ECTOCELLULAR GLUCOSIDASE AND PEPTIDASE ACTIVITY OF THE MIXOTROPHIC DINOFLAGELLATE PROROCENTRUM MINIMUM (DINOPHYCEAE)

The activities of the enzymes alpha- and beta-glucosidase, and leucine aminopeptidase were measured in cultures of the dinoflagellate Prorocentrum minimum (Pavill.) J. Schiller and in field samples collected during dinoflagellate blooms occurring in tributaries of the Chesapeake Bay, Maryland, USA. Activities were measured using fluorogenic artificial substrates and partitioned among the > 5 mu m size fraction, small microbes fraction (0.1-5 mu m), and dissolved phase (< 0.1 mu m). P. minimum and most other photosynthetic dinoflagellates are > 5 mu m in size and thus can be separated from the small microbes fraction, which contains most bacteria. Little to no glucosidase activity was detected associated with the > 5 mu m size fraction in cultures or in field samples, with most of the activity (67% to 93% in cultures, 54% to 100% in field samples) in the small microbes size fraction for both alpha and beta glucosidase. In contrast, 67% to 90% of the total leucine aminopeptidase (LAP) activity in cultures was measured in the > 5 mu m fraction. Within a culture, LAP activity in the size fraction containing P. minimum decreased in response to ammonium and urea additions, but not in response to nitrate. In field samples, LAP activity was positively correlated with dinoflagellate abundance and chl a, and negatively correlated with ammonium concentration. During blooms, up to 34% of LAP activity was associated with the > 5 mu m fraction, indicating that when abundant, dinoflagellates may make a substantial contribution to ectocellular LAP activity in the water column.