Selfish bacteria take up polysaccharides under deep ocean pressure: insights from in situ and ex situ measurements

Heterotrophic bacteria process a large fraction of the polysaccharides produced by phytoplankton in the surface ocean. Some of these polysaccharides are taken up by "selfish" bacteria, which bind, partially hydrolyze, and transport large fragments of polysaccharides into the cell with little loss of hydrolysis products. Recently, selfish bacteria have also been identified in deep ocean water. Whether these bacteria effectively capture polysaccharides under hydrostatic pressures typical of the deep ocean, however, remained an open question. Here, for the first time, we measured the extent of selfish uptake of laminarin-a common marine polysaccharide-in surface and deep waters, under pressures ranging from atmospheric up to ~50 MPa (equivalent to 5000 m depth). We incubated seawater from Japan, Denmark, and the western North Atlantic Ocean in pressure vessels and found that the extent of selfish uptake of laminarin varied somewhat by site but was insensitive to hydrostatic pressure. Deployment of an in situ incubator in the North Atlantic Ocean also showed evidence of selfish uptake in situ. These results suggest that selfish uptake can play a key role in polysaccharide degradation, especially in the deep ocean where other modes of organic matter degradation may be more inhibited by high hydrostatic pressure.