EXTRACELLULAR AMINO-ACID OXIDATION BY MICROPLANKTON - A CROSS-ECOSYSTEM COMPARISON

Rates of extracellular amino acid oxidase activity in natural phytopla nkton, cyanobacterial, and bacterial assemblages were measured using a fluorescent analog of the amino acid lysine. Activity was measured in a variety of ecosystems with different levels of nutrient enrichment and diverse community composition. Sites included a station in Shinnec ock Bay, Long Island Sound, New York (USA); the Chesapeake Bay, Maryla nd (USA); the NW Atlantic Ocean near the Bahamas and the Caribbean Sea ; Brazilian coastal waters; and 2 estuarine mesocosms. Highest rates o f amino acid oxidase activity (25 to 30 nM h(-1)) were found in the su mmer mesocosm experiments when NH4+ concentrations were near the Limit of detection, and biomass levels were indicative of an algal bloom. L ower rates of amino acid oxidase activity were found during a bloom of Aureococcus anophagefferens and in oligotrophic oceanic waters. High rates of amino acid oxidase activity (up to 20 nM h(-1)) were also fou nd in oceanic samples enriched with colonies of the diazotrophic cyano bacteria Trichodesmium. No activity was observed in samples from oligo trophic environments that were prefractionated through 1.0 mu m filter s; however, when amended with glucose or an amino acid mixture, oxidat ion rates of up to 8 nM h(-1) were observed. No activity was found dur ing a diatom-dominated, autumnal bloom in Chesapeake Bay. Overall, ami no acid oxidation represented a higher percentage of NH4+ uptake in th e oligotrophic waters (up to 10%) than in the coastal waters studied. In oligotrophic waters, where ambient inorganic nitrogen concentration s are low and consequently uptake rates are low, this pathway appears to represent a potentially important source of nitrogen for phytoplank ton and the diazotrophic cyanobacteria Trichodesmium.