Bacterial mediation of carbon fluxes during a diatom bloom in a mesocosm

Abstract

Bacteria-diatom interactions were studied during a diatom bloom produced in a mesocosm, in the absence of metazoan grazers, in order to examine the significance of bacterial hydrolytic ectoenzymes in mediating carbon fluxes and influencing diatom aggregation. The abundances of bacteria and protozoa, the production rates and hydrolytic ectoenzyme activities (protease, a and B glucosidase and chitobiase) of attached and free bacteria, were followed as well as the dynamics of the dissolved organic carbon (DOC) pool. An intense diatom bloom occurred with chlorophyll a (chl a) concentrations reaching 132yg liter-' prior to aggregation. The diatoms were colonized by bacteria early on in the bloom and remained colonized throughout the bloom, yet they grew rapidly (>l day-'). Attached bacteria were numerically a small fraction of the total. but they also grew very rapidly & = 4-16 day-') and were generally responsible for the majority of bacterial carbon demand, BCD, (46-92%) and hydrolytic enzyme activities (41-990/o). BCD accounted for an estimated 4060% of the total carbon fixed during the bloom; thus, roughly one- half of the primary production was channeled. via the DOC pool, into bacteria. The high ectohydrolase activities of bacteria attached to the surface of diatoms suggests that the hydrolysis of diatom surface mucus could be responsible for a major flux into the DOC pool making it a significant. but previously unrecognized, mechanism of DOM production. Enzymatic hydrolysis of surface mucus may also have inhibited diatom aggregation. Addition of purified glucosidase and proteasc to samples from the mesocosm inhibited diatom aggregation in cxpcriments designed to induct aggregation. It is hypothesized that the action of bacterial ectoenzyme on diatom surfaces inhibited diatom aggregation by reducing stickiness, thus prolonging the bloom and allowing the accumulation of extremely high chl a levels prior to aggregation. Future studies should consider bacterial hydrolytic ectoenzymes as a potentially important variable influencing carbon Rux pathways, particle aggregation, and the size and duration of diatom blooms in the ocean.