A comparison of bacterial community structure, activity and microcystins associated with formation and breakdown of a cyanobacterial scum

Toxic Cyanobacteria-dominated blooms are a global phenomenon that poses a health risk to humans and animals. These blooms harbor a diverse range of heterotrophic bacteria which are involved in growth-promoting and decomposition processes. In the present study, we investigated microbial communities and microcystins in a lake cyanobacterial scum at a single point in time. The outer edges of the scum (ca. 500 cm from shore) were freshly formed, while those closest to the shore showed signs of cyanobacterial cell lysis and degradation. Samples were collected from 5 sites across the scum and from 2 separate bays. We hypothesized that cyanobacterial genera, bacterial communities and microcystin quota (toxin content per cell) would be significantly different in the degrading scums versus where the scum had freshly formed. Samples were analyzed using 16S rRNA metabarcoding (DNA and RNA), and a range of physicochemical para meters were determined. Microcystis transcripts were more abundant than Dolichospermum in the breaking-down scum, suggesting they are better suited to tolerating the harsh physicochemical conditions encountered within scums. Multivariate analysis of operational taxonomic units (excluding Cyanobacteria) showed significant differences in bacterial community structures across the scums. Proteobacteria was the most abundant phylum, among which Aeromonas, Caulobacter and Brevundimonas dominated. No relationships were observed between microcystin quotas and bacterial community structure or position in the scum.