BACTERIAL MEDIATION OF CARBON FLUXES DURING A DIATOM BLOOM IN A MESOCOSM

Bacteria-diatom interactions were studied during a diatom bloom produc ed in a mesocosm, in the absence of metazoan grazers, in order to exam ine the significance of bacterial hydrolytic ectoenzymes in mediating carbon fluxes and influencing diatom aggregation. The abundances of ba cteria and protozoa, the production rates and hydrolytic ectoenzyme ac tivities (protease alpha and beta glucosidase and chitobiase) of attac hed and free bacteria, were followed as well as the dynamics of the di ssolved organic carbon (DOC) pool. An intense diatom bloom occurred wi th chlorophyll a (chl a) concentrations reaching 132 mu g liter(-1) pr ior to aggregation. The diatoms were colonized by bacteria early on in the bloom and remained colonized throughout the bloom, yet they grew rapidly (>1 day(-1)). Attached bacteria were numerically a small fract ion of the total, but they also grew very rapidly (mu = 4-16 day(-1)) and were generally responsible for the majority of bacterial carbon de mand, BCD, (46-92%) and hydrolytic enzyme activities (41-99%). BCD acc ounted for an estimated 40-60% of the total carbon fixed during the bl oom; thus, roughly one-half of the primary production was channeled, v ia the DOC pool, into bacteria. The high ectohydrolase activities of b acteria attached to the surface of diatoms suggests that the hydrolysi s of diatom surface mucus could be responsible for a major flux into t he DOC pool making it a significant, but previously unrecognized, mech anism of DOM production. Enzymatic hydrolysis of surface mucus may als o have inhibited diatom aggregation. Addition of purified glucosidase and protease to samples from the mesocosm inhibited diatom aggregation in experiments designed to induce aggregation. It is hypothesized tha t the action of bacterial ectoenzyme on diatom surfaces inhibited diat om aggregation by reducing stickiness, thus prolonging the bloom and a llowing the accumulation of extremely high chl a levels prior to aggre gation. Future studies should consider bacterial hydrolytic ectoenzyme s as a potentially important variable influencing carbon flux pathways , particle aggregation, and the size and duration of diatom blooms in the ocean.