A ubiquitous Microcoleus species causes benthic cyanotoxic blooms worldwide

In the last ten years, blooms of benthic cyanobacteria producing derivatives of the potent neurotoxin anatoxin-a have been repeatedly reported in rivers of temperate regions all over the world. For three series of events, in New Zealand, the USA, and Canada, a cohesive cluster of Microcoleus strains was found to be responsible for the toxin synthesis. Here, we document a similar toxigenic event that occurred at the mouth of the river Areuse in the lake of Neuchâtel (Switzerland) and caused the death of several dogs. Using 16S RNA-based community analysis, we show that benthic communities in the river are dominated by Oscillatoriales and especially by the Microcoleus genus. We correlate the detection of one specific sequence variant with the presence of anatoxin-a derivatives and use metagenomics to assemble a complete circular genome of the strain of interest. The strain is distinct from the ones isolated in New Zealand, the USA, and Canada, but belongs to the same species (>94% average nucleotide identity); it shares significant genomic traits with them, in particular a relatively small genome and incomplete vitamin biosynthetic pathways. Overall, our results suggest that the major anatoxin-a-associated benthic proliferations worldwide can be traced back to a single ubiquitous Microcoleus species rather than to a diversity of cyanobacterial lineages. We recommend that this species be monitored internationally and studied in detail in order to help predict and mitigate similar cyanotoxic events. ### Competing Interest Statement The authors have declared no competing interest.