AME 15:141-152 (1998)  -  doi:10.3354/ame015141

Extracellular amino acid oxidation by microplankton: a cross-ecosystem comparison

Margaret R. Mulholland1,*, Patricia M. Glibert2, Gry Miné Berg2, Laurie Van Heukelem2, Silvio Pantoja3,**, Cindy Lee3

1University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory, PO Box 38, Solomons, Maryland 20688, USA
2University of Maryland Center for Environmental Science, Horn Point Laboratory, PO Box 775, Cambridge, Maryland 21613, USA
3Marine Sciences Research Center, SUNY, Stony Brook, New York 11794-5000, USA
**Present address: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA

ABSTRACT: Rates of extracellular amino acid oxidase activity in natural phytoplankton, 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 Shinnecock Bay, Long Island Sound, New York (USA); the Chesapeake Bay, Maryland (USA); the NW Atlantic Ocean near the Bahamas and the Caribbean Sea; Brazilian coastal waters; and 2 estuarine mesocosms. Highest rates of amino acid oxidase activity (25 to 30 nM h-1) were found in the summer mesocosm experiments when NH4+ concentrations were near the limit of detection, and biomass levels were indicative of an algal bloom. Lower 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 found in oceanic samples enriched with colonies of the diazotrophic cyanobacteria Trichodesmium. No activity was observed in samples from oligotrophic environments that were prefractionated through 1.0 μm filters; however, when amended with glucose or an amino acid mixture, oxidation rates of up to 8 nM h-1 were observed. No activity was found during a diatom-dominated, autumnal bloom in Chesapeake Bay. Overall, amino acid oxidation represented a higher percentage of NH4+ uptake in the oligotrophic waters (up to 10%) than in the coastal waters studied. In oligotrophic waters, where ambient inorganic nitrogen concentrations are low and consequently uptake rates are low, this pathway appears to represent a potentially important source of nitrogen for phytoplankton and the diazotrophic cyanobacteria Trichodesmium.

KEY WORDS: Amino acid oxidase activity · Nitrogen uptake · Marine phytoplankton · Marine cyanobacteria · Oxidative deamination

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