Cyanobacterial blooms alter the relative importance of neutral and selective processes in assembling freshwater...

CyanoHABs have substantial impacts on the functioning and sustainability of freshwater ecosystems by restricting light penetration, depleting dissolved oxygen, and producing various toxins. This study combined physicochemical variable measurements, 16S rRNA gene sequencing and microscopy observations to examine mechanisms that govern the assembly of bacterioplankton communities following the progress of cyanobacterial blooms in a freshwater reservoir. Throughout the sampling season, bacterioplankton distribution patterns were well predicted by a neutral model, which assumes passive dispersal and ecological drift as the predominate mechanisms for community assembly. The neutral model consistently explained the distribution of over 67% of bacterioplankton OTUs and its fit was weaker during the cyanobacterial blooms (R² = 0.322) than the before- (R² = 0.549) and after-bloom stages (R² = 0.535). Variations of environmental factors, acting as selective pressures, explained shifts of non-neutral OTUs (above/under neutral prediction) (63.9%) better than neutral OTUs (34.5%). Co-occurrence network analysis organized microbial communities into modules and revealed strong positive correlations between bacterioplankton and cyanobacteria than with planktonic algae and zooplankton. Overall, our results suggest that neutral processes play significant roles in assembling bacterioplankton communities over a cyanobacterial bloom succession and its relative importance may be weakened by biotic pressures (interspecific interactions) during the bloom period. Our results also indicate that among biotic factors, cyanobacteria had greater impacts on bacterioplankton community assembly than planktonic algae and zooplankton.