Metabolic activity, population dynamics and community structure of particle-attached and unattached microbes during differing phytoplankton blooms and natural conditions

Bacterioplankton communities can have a significant impact on the resulting biogeochemistry of a phytoplankton bloom (Smith et al. 1995). We followed three distinct phytoplankton blooms dominated by dinoflagellates, phaeocystis and diatoms, respectively in a 1200-liter mesocosm. Bacterial abundance was highly coupled to phytoplankton abundance in the dinoflagellate bloom but was less coupled in the other blooms. Microbial activity, as measured by carbon production and hydrolytic enzyme activity, varied. The dinoflagellate-bloom generated the most microbial activity and the phaeocystis-bloom generated the least activity. Thus the respective phytoplankton blooms led to three distinct microbial communities.; Bacteria from the Cytophaga/Flavobacter/Bacteriodes phylum (CFB) are known to comprise a substantial fraction of the bacterial communities attached to “marine snow” particles (DeLong et al. 1993), and are know to produce exo-cellular enzymes that can hydrolyze a wide range of substrates (Reichenbach, 1991). Marine snow particles were generated during the flocculation of the diatom bloom induced in the 1200 liter mesocosm. Total hydrolytic activity during the flocculation phase of the bloom was greater in the aggregate samples than in free-living bacteria. The FL CFB relative rRNA abundance reached 57.6%, and the AGG relative rRNA abundance reached 27.2%. Thus, CFB are important components of both FL and AGG bacterial communities associated with diatom blooms and are involved in the hydrolytic degradation of marine snow.; Aggregate-attached bacteria are phylogenetically distinct microorganisms from the predominant planktonic microbial species. We collected aggregate-attached (AGG) and free-living (FL) bacterial assemblages to compare community structures. Bacterial phyla detected in AGG clone libraries ranked from most to least frequently recovered were the CFB, Cyanobacteria, Proteobacteria, Planctomyces, MC 18-like (Liesack and Stackebrandt, 1992), Gram-positive bacteria and Fibrobacter. In contrast, the most commonly recovered clones in the FL fraction were the α- and γ-Proteobacteria. When both AGG and FL clones were grouped phylogenetically, clones from the different habitats often comprised separate clusters. These data suggest that these bacterial groups may be important in the biogeochemical transformation of sinking particulate matter in the sea.