Dynamics of bacterial community composition and activity during a mesocosmdiatom bloom

Bacterial community composition, enzymatic activities, and carbon dynamics were examined during diatom blooms in four 200-liter laboratory seawater me socosms, The objective was to determine whether the dramatic shifts in grow th rates and ectoenzyme activities, which are commonly observed during the course of phytoplankton blooms and their subsequent demise, could result fr om shifts in bacterial community composition. Nutrient enrichment of metazo an-free seawater resulted in diatom blooms dominated by a Thalassiosira sp. , which peaked 9 days after enrichment (approximate to 24 pg of chlorophyll a liter(-1)). At this time bacterial abundance abruptly decreased from 2.8 x 10(6) to 0.75 x 106 ml(-1), and an analysis of bacterial community compo sition, by denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene fragments, revealed the disappearance of three dominant phylo types, Increased viral and flagellate abundances suggested that both lysis and grazing could have played a role in the observed phylotype-specific mor tality. Subsequently, new phylotypes appeared and bacterial production, abu ndance, and enzyme activities shifted from being predominantly associated w ith the <1.0-mu m size fraction towards the >1.0-mu m size fraction, indica ting a pronounced microbial colonization of particles. Sequencing of DGGE b ands suggested that the observed rapid and,extensive colonization of partic ulate matter was mainly by specialized alpha-Proteobacteria- and Cytophagal es-related phylotypes, These particle-associated bacteria had high growth r ates as well as high cell-specific aminopeptidase, beta-glucosidase, and li pase activities. Rate measurements as well as bacterial population dynamics were almost identical among the mesocosms indicating that the observed bac terial community dynamics were systematic and repeatable responses to the m anipulated conditions.