Bacterial and phytoplankton succession during a freshwater bloom. Microbial succession during a diatom bloom

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 while in situ studies have typically not been sampled frequently enough to capture the full details of the transient dynamics in the plankton. To address this gap in knowledge, we followed bacterial community shifts at high temporal resolution during the onset and decline of a springtime 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 “Ca Fonsibacter” (LD12) and betI were dominant. 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 and peaked in synchrony with a rise in chlorophyll. The bloom caused populations with inferred copiotrophic lifestyles sush as Bacteroidetes Flavo-A3 and bacI-A1 and Betaproteobacterial tribes PnecC and betIII-A1, to increase in relative abundance along with the ultramicrobacteria acI-A6 (Actinobacteria). In contrast, nutrient depletion at the collapse of the bloom favoured ultramicrobacteria with oligotrophic lifestyles, such as “Ca Fonsibacter” (LD12) and members of Actinobacteria acI and acIV. Focusing on abundant bacterial freshwater bacterial groups, we observed diverse responses in the bacterioplankton community to the environmental shifts caused by the annually recurring phytoplankton spring bloom. These diverse responses likely reflect contrasting pelagic lifestyles and traits, including varying potential to profit from the organic substrates released from phytoplankton. Our work puts the spotlight on a dynamic but so far poorly studied period in the annual cycle of seasonally ice covered lakes and calls for more research on the biotic interactions that control the initiation and termination of phytoplankton blooms.