (Grant agreement No. 669947; HADES‐ERC) and the Danish National Research Foundation through the Danish Center for Hadal Research (HADAL, Grant no. DNRF145). S. marinoi
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