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Coastal lagoons are extremely sensitive to increasing temperature, especially in the Mediterranean basin, which has been identified as a hotspot for global warming. The warming effects on the abundance and size structure of a Mediterranean coastal lagoon plankton community were investigated in this study. Water from Cabras Lagoon (Italy) was incubated in laboratory for 16 days in winter, excluding mesozooplankton. Three temperature treatments were applied: (i) the in situ winter mean water temperature; (ii) + 3 °C, as forecasted for the Mediterranean region by the next century; (iii) + 6 °C, as forecasted for the Mediterranean region by the next two centuries. A direct and positive effect of warming on ciliate density was observed in absence of their predators, as well as a taxonomic composition shift from Oligotrichs to Scuticociliatida. In turn, a decrease in heterotrophic nanoflagellate density was detected under warming. Phytoplankton (autotrophic cell size > 2 µm) density increased significantly, while their mean cell size decreased strongly throughout the experiment at the highest temperature. A significant change in phytoplankton class composition, consisting of the increase of smaller Chlorophyceae which replaced larger Bacillariophyceae, was observed under heating. Considering picoplankton (cell size < 2 µm), a decrease in larger autotrophic cell density was contemporary to an increase in smaller heterotrophic cell density, especially at the highest temperature. This work adds novel information to the predictions about plankton community responses on warming considering several trophic levels, which has been little studied in shallow coastal lagoons and in the Mediterranean basin.

1. Planktonic bacterial community dynamics over short timescales can be of great importance for food webs and ecosystem functioning but are rarely described when microbial community and composition are assessed. To study the significance of such dynamics we sampled the surface water at the deepest point of a mesotrophic lake (Lake Erken, Sweden) every third hour over two days. 2. By combining 454 pyrosequencing of 16S rRNA genes with bromodeoxyuridine immunocapturing of DNA, replicating populations were identified and compared to the community retrieved from total DNA samples. This comparison revealed a significant difference between the actively replicating and total community. 3. The high-frequency diurnal sampling was compared to a year-long survey conducted in the same lake in order to compare the diurnal and seasonal variation in bacterioplankton community composition. At the diurnal-scale, the variation was significantly higher in the replicating than in the total community. However, variation in both active and total diurnal community was significantly lower than the variation in the seasonal total community. 4. Our analysis revealed pronounced short-term dynamics of individual bacterial populations uncoupled from the diurnal light cycle. For example, the proliferating fraction of the most abundant bacterial tribe (LD12) followed a cyclic pattern that covaried with viral abundance. This implies that environmental factors other than light may act as important drivers of microbial community composition, at least in mesotrophic Lake Erken., QC 20170125

Predicting the responses of organisms is a complex challenge especially when water temperature is expected to increase over the coming decades, as a result of global warming. In this work the effects of warming on phytoplankton communities were investigated. An indoor experiment was performed, where water from a Mediterranean lagoon was incubated at different temperatures. Three treatments were applied in triplicate incubation units: the control (11 °C), 3 °C increase (14 °C), and 6 °C increase (17 °C). Our results showed significant effects by warming on phytoplankton. The abundance of relatively smaller taxa (Chlorella sp. and Planktothrix agardhii–rubescens group) increased at 17 °C, whereas the abundance of relatively larger species (Cyclotella sp. and Thalassiosira sp.) decreased, compared with the control. This shift towards smaller taxa resulted in a higher total biomass but lower chlorophyll a concentrations at the highest temperature., Short communication.

Aquatic bacterial communities experience rapid turnover as they have fast reproductive rates and are highly susceptible to predation and other mortality factors. Short-lived changes in the environment, such as the spring phytoplankton bloom which alter both resource supply and food web interactions can therefore cause rapid shifts in bacterioplankton community composition and function. Information about the temporal dynamics of such successional changes is still limited and has for the most part been obtained in studies of experimental enclosures with limited ability to extrapolate findings to complex natural ecosystems. To address this gap in knowledge, we followed bacterial community shifts at high temporal resolution during the onset and decline of a spring diatom bloom in a dimictic temperate lake. Water was collected from shortly before ice-off until the onset of summer stratification. Bacterioplankton community composition was assessed using 16S rRNA amplicon sequencing while parallel samples were collected for chemical analyses and microscopy-based phytoplankton community composition. In agreement with previous work, Proteobacteria, Actinobacteria and Bacterioidetes were the dominant bacterial phyla throughout the study period. Under the ice proteobacterial LD12 (Fonsibacter) and betI were dominant tribes. At ice-off there was a rapid initiation of the phytoplankton spring bloom and concurrently an immediate positive response in bacterial abundances which lasted throughout the bloom. Tribes Flavo-A3 is and acI-A6 responded positively to the bloom and at the maximum bloom intensity, acI-A6 was very abundant while LD12 were scarce. Immediately after the phytoplankton bloom, the bacterial community went into a more stable phase without major changes in bacterial community composition except that LD12 and bacI-A1 (FukuN47) became more abundant. Focusing on abundant bacterial freshwater bacterial groups, we observed diverse responses in the bacterioplankton comm

In surface waters, solar radiation transforms complex dissolved organic compounds into photoproducts that are more readily available for bacterial metabolic use. In this way, photochemical processes can influence both carbon cycling and freshwater bacterioplankton communities. We experimentally studied photochemical transformation of dissolved organic carbon (DOC) in the presence or absence of nitrate added as a photosensitizer and followed how this affected growth and community composition of heterotrophic bacteria. As expected, bleaching of chromophoric dissolved organic matter (cDOM) was strongly positively correlated to total energy content of the solar irradiance. In contrast, the addition of nitrate had no significant impact on neither DOC photodegradation or bleaching. Bacterial growth assays revealed extensive solar-driven transformation of biologically recalcitrant carbon compounds into more labile forms that led to enhanced bacterial growth. Based on 16S rRNA community analyses applied to regrowth cultures, we demonstrate that solar exposure and to a lesser extent also the nitrate-mediated photosensitization, had a significant influence on the taxonomic composition of bacterioplankton communities. All communities were dominated by class Proteobacteria, and notably known bacterivorous taxa such as Bdellovibrio and like organisms (BALOs) within the class of Oligoflexia were strongly favored by DOM exposed to solar irradiation while this effect was not seen when irradiation was combined with nitrate as photosensitizer. In addition, taxa which seem to have a copiotrophic lifestyle such as Acin (Gammaproteobacteria) showed a positive response to the irradiation and nitrate addition while the opposite was found for LD12, acI-A and alfVI that are considered to have a more oligotrophic one. Further, ambiguous responses from the treatments were observed as bal-A1 (FukuN47) belonging to Bacteroidetes only responded positively to the irradiation in the absence of nit