Cell-surface proteolytic activity of photosynthetic dinoflagellates

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 mu m fraction, indicating association with the dinoflagellates rather than with bacteria. At 20 degree C, estimated activity ranged from 0.04 pmol cell super(-1) h super(-1) in the smallest species, K. micrum, to 2.56 pmol cell super(-1) h super(-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 mu m 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.