Genomic Identification and Characterization of Saxitoxin Producing Cyanobacteria in Western Lake Erie Harmful Algal Blooms

Saxitoxins (STXs), a group of closely related neurotoxins,
are
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 Laurentian Great Lakes have not been identified.
We identified a full suite of STX biosynthesis genes in a Dolichospermum metagenome-assembled genome (MAG). The content
of sxt genes suggest that this organism can produce
STX, decarbamoyl and deoxy-decarbamoyl saxitoxins, and other congeners.
The absence of sxtX indicates this organism is unable
to produce neosaxitoxin, a potent congener. However, a distinct, lower
abundance sxt operon from an unidentified organism
did contain sxtX, indicating neosaxitoxin biosynthesis
potential. Metatranscriptomic data confirmed STX biosynthesis gene
expression. We also recovered highly similar Dolichospermum MAGs lacking sxt genes, implying gene loss or horizontal
gene transfer. sxtA was detected by quantitative
polymerase chain reaction during 47 of 76 sampling dates between 2015
and 2019, demonstrating higher sensitivity than metagenomic approaches. sxtA gene abundance was positively correlated with temperature
and particulate nitrogen:phosphorus ratio and negatively correlated
with ammonium concentration. All Dolichospermum MAGs
had genes required for nitrogen fixation. Collectively, this study
provides a foundation for understanding potential new threats to Lake
Erie water quality.