Phytoplankton, traditionally viewed as primary producers at the base of aquatic food webs, provide an energy source for higher trophic levels. However, some phytoplankton species function as both primary producers and heterotrophic secondary consumers. Phytoplankton that are photosynthetically competent but also take up and assimilate organic compounds are classified as facultative mixotrophs or, more simply, photomixotrophs. Unfortunately, we currently have few estimates of the proportion of the phytoplankton community that function as photomixotrophs, their rate of secondary production, or their temporal variation in abundance. Current paradigms about trophodynamics in marine systems do not consider this potentially important alternative pathway for energy flow for phytoplankton. The implication is that we may be missing a significant, fundamental process that affects carbon cycling and trophodynamics in estuarine systems. Furthermore, changes in the DOC composition due to anthropogenic alterations may result in changes in phytoplankton community structure and possibly promote the proliferation of harmful algal bloom species. In terms of ecosystem function, even moderate rates of photomixotrophy could potentially alter our current understanding of phytoplankton productivity, overall C turnover, competitive interactions, and energy transfer in estuarine environments. This project will use a novel approach to provide quantitative measures of the in situ rates and magnitudes of facultative heterotrophy in natural, estuarine phytoplankton communities over seasonal time scales in a representative estuarine ecosystem. The project will utilize a unique 14C radiolabeling technique to quantify the in situ assimilation rates of DOC by estuarine photomixotrophs and estimate the amount of DOC converted to phytoplankton biomass by photomixotrophy over seasonal time scales. This information will provide new insights into carbon dynamics in estuaries, the contribution of DOC to estuarine food webs, and the importance of photomixotrophy in determining the structural and functional characteristics of estuarine phytoplankton communities.
INTELLECTUAL MERIT. The research will provide new insights into the potential importance of an alternative source of C (DOC) and pathway for phytoplankton production. Successful demonstration that a significant fraction of the production of estuarine phytoplankton is by photomixotrophy would have major implications for the way we currently view C flow in these systems. The project has a reasonable likelihood of altering the current paradigms for estuarine primary productivity as well as providing new evidence for mechanisms of competitive interactions between phytoplankton species in mixed, natural assemblages. The study will also provide first-order approximations of labile DOC turnover by phytoplankton and will be one of the few measurements ever obtained for an estuary. Research results will contribute to an improved knowledge of relevant and underlying ecophysiological, organismal, and community-level processes involving carbon cycling in an estuarine environment.
BROADER IMPACTS. This project will address the broader impacts criterion of advancing discovery and understanding while promoting teaching, training, and learning. The project will provide support for a graduate student, summer support for an undergraduate assistant, and involve upper level undergraduates as lab interns. Undergraduate interns, recruited from the Marine Science Program at USC, will participate in sample analyses and learn to operate laboratory instrumentation. Furthermore, the field bioassays will be added as a module in a required Marine Science undergraduate field course, MSCI 460 - Field & Lab Investigations in MSCI.
