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

Bacterial biomass, potential bacterial production and ectoenzymatic ac tivity were measured at 4 stations along a trophic gradient at the Atl antic Barrier Reef off Belize (Central America). We investigated the c hanges in the trophic status of this system, which we hypothesized wou ld also lead to changes in the bacterial community. The bacterial comm unity of the oligotrophic station was characterized by low biomass (me an 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 eutroph ic station bacterial biomass (mean 18.6 mug C l-1) as well as potentia l bacterial production (mean 15.5 mug C l-1 d-1) was significantly hig her. Oligotrophic bacteria exhibit high potential turnover rates (1.5 d-1). Ectoenzymatic activity measurements with fluorogenic-substrate a nalogs showed distinct differences in the expression of certain enzyme s at different trophic conditions. Alpha- and beta-D-glucosidase activ ity declined from eutrophic to oligotrophic in both per volume and per cell while leucine-aminopeptidase activity per cell was inversely cor related with the trophic status of the environment. Differences in ect oenzymatic 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 w aters by 2 orders of magnitude as compared to eutrophic environments. Potential activity (v(max)) of the extracellular enzymes tested was fo und to be correlated to potential bacterial production and bacterial b iomass.