ABSTRACT: We used the artificial substrate l-leucine 7-amido-4-methyl-coumarin (Leu-AMC) to measure leucine aminopeptidase (LAP) of dinoflagellates. Axenic cultures of Alexandrium tamarense, Heterocapsa triquetra and Prorocentrum minimum had considerable LAP associated with the cell surface. In non-axenic cultures of Akashiwo sanguinea, Gonyaulax grindleyi, Gyrodinium uncatenum, Karlodinium micrum and P. minimum, 60 to 99% of the total LAP activity was found in the >5 µm fraction, indicating association with the dinoflagellates rather than with bacteria. At 20°C, estimated activity ranged from 0.04 pmol cell^-1 h^-1 in the smallest species, K. micrum, to 2.56 pmol cell^-1 h^-1 in the largest species, G. grindleyi. Activity per cell could be predicted from cell size. During a mixed species dinoflagellate bloom in the Choptank River, a tributary of the Chesapeake Bay, total LAP activity was positively correlated with dinoflagellate concentration. In 'red-water' samples, up to 76% of LAP activity was in the >2 µm fraction. We calculate that in red-water events, dinoflagellates may account for 50% or more of the in situ LAP activity. Cell-surface proteases may play a role in nutrition of mixotrophic dinoflagellates by providing amino acids for assimilation. Alternatively, released amino acids may be degraded by cell-surface amino acid oxidases to provide ammonium which can be taken up as a source of nitrogen.

KEY WORDS: LAP · Ectoenzyme · Mixotrophy · Osmotrophy

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Published in AME Vol. 30, No. 2 (2003) on January 7
Print ISSN: 0948-3055; Online ISSN: 1616-1564. Copyright © Inter-Research, Oldendorf/Luhe, 2003