An experimental test of cyanotoxins as a potential driver of microbial community structure.

Cyanobacterial harmful algal blooms (CyanoHABs) are common biological disturbances in freshwater ecosystems, impacting microbial community diversity and composition. While extensive research has focused on these blooms, the direct effects of cyanotoxins on microbial communities remain less understood. In this study, we investigated the impact of various cyanotoxins on the microbial community of an oligotrophic lake in Quebec, Canada (45.99°N, 74.00°W). Water samples were exposed to different concentrations of MC-LR, MC-RR, MC-LF, and CYN, both individually and in combination. These toxins were selected based on their prevalence, toxicity, and distinct chemical properties. Toxin concentrations were chosen in relation to the World Health Organization (WHO) regulatory thresholds, 1 μg/L as indicative of low toxin exposure (drinking water limit) and 1000 μg/L as indicative of high exposure (lake threshold). We performed a longitudinal analysis of 16S rRNA to assess changes in microbial community diversity and composition at 24-h, 48-h, and 72-h intervals. Our findings showed a significant change in alpha and beta diversity, highlighting shifts in community structure in response to high cyanotoxin doses. Conversely, no significant changes were detected across diverse cyanotoxin compositions. We then performed a differential analysis and identified several amplicon sequence variants (ASVs) with significant changes in relative abundance across cyanotoxin doses. This analysis highlighted potential cyanotoxins degrading bacteria, such as Paucibacter and Ideonella. Overall, our results showed that the changes were more associated with cyanotoxin doses than with composition. Understanding how cyanotoxins could impact oligotrophic lakes is essential for better predicting their ecological impacts, especially as these lakes are increasingly affected by cyanobacterial blooms.