This renewal proposal from Dr. Fuhrman is for an extension of studies on dissolved free amino acids (DFAAs), with new emphasis on the open sea. The DFAAs are important sources of organic C and N, and represent excellent models of biologically- labile dissolved organic matter (DOM). DOM flux is now known to be a major part of the marine C and N cycles. Recent work demonstrating very rapid turnover of small DFAA pools has shown a need to revise previous ideas in this area. This proposal includes extensive studies of DFAA turnover, flux, and availability in the oligothrophic and deep ocean, from which reliable data are sparse. Other proposed work includes measuring DFAA release from zooplankton fecal pellets, the microbial responses to sudden DFAA enrichment as might be found in "enriched microzones," algal assimilation of DFAAs, ammonium regeneration from DFAAs, and degradation of peptides and proteins. In addition to the usual tritiated tracers, for some of this work the fate of the amino acid nitrogen will be traced with the short-lived isotope N-13. This isotope has a very high specific activity, so it can be added at true trace levels with great sensitivity. But due to its brief useful lifetime (ca. 90 min.), samples must be transported to the cyclotron, so for these experiments the use of a helicopter to sample offshore waters is required.
