Speed Bumps in the Carbon Cycle: Enzymatic Hydrolysis and Carbon Flow in Marine Systems

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ABSTRACTOCE-0323975Most organic carbon in marine systems originates from phytoplankton, which biosynthesize cellular macromolecules such as proteins, lipid complexes, and polysaccharides from CO2. These phytoplankton macromolecules are sequentially transformed and for the most part remineralized back to CO2 as carbon passes through the marine food web. Microbes play a key role in these remineralization processes, cycling ca. 50% of marine primary productivity (Azam 1998). Although their importance is clear, the specific rates and means by which microbes transform phytoplankton-derived carbon to CO2 are largely unexplored. Microbial activities and the carbon transformations they catalyze therefore are often relegated to the black box. of carbon cycling, whose inner workings are unknown.On this project, researchers at the University of North Carolina at Chapel Hill will conduct investigations to illuminate the workings of the black box by determining the rates and means by which carbon initiates cycling through the activities of extracellular enzymes. The assertion has been made that extracellular enzymatic hydrolysis is .the. rate-limiting step in carbon cycling (e.g. Arrieta and Herndl 2002) Previous work (Arnosti et al. 1994), however, demonstrates that this assertion is not a priori true: a more differentiated perspective is required. In some cases, hydrolysis of a given macromolecular substrate in the water column is extremely rapid, while the same substrate is not detectably hydrolyzed in seawater at other locations (Arnosti et al. submitted). There is also evidence for systematic differences in the nature and rates of extracellular enzymatic activities of seawater and of sedimentary heterotrophic microbes (Arnosti, 2000). What factors control these differences? The proposed work is intended to examine systematically factors that may control the patterns and activities of extracellular enzymes among seawater and sedimentary microbial communities. In an effort to identify and constrain some of the speed bumps. in the carbon cycle, enzymatic hydrolysis will also be linked quantitatively with subsequent carbon transformation steps in anoxic sediments, where knowledge of spatial and temporal connections of carbon flow is particularly sparse.To accomplish these goals, field measurements of enzymatic hydrolysis rates and patterns will be coupled with laboratory studies of specific factors that may affect enzyme expression in heterotrophic microbial communities. Since measurements of enzyme activities have been restricted by the limited number of commercially available substrates and substrate proxies, this project will also expand the spectrum of well-characterized phytoplankton macromolecules whose hydrolysis and subsequent transformations can be studied in detail. These new substrates will be used in field experiments as well as in laboratory investigations to quantitatively measure carbon flow via transformations carried out by microbial communities in anoxic sediments.