Genetic Characterization of Saxitoxin Producing Cyanobacteria Associated with Western Lake Erie Harmful Algal Blooms

Abstract

Saxitoxins (STXs) are a group of closely related neurotoxins and among the most potent natural toxins known. While genes encoding STX biosynthesis have been observed in Lake Erie, the organism(s) responsible for producing STXs in the region have not been confirmed. We used metagenomic tools to identify a full suite of STX biosynthesis genes in a high-quality metagenome-assembled genome (MAG) from the Anabaena-Dolichospermum-Aphanizomenon (ADA) clade of cyanobacteria. The order and sequence of sxt genes suggest the Lake Erie MAG is capable of producing STX and STX congeners. The absence of a sxtX gene suggests an inability to produce neoSTX, one of the most potent congeners of STX. We also recovered highly similar ADA MAGs that did not contain STX genes, implying gene loss or horizontal gene transfer. The full suite of STX biosynthesis genes was observed in metagenomic datasets across 13 of 123 unique sampling dates between 2014 and 2022, with occurrences spanning from July 6 to September 19. Our results also reveal trends in the increasing abundance of ADA MAGs during late-season blooms. Collectively, this study provides a foundation for understanding a potential new front of threats to Lake Erie water quality.