HETEROTROPHIC NANOPLANKTON BIOMASS MEASURED BY A GLUCOSAMINIDASE ASSAY

Bacterivorous protozoa secrete glucosaminidase enzymes which digest pe ptidoglycans in bacterial cell walls. Phagotrophic protozoa store dige stive enzymes in lysosomes before release into food vacuoles where dig estion takes place initially at acidic pH. The quantity of glucosamini dase can be assayed by measuring the rate of formation of the fluoresc ent product 4-methylumbelliferone by cleavage of -methylumbelliferyl-n -acetyl-beta-D-glucosaminide. Flagellate and ciliate cells lysed with detergent possessed glucosaminidase activity which remained steady ove r at least 24 h. The enzyme activity increased in direct proportion to the biomass for each species tested, although the level of activity v aried in different species in a way that probably relates to the ecolo gical strategy of the species. Heterotrophic nanoplankton sampled at e ight stations in waters of widely different trophic status along an At lantic Meridional Transect showed a very strong correlation between gl ucosaminidase activity and estimated biomass. We conclude that this ca librated glucosaminidase assay can provide a quick and effective metho d of estimating the biomass of heterotrophic nanoplankton. (C) 1998 Fe deration of European Microbiological Societies. Published by Elsevier Science B.V.