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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 ...

Abstract. The predicted rise in anthropogenic CO 2 emissions will increase CO 2 concentrations and decrease seawater pH in the upper ocean. Recent studies have revealed effects of pCO 2 induced changes in seawater chemistry on a variety of marine life forms, in particular calcifying organisms. To test whether the predicted increase in pCO 2 will directly or indirectly (via changes in phytoplankton dynamics) affect abundance, activities, and community composition of heterotrophic bacteria during phytoplankton bloom development, we have aerated mesocosms with CO 2 to obtain triplicates with three different partial pressures of CO 2 (pCO 2 ): 350 µatm (1×CO 2 ), 700 µatm (2×CO 2 ) and 1050 µatm (3×CO 2 ). The development of a phytoplankton bloom was initiated by the addition of nitrate and phosphate. In accordance to an elevated carbon to nitrogen drawdown at increasing pCO 2 , bacterial production (BPP) of free-living and attached bacteria as well as cellspecific BPP (csBPP) of attached bacteria were related to the C:N ratio of suspended matter. These relationships significantly differed among treatments. However, bacterial abundance and activities were not statistically different among treatments. Solely community structure of free-living bacteria changed with pCO 2 whereas that of attached bacteria seemed to be independent of pCO 2 but tightly coupled to phytoplankton bloom development. Our findings imply that changes in pCO 2 , although reflected by changes in commuCorrespondence to: H.-P. Grossart (hgrossart@igb-berlin.de) nity structure of free-living bacteria, do not directly affect bacterial activity. Furthermore, bacterial activity and dynamics of heterotrophic bacteria, especially of attached bacteria, were tightly correlated to phytoplankton development and, hence, may also potentially depend on changes in pCO 2 .

Abstract. Heterocystous cyanobacteria of the genus Nodularia form extensive blooms in the Baltic Sea and contribute substantially to the total annual primary production. Moreover, they dispense a large fraction of new nitrogen to the ecosystem when inorganic nitrogen concentration in summer is low. Thus, it is of ecological importance to know how Nodularia will react to future environmental changes, in particular to increasing carbon dioxide (CO2) concentrations and what consequences there might arise for cycling of organic matter in the Baltic Sea. Here, we determined carbon (C) and dinitrogen (N2) fixation rates, growth, elemental stoichiometry of particulate organic matter and nitrogen turnover in batch cultures of the heterocystous cyanobacterium Nodularia spumigena under low (median 315 μatm), mid (median 353 μatm), and high (median 548 μatm) CO2 concentrations. Our results demonstrate an overall stimulating effect of rising pCO2 on C and N2 fixation, as well as on cell growth. An increase in pCO2 during incubation days 0 to 9 resulted in an elevation in growth rate by 84 ± 38% (low vs. high pCO2) and 40 ± 25% (mid vs. high pCO2), as well as in N2 fixation by 93 ± 35% and 38 ± 1%, respectively. C uptake rates showed high standard deviations within treatments and in between sampling days. Nevertheless, C fixation in the high pCO2 treatment was elevated compared to the other two treatments by 97% (high vs. low) and 44% (high vs. mid) at day 0 and day 3, but this effect diminished afterwards. Additionally, elevation in carbon to nitrogen and nitrogen to phosphorus ratios of the particulate biomass formed (POC : POP and PON : POP) was observed at high pCO2. Our findings suggest that rising pCO2 stimulates the growth of heterocystous diazotrophic cyanobacteria, in a similar way as reported for the non-heterocystous diazotroph Trichodesmium. Implications for biogeochemical cycling and food web dynamics, as well as ecological and socio-economical aspects in the Baltic Sea are discussed.