DISSOLVED ORGANIC NITROGEN DYNAMICS AND BACTERIAL AMINO ACID METABOLISM IN CASTLE LAKE, CALIFORNIA (MINERALIZATION, REGENERATION, BACTERIOPLANKTON)

Heterotrophic bacterioplankton can play an important role in the mineralization of dissolved organic nitrogen (DON) in aquatic ecosystems. The mineralization process depends on the nutrient requirements of bacterial populations, as well as the composition, concentration and flux rates of inorganic and organic compounds. The metabolism of organic compounds varies amongst microorganisms resulting in different mineralization efficiences for various substrates. For this reason, the dissolved organic nitrogen pool is comprised of individual compounds which have different sources and fates in the aquatic environment.;The concentration and composition of DON varied seasonally. Dissolved free amino acids comprised 5-8% of the DON. Early season pulses of DON (13-16 ug atom N('-1)) were correlated with the decline of diatom blooms, but phytoplankton excretion or death could not account for the pulse of DON. Sediment release appeared to be the most important source of the observed increase of DON.;The mineralization of amino acid carbon and nitrogen was measured with ('14)C- and ('15)N-labeled amino acids. Glutamate carbon was preferentially assimilated relative to nitrogen, indicating that nitrogen was mineralized. Nitrogen mineralization rates were also measured for aspartate, glycine, glutamine and lysine. The mineralization of nitrogen from these substrates implies that the bacteria were not nitrogen-limited; and therefore, do not compete with phytoplankton for inorganic nitrogen.;Total amino acid nitrogen flux was calculated and a conservative estimate showed that amino acid mineralization could supply a minimum of 2% of the phytoplankton N-requirements. However, amino acids are only one component of DON and there are many other substrates available for bacterial mineralization. Further research on the differential carbon and nitrogen metabolism of other components of DON are still needed. This approach provides useful information on the role of DON in the nitrogen dynamics of aquatic ecosystems.;The study was designed to detail the dynamics of DON in N-deficient Castle Lake, a meso-oligotrophic subalpine lake located in the Klamath Mountains of northern California. During the summer growing season, dissolved inorganic nitrogen in the epilimnion of Castle Lake is often below detection, thus phytoplankton production is dependent upon N-regeneration mechanisms. The specific objectives were threefold: (1) analyze seasonal variation of dissolved organic nitrogen in relation to primary productivity and biomass, (2) determine the fraction of dissolved organic nitrogen composed of dissolved free amino acids, and (3) determine mineralization rates of amino acid nitrogen.