The goal of this study was to attain an integrated understanding of the dynamics of attached microbial communities in nature as governed by the component processes of particle formation, colonization, detachment, growth and interspecific interactions among the microbes. The study was conducted in mesocosms in a Danish fjord over a 2 wk period in April to May 2005. Despite nutrient additions, chlorophyll and particle concentrations were low and no distinct phytoplankton blooms were observed. Particle volume concentration was dominated by particles >100 µm. The abundance of attached bacteria was weakly but positively correlated with total particle volume. Phylogenetic composition of attached bacteria became increasingly different from that of free bacteria through time. Growth rates of free and attached bacteria were 0.31 and 0.023 d -1 , respectively. Colonization experiments with model agar aggregates showed that the bacterial community had limited apparent diffusivity but strong chemotaxis, which resulted in an up to 8-fold increase in bacterial colonization rates with organic-enriched aggregates. Estimated residence time of attached bacteria on aggregates was 21 min or less. Grazing mortality derived from temporal changes in attached bacterial abundance in the presence or absence of grazing activities ranged from 0 to 9.8 d , suggesting that the bacterivorous flagellates were more adapted to grazing on surfaces than in free suspension. Very few attached flagellates were observed in the experiments, and microscopic observations revealed that many flagellates were attached only temporarily to surfaces. Our study showed that grazing mortality is a critical but often underappreciated factor in regulating marine snow bacterial population abundance. The loose association of bacterivorous flagellates with aggregates requires that their role be evaluated by direct grazing measurements instead of simple quantification of attached flagellate abundance.

Fluxes of particulate matter to depth and dynamics of dissolved organic matter in the water column are influenced by microbial processes associated with organic aggregates like marine snow. These microscale processes include the encounter between bacteria and aggregates, which has been previously modeled and tested with well-fed and actively growing bacteria. In the present study, we investigated the effects of starvation on initial bacterial colonization of aggregates by measuring colonization and detachment of 6 isolates in different physiological states (fed vs. starved) using model aggregates. Because aggregate encounter depends on motility, the motility behaviors of fed and starved bacteria of 3 selected strains were also compared using image analysis. All 6 fed isolates colonized faster and achieved significantly higher steady-state abundances on model aggregates than those that were starved. However, there was no difference in detachment rates between fed and starved bacteria. The 3 selected strains had significantly lower average swimming speeds when starved. Diffusivities calculated from motilities of 2 starved isolates were more than 6 times lower than those of their fed counterparts. Our results show that starvation significantly affects bacterial behavior and bacteria-aggregate interactions, which may lead to differences in particulate and dissolved organic matter fluxes and cycling under different productivity regimes.

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