ECTOENZYMATIC ACTIVITY AND BACTERIAL DYNAMICS ALONG A TROPHIC GRADIENT IN THE CARIBBEAN SEA

Bacterial biomass, potential bacterial production and ectoenzymatic activity were measured at 4 stations along a trophic gradient at the Atlantic Barrier Reef off Belize (Central America). We investigated the changes in the trophic status of this system, which we hypothesized would also lead to changes in the bacterial community. The bacterial community of the oligotrophic station was characterized by low biomass (mean 7.2 mug C l-1) and low potential production rates (mean 9.2 mug C l-1 d-1) as determined by seawater cultures whereas at the most eutrophic station bacterial biomass (mean 18.6 mug C l-1) as well as potential bacterial production (mean 15.5 mug C l-1 d-1) was significantly higher. Oligotrophic bacteria exhibit high potential turnover rates (1.5 d-1). Ectoenzymatic activity measurements with fluorogenic-substrate analogs showed distinct differences in the expression of certain enzymes at different trophic conditions. Alpha- and beta-D-glucosidase activity declined from eutrophic to oligotrophic in both per volume and per cell while leucine-aminopeptidase activity per cell was inversely correlated with the trophic status of the environment. Differences in ectoenzymatic activity are indicated by differences in both v(max) and K(m). K(m) values of leucine-aminopeptidase clearly reflect the trophic situation. Substrate affinity was found to be higher in oligotrophic waters by 2 orders of magnitude as compared to eutrophic environments. Potential activity (v(max)) of the extracellular enzymes tested was found to be correlated to potential bacterial production and bacterial biomass.