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Abstract Heterotrophic microorganisms are responsible for transforming and respiring a substantial fraction of the organic matter produced by phytoplankton in the surface ocean. Much of this organic matter is composed of polysaccharides, high‐molecular weight (HMW) sugars. To initiate degradation of polysaccharides, microorganisms must produce extracellular enzymes of the right structural specificity to hydrolyze these complex structures. To date, most measurements of enzyme activities are made at in situ temperatures, but at atmospheric pressure. However, previous studies have shown that hydrostatic pressure can impact the functionality of enzymes. Since deep sea communities may be seeded by microbes from shallow waters, we aimed to determine if pressure affects the performance of enzymes from coastal waters. To determine the extent to which enzymatic activities of coastal microbial communities are affected by pressure, we quantified the degradation of seven polysaccharides under pressures ranging from 0.1 MPa (atmospheric) to 40 MPa (equivalent to 4,000 m). Enzyme activities of pelagic communities were inhibited with increased pressure, while enzyme activities of benthic microbial communities were more resistant to increased pressure. Addition of HMW organic matter resulted in communities with enzyme activities that were more resistant to increased pressure. However, the freely‐dissolved enzymes (<0.2 μm) produced by these communities were strongly inhibited by increased hydrostatic pressure, suggesting that the pressure‐resistant enzymes were cell‐surface attached. Because pressure inhibition of enzyme activities varied strongly by polysaccharide, we surmise that the structural complexity of a polysaccharide—and therefore the number of distinct enzymes required for hydrolysis—is likely closely associated with pressure inhibition. 

Abstract Heterotrophic bacteria hydrolyze high molecular weight (HMW) organic matter extracellularly prior to uptake, resulting in diffusive loss of hydrolysis products. An alternative ‘selfish’ uptake mechanism that minimises this loss has recently been found to be common in the ocean. We investigated how HMW organic matter addition affects these two processing mechanisms in surface and bottom waters at three stations in the North Atlantic Ocean. A pulse of HMW organic matter increased cell numbers, as well as the rate and spectrum of extracellular enzymatic activities at both depths. The effects on selfish uptake were more differentiated: in Gulf Stream surface waters and productive surface waters south of Newfoundland, selfish uptake of structurally simple polysaccharides increased upon HMW organic matter addition. The number of selfish bacteria taking up structurally complex polysaccharides, however, was largely unchanged. In contrast, in the oligotrophic North Atlantic gyre, despite high external hydrolysis rates, the number of selfish bacteria was unchanged, irrespective of polysaccharide structure. In deep bottom waters (> 4000 m), structurally complex substrates were processed only by selfish bacteria. Mechanisms of substrate processing—and the extent to which hydrolysis products are released to the external environment—depend on substrate structural complexity and the resident bacterial community.