Macroscopic aggregates (marine snow) contribute to new production and nutrient dynamics in the upper ocean and vertical fluxes of organic matter to the deep ocean. To test whether microorganisms play a significant role in phytoplankton aggregate formation we studied particle abundance and size as well as abundance, colonization behaviour, and community composition of bacteria during the growth of 2 marine diatoms (Thalassiosira weissflogii and Navicula sp.) under axenic and non-axenic conditions. Community composition of free-living and attached bacteria during phytoplankton growth and aggregation was studied by amplification of 16S rRNA gene fragments and denaturing gradient gel electrophoresis (DGGE). Our results show that the presence of bacteria was a prerequisite for aggregation of T. weissflogii but not of Navicula sp. Occurrences of distinct populations of free-living and attached bacteria depended on phytoplankton growth and aggregation dynamics. The community composition of especially attached bacteria significantly differed between the 2 algal cultures. Our study suggests that phytoplankton aggregation and vertical fluxes are closely linked to interactions between the marine phytoplankton and the ambient microbial community. KEY WORDS: Heterotrophic bacteria · Marine diatoms · Aggregates · DGGE · Image analysis · Bacterial behaviour Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 42: [19][20][21][22][23][24][25][26] 2006 ria that can grow in suspension as well as on particles ('generalists'). The predominance of one functional type of bacteria may be indicative of specific interactions between bacterial and algal communities in the course of phytoplankton blooms.The production of source particles and their subsequent aggregation is under continuous influence of the aforementioned microbial processes in the ocean. In previous studies we have developed an encounter model to predict the rate at which bacteria colonize marine snow particles (Kiørboe et al. 2002). Using agar spheres as model aggregates, we studied bacterial attachment, detachment, growth, cell-cell interactions, and predation in detail (Kiørboe et al. 2001, 2002, Grossart et al. 2003b. In the present study we examined how the microbial community affect marine snow formation, and in turn how marine snow affects microbial community composition and activities. We tested the following specific hypotheses: (1) aggregate formation is enhanced by the presence of attached bacteria; (2) the ambient bacterial community will shift from one dominated by 'generalists' to 2 distinct populations, one of 'particle specialists' and one of freeliving bacteria, as aggregate abundance increases; (3) the attached bacterial community is specific to the nature of POM (e.g. algal species). Our results show that the presence of bacteria significantly changes phytoplankton aggregation, suggesting that the fate of algal aggregates may hinge on complex mutual effects between the marine phytoplankton and the ...

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