Award: OCE-1736772

Dissolved organic carbon in the ocean is one of the largest and biologically most active carbon reservoirs on earth. A considerable fraction of the dissolved organic carbon pool consists of large molecules (high molecular weight organic carbon). This high molecular weight carbon is known to be fresher and more biologically reactive than the rest of the dissolved organic carbon pool, but the reasons for higher biological reactivity were unknown. This project focused specifically on the mechanisms by which bacteria process and transform high molecular weight polysaccharides, a major part of the high molecular weight dissolved organic carbon pool. Polysaccharides are too large to be eaten directly by bacteria; instead, they must first be cut to smaller sizes before uptake. Typically, the polysaccharides are cut to smaller pieces outside the cell by enzymes (termed external hydrolysis); the resulting smaller pieces of organic carbon can be taken up by the organism that produced the enzymes, as well as by other bacteria that did not produce enzymes. We discovered an addition mechanism of polysaccharide processing: some members of bacterial communities can bind, cut, and take up high molecular weight polysaccharides without losing pieces of organic matter to the external environment (referred to as a selfish mechanism of uptake). Using a selfish uptake mechanism essentially guarantees a payback for the investment in the enzymes used to bind and cut their target food. As part of this project, we compared the rates and types of substrates processed via selfish uptake and external hydrolysis in different parts of the ocean. We expected the selfish mechanism to be used by specific bacteria that are relatively common in the upper ocean, in locations where much fresh organic matter is produced. Much to our surprise, we found that selfish uptake is found not only in the surface ocean, but also in the deep ocean. Furthermore, some highly complex organic matter that otherwise is untouched in the deep ocean is apparently processed only by selfish mechanisms. Rapid selfish uptake of high molecular weight organic carbon appears to be quite widespread in the ocean, carried out by a broad range of different types of bacteria: this mechanism is not restricted to a few types of organisms. These findings are important because bacteria consume and transform much of the organic matter produced in the ocean. Understanding the mechanisms by which bacteria consume organic matter, and the factors controlling which organic matter is eaten in any given location, helps explain the ecology of bacterial communities that process organic matter, as well as the distribution of organic carbon in the ocean, and thus is central to the workings of the global carbon cycle. The ecological insights and methods refined as part of this project to identify specific bacteria that selfishly take up polysaccharides have implications far beyond the field of oceanography: bacterial communities in the guts of living organisms are essential to digestive processes. The technical approaches developed and refined in this project are already being applied also in biomedicine and veterinary studies. Last Modified: 12/31/2022 Submitted by: Carol Arnosti