Award: OCE-2022952

Almost half of the organic carbon produced in the ocean is processed by bacteria. Bacteria use extracellular (outside the cell) enzymes to break down large organic molecules to small sizes that can be transported into their cells. It has recently been discovered that bacteria use extracellular enzymes in two ways: selfish uptake and external hydrolysis. External hydrolysis releases low molecular weight products to the environment where they can be used by other organisms. Selfish uptake releases little or no products. The extent of selfish uptake affects the distribution of organic carbon in the ocean, the flow of small organic molecules to feed a wider range of bacteria, and the composition and dynamics of the bacterial community.

Our recent research shows that selfish bacteria are active in deep ocean waters, where they take up complex polysaccharides (sugars) that are not hydrolyzed externally. These results inspired a new model that links selfish uptake and external hydrolysis to the amount and complexity of the organic matter that is used by bacteria. As part of this project, we tested the model by investigating the relationships between organic matter abundance, structural complexity, and extracellular enzyme use. We have found that addition of high molecular weight organic matter can indeed tip the balance between selfish uptake and external hydrolysis, typically with external hydrolysis becoming more important with addition of easily-metabolized high molecular weight organic matter. The most complex substrates, however, remain the target of selfish bacteria, whose activities appear independent of organic matter addition.

A further significant finding was that the ability to degrade a wide range of polysaccharides in the ocean is, in comparison to protein degradation, often a function of the specific enzymatic capabilities of microbial communities. We discovered that microbial communities from a wide range of locations and depths could degrade all of the peptide substrates tested, but their abilities to degrade polysaccharides, especially in deep ocean waters, varied greatly with location. Polysaccharide degradation as a broad function therefore has limited functional redundancy: not all communities have the same enzymatic capabilities.

Understanding the factors and actors that control cycling of organic matter in the ocean is critically important in order to accurately measure and model carbon cycling in the ocean (and on earth in general). The activities of selfish bacteria have previously not been accounted for (or indeed, have not even been measured with standard measurements used to quantify bacterial activity). By determining the conditions under which selfish bacteria are highly active (or not) we have taken a significant step forward in our understanding of microbially- driven carbon cycling.

From a wider perspective, selfish uptake of polysaccharides is an important area of research in gut microbiomes, human as well as animal. The activities of organisms in the gut have profound effects on organism health and disease susceptibility. Therefore, the tools, techniques, and perspectives that we develop are also significant to these other areas of research.

Two graduate students and three undergraduate students contributed significantly to the work carried out as part of this project. 

Last Modified: 09/29/2025
Modified by: Carol Arnosti