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In parts of the Baltic Sea, the phytoplankton spring bloom communities, commonly dominated by diatoms, are shifting toward the co‐occurrence of diatoms and dinoflagellates. Although phytoplankton are known to shape the composition and function of associated bacterioplankton communities, the potential bacterial responses to such a decrease of diatoms are unknown. Here we explored the changes in bacterial communities and heterotrophic production during the spring bloom in four consecutive spring blooms across several sub‐basins of the Baltic Sea and related them to changes in environmental variables and in phytoplankton community structure. The taxonomic structure of bacterioplankton assemblages was partially explained by salinity and temperature but also linked to the phytoplankton community. Higher carbon biomass of the diatoms Achnanthes taeniata, Skeletonema marinoi, Thalassiosira levanderi, and Chaetoceros spp. was associated with more diverse bacterial communities dominated by copiotrophic bacteria (Flavobacteriia, Gammaproteobacteria, and Betaproteobacteria) and higher bacterial production. During dinoflagellate dominance, bacterial production was low and bacterial communities were dominated by Alphaproteobacteria, mainly SAR11. Our results suggest that increases in dinoflagellate abundance during the spring bloom will largely affect the structuring and functioning of the associated bacterial communities. This could decrease pelagic remineralization of organic matter and possibly affect the bacterial grazers communities. [ABSTRACT FROM AUTHOR]

We investigated the short-term effects of variable solar irradiance and spectrum on the gross biological production of dimethylsulfide (DMS), a trace gas with potential climatic effects, in eight experiments performed at different times of the year in a northwest Mediterranean coastal site. Experimentally determined net community DMS production, DMS photolysis, and dark microbial DMS consumption rates were used to calculate gross community DMS production by budgeting. In addition, the composition of the bacterioplankton and phytoplankton communities in the initial samples, and the photoinhibition of bulk bacterial heterotrophic activity and phytoplankton photosynthetic efficiency were monitored. Our results show that: (1) gross DMS production is irradiance dependent, with a maximum short-term stimulation factor of 2- to 6-fold compared to dark incubations; (2) its spectral shape is variable but generally similar to that of phytoplankton photoinhibition or photodamage, with more effective stimulation at shorter ultraviolet wavelengths; and (3) stronger stimulation occurs when samples are overexposed with respect to their prior exposure. Remarkably, the photoresponse of gross DMS production was in most cases strong enough to (at least) compensate the photochemical DMS loss at the water subsurface. Such response would prevent DMS depletion in stratified and highly irradiated waters. Since the initial microbial communities were representative of meso- to oligotrophic conditions, our observations should apply to a wide variety of oceanic regimes. [ABSTRACT FROM AUTHOR]

The effect of photosynthetically available radiation (PAR; 400-700 nm) and ultraviolet radiation (UVR; 280-400 nm) on marine bacterial heterotrophic activity was assessed monthly throughout a seasonal cycle in Blanes Bay (northwestern Mediterranean Sea). Seawater samples amended with ³H-leucine were exposed to solar radiation under three radiation treatments: PAR +UVR (280-700 nm), PAR +UVA (320-700 nm), and PAR only. Parallel reference incubations in the dark and under a fixed artificial light source (PAR only) were also performed. Exposure to high UVR doses caused strong inhibition of ³H-leucine incorporation rates (LIR), whereas natural PAR doses did not cause overall significant effects. Within UVR, UVA radiation accounted for most of the reduction in LIR, and this effect was modulated by the proportionality of the experimental light to the previous light exposure history of the samples. Constant (artificial) PAR-only exposure led to a general but seasonally variable increase in bacterial heterotrophic production compared to the dark controls, with large increases in spring and lower changes during summer. This pattern was likely caused by the stimulation of the bacterial group Gammaproteobacteria, which showed higher numbers of cells active in ³Hleucine uptake after light exposure. Again, the previous light history of the samples seemed to partly explain the measured effects. Overall, our results show variable responses of bacterial activities to light manipulations, depending on seasonally changing light conditions and communities, and stress the importance of realistic simulation of light exposure conditions for ecosystem-relevant photobiological studies with microbial plankton. [ABSTRACT FROM AUTHOR]

Two diel cycle studies were conducted to determine the effect of day-night light changes on winter bacterial activity in the coastal Mediterranean (Blanes Bay Microbial Observatory). Bacterial abundances, bacterial heterotrophic activity, and flagellate grazing counts were determined at 4-h intervals during two 3-d periods separated by 2 d. Twice a day, the single-cell activity of major bacterial groups was further analyzed by applying microautoradiography combined with catalyzed reporter deposition-fluorescence in situ hybridization. During the first cycle, all the measured parameters (including the number of active cells in most groups) varied synchronously, with higher values at night and lower during the day. An episode of strong winds between the two studied periods disrupted this periodicity at the onset of the second cycle. The bulk incorporation of ³H-leucine recovered the diel pattern after 2 d, mostly driven by the activity of Gammaproteobacteria. Among the possibilities considered, the observed nighttime increases in the grazing activity of heterotrophic nanoflagellates appeared to partially drive the activity of all bacterial taxa through potential periodic release of dissolved organic matter. [ABSTRACT FROM AUTHOR]