Phagotrophic nanoflagellates contribute to occurrence of a-glucosidase and aminopeptidase in marine environments

Abstract

Using fluorogenic substrate analogs, the dynamics of extracellular enzyme activities (EEA) of free and particle-bound a-glucosidase and aminopeptidase were investigated in mixed cultures of marine phagotrophic nanoflagellates and bacteria. Flagellate growth on either live or heatkilled bacteria led to an expression of free aminopeptidase activity with up to 94 % of total EEA found in the dissolved phase. No significant release of free a-glucosidase or free aminopeptidase was found in flagellate-free bacterial cultures. Fed with heat-killed bacteria, flagellates seemed to exhibit both cell wall bound a-glucosidase and aminopeptidase actlv~ties. Flagellate biovolume-specific EEA In flagellate assemblages grown on heat-killed bacteria was similar in magnitude to bacterial biovolumespecific EEA in flagellate-free bacterial cultures. The significance of these results is further discussed according to current theories on enzyme-catalyzed organic matter cycling.