Extracellular enzymes in aggregates and contributions of free enzymes to hydrolytic activities: Consequences for organic mater remineralizations in marine systems

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Aggregates play a central role in organic matter dynamics in the ocean, harboring diverse communities of microorganisms that mediate the transformations and transport of colloidal, particulate, and dissolved organic matter. Because these aggregates can be hot spots of carbon remineralization, microbial dynamics in aggregates have been a focus of intense investigation. High rates of enzymatic activities have been measured in aggregates, but only a limited range of substrate proxies have been used to assess these activities. The structural specificities of microbial extracellular enzymes are critical, however, since they determine the nature of organic macromolecules that can be metabolized by heterotrophic communities. In this project, investigators at the University of North Carolina at Chapel Hill will probe the enzymatic capabilities of aggregate-associated microbial communities and compare them with non-aggregate associated communities using phytoplankton extracts, dissolved organic carbon, and polysaccharides as substrates. The composition of the microbial communities will be compared in parallel to determine whether differences in the spectrum of enzyme activities are related to compositional differences in the active microbial communities. Since organic aggregates particularly as components of sinking organic matter, in the nepheloid layer, or resuspended from surface sediments often include mineral particles with associated organic matter, they will investigate the extent to which surface-associated organic matter can be hydrolyzed by aggregate-associated microbial communities. Preliminary investigations further suggest that specific enzymes are released into the water column while other enzymes remain associated with cells and/or aggregates, so they will measure the activities as well as the structural specificities of free enzymes in aggregate-free samples. Models suggest that active release of free enzymes should occur when microorganisms receive a reasonable return. The investigators will assess factors affecting enzyme lifetimes, critical experimental data that could be used to test conflicting model predictions about microbial extracellular enzymes. Broader Impacts: This project will contribute to teaching and learning, with two undergraduates, a graduate student, and a postdoc as integral members of the research team. Results of the project will be presented at national meetings and will be published in the scientific literature. Results of this project will be incorporated into classes the PI teaches at both graduate and undergraduate levels. Two of the current undergraduate research assistants are women.