Sources and Biogeochemical Cycling of Intact Membrane Lipids in the Upper Ocean

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Cell membranes make up 10-25% of the carbon biomass in the upper ocean. These important structural components of planktonic cells are dominated by intact polar lipids (IPLs), and a significant fraction of the organic carbon that is exported from the upper ocean is derived from IPLs. The primary tool for analyzing IPLs has been the gas chromatographic (GC) analysis of their constituent fatty acids. This approach has provided many valuable fatty acid biomarkers for specific groups of planktonic organisms. Yet GC is insensitive to an immense degree of structural diversity associated with the larger IPL molecules since fatty acids must be cleaved from polar "headgroups" prior to GC analysis. In this project, researchers at the Woods Hole Oceanographic Institution will study IPLs in the upper ocean with the goal of definitively answering the following two research questions: 1. Do the major classes of IPLs present in the upper ocean have specific taxonomically- or functionally-defined biological sources? 2. Do the major classes of IPLs in the upper ocean turnover at rates that are consistent with those of the living biomass from which they are derived? The research team will apply new high performance liquid chromatography/mass spectrometry (HPLC/MS) methods that allow IPLs to be identified and quantified while still in intact form. This approach has revealed a broad and unrecognized diversity of IPL molecules in the upper ocean. The source organisms of these IPLs are largely unknown, representing a significant gap in our understanding of the upper ocean carbon cycle. Furthermore, IPLs have an immense potential as new biogeochemical tracers for specific groups of microbial plankton. The project will involve three distinct approaches: First, HPLC/MS be used to will characterize the distribution of IPLs in major taxonomic groups of plankton. These groups isolated by flow cytometry from natural seawater collected during four cruises in the North Atlantic and North Pacific. Second, on these cruises the team will conduct incubations to trace 13C-labeled CO2 and organic compounds into specific IPL molecules. This information will allow us to constrain functionally-defined sources of IPLs. Last, they will use isotope tracer incubations to target the headgroups of IPLs and thereby determine the turnover rates of the intact molecules. This information will be used to establish whether IPLs are a signal of living or senescent biomass. This work will have several broader impacts. First, the proposed project will constitute the heart of an MIT/WHOI Joint Program student's dissertation research. This student will be involved in all aspects of the project. Second, a student will be recruited from a group underrepresented in oceanography to participate in this project. The WHOI Minority Fellowship Program draws applications from the brightest undergraduates in the country and WHOI has agreed to support a summer Minority Fellowship specifically for our proposed project.