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Heterotrophic bacteria in the ocean invest carbon, nitrogen, and energy in extracellular enzymes to hydrolyze large substrates to smaller sizes suitable for uptake. Since hydrolysis products produced outside of a cell may be lost to diffusion, the return on this investment is uncertain. Selfish bacteria change the odds in their favor by binding, partially hydrolyzing, and transporting polysaccharides into the periplasmic space without loss of hydrolysis products. We expected selfish bacteria to be most common in the upper ocean, where phytoplankton produce abundant fresh organic matter, including complex polysaccharides. We, therefore, sampled water in the western North Atlantic Ocean at four depths from three stations differing in physiochemical conditions; these stations and depths also differed considerably in microbial community composition. To our surprise, we found that selfish bacteria are common throughout the water column of the ocean, including at depths greater than 5500 m. Selfish uptake as a strategy thus appears to be geographically—and phylogenetically—widespread. Since processing and uptake of polysaccharides require enzymes that are highly sensitive to substrate structure, the activities of these bacteria might not be reflected by measurements relying on uptake only of low molecular weight substrates. Moreover, even at the bottom of the ocean, the supply of structurally-intact polysaccharides, and therefore the return on enzymatic investment, must be sufficient to maintain these organisms.

Seamounts, often rising hundreds of metres above surrounding seafloor, obstruct the flow of deepocean water. While the retention of deep-water by seamounts is predicted from ocean circulation models, its empirical validation has been hampered by large scale and slow rate of the interaction. to overcome these limitations we use the growth of planktonic bacteria to assess the retention time of deep-ocean water by a seamount. the selected tropic Seamount in the north-eastern Atlantic is representative for the majority of isolated seamounts, which do not affect the surface ocean waters. We prove deep-water is retained by the seamount by measuring 2.4× higher bacterial concentrations in the seamount-associated or 'sheath'-water than in deep-ocean water unaffected by seamounts. Genomic analyses of flow-sorted, dominant sheath-water bacteria confirm their planktonic origin, whilst proteomic analyses of the sheath-water bacteria, isotopically labelled in situ, indicate their slow growth. According to our radiotracer experiments, it takes the sheath-water bacterioplankton 1.5 years to double their concentration. therefore, the seamount should retain the deep-ocean water for 1.8 years for the deep-ocean bacterioplankton to grow to the 2.4× higher concentration in the sheathwater. We propose that turbulent mixing of the seamount sheath-water stimulates bacterioplankton growth by increasing cell encounter rate with ambient dissolved organic molecules.The 1,000-year-long global thermohaline circulation 1 connects the bulk deep water of the modern World Ocean, irrespective of barriers erected by continents, islands and thousands of seamounts 2-4 . While continents and islands shape the circulation, seamounts affect this deep-water flow by creating enclosed circulation cells 5 , thereby reducing exchange between the so-called 'sheath-water' retained by seamounts and the surrounding deep water.This does not, however, mean that the sheath-water is stagnant. The interaction of seamounts with deep water currents and waves (internal and tidal) causes complex sheath-water dynamics 6 , specified by the unique geometry of individual seamounts 5 . The complexity arises from interactions of parallel, rapid, turbulent mixing at centimetre-scales on seamount slopes 7,8 and much slower flowing circulations (including Taylor columns) at the seamount-scale 9 . The complex sheath-water dynamics shapes seamount habitats for resident benthos and plankton 5,10,11 , causes erosion, controls sedimentation and affects ferromanganese crust formation on seamount slopes 12 .A number of seamounts peak close to the ocean surface and mix nutrient-rich deep water with the nutrient-poor surface water enhancing local phytoplankton growth 13 and causing a surface seamount effect 5,13,14 ; retention of the produced organic matter in the seamount proximity raises productivity and enriches diversity of the entire seamount-associated ecosystem 9,10,[15][16][17][18][19] . The majority of seamounts do not cause the surface seamount effect because their summits are ...

Summary Marine heterotrophic bacteria contribute considerably to global carbon cycling, in part by utilizing phytoplankton‐derived polysaccharides. The patterns and rates of two different polysaccharide utilization modes – extracellular hydrolysis and selfish uptake – have previously been found to change during spring phytoplankton bloom events. Here we investigated seasonal changes in bacterial utilization of three polysaccharides, laminarin, xylan and chondroitin sulfate. Strong seasonal differences were apparent in mode and speed of polysaccharide utilization, as well as in bacterial community compositions. Compared to the winter month of February, during the spring bloom in May, polysaccharide utilization was detected earlier in the incubations and a higher portion of all bacteria took up laminarin selfishly. Highest polysaccharide utilization was measured in June and September, mediated by bacterial communities that were significantly different from spring assemblages. Extensive selfish laminarin uptake, for example, was detectible within a few hours in June, while extracellular hydrolysis of chondroitin was dominant in September. In addition to the well‐known Bacteroidota and Gammaproteobacteria clades, the numerically minor verrucomicrobial clade Pedosphaeraceae could be identified as a rapid laminarin utilizer. In summary, polysaccharide utilization proved highly variable over the seasons, both in mode and speed, and also by the bacterial clades involved.

Heterotrophic bacteria use extracellular enzymes to hydrolyze high molecular weight (HMW) organic matter to low molecular weight (LMW) hydrolysis products that can be taken into the cell. These enzymes represent a considerable investment of carbon, nitrogen, and energy, yet the return on this investment is uncertain, since hydrolysis of a HMW substrate outside a cell yields LMW products that can be lost to diffusion and taken up by scavengers that do not produce extracellular enzymes1. However, an additional strategy of HMW organic matter utilization, selfish uptake2, is used for polysaccharide degradation, and has recently been found to be widespread among bacterial communities in surface ocean waters3. During selfish uptake, polysaccharides are bound at the cell surface, initially hydrolyzed, and transported into the periplasmic space without loss of hydrolysis products2, thereby retaining hydrolysate for the selfish bacteria and reducing availability of LMW substrates to scavenging bacteria. Here we show that selfish bacteria are common not only in the sunlit upper ocean, where polysaccharides are freshly produced by phytoplankton, but also deeper in the oceanic water column, including in bottom waters at depths of more than 5,500 meters. Thus, the return on investment, and therefore also the supply of suitable polysaccharides, must be sufficient to maintain these organisms.