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During the warm summer months in Lake Erie, cyanobacteria, commonly referred to as blue-green algae, can multiply uncontrollably, resulting in algal blooms that generate toxins capable of harming both wildlife and human health.

Recently, researchers from the University of Michigan have pinpointed the microorganism responsible for these toxins: a form of cyanobacteria known as Dolichospermum.

Harmful algal blooms, abbreviated as HABs, can consist of various types of cyanobacteria, each of which may generate different kinds of toxins. Understanding which species produces specific toxins can aid scientists in monitoring and managing harmful algal blooms.

An algal bloom in 2014 generated a toxin identified as microcystin, which posed a threat to Toledo’s drinking water. Evidence of a potent toxin called saxitoxin was first discovered in Lake Erie in 2007, but the organism responsible for its production remained uncertain. Saxitoxins are a category of closely related neurotoxins recognized as some of the most powerful natural toxins available.

“The primary benefit of knowing which organism produces the toxin is that it enhances our understanding of the conditions that trigger toxin production—essentially, what makes these organisms thrive,” explained Gregory Dick, a professor in earth and environmental sciences as well as environment and sustainability. “This information could potentially inform policy and management, although we still have a significant journey ahead in this regard.”

To identify the cyanobacteria, University of Michigan researchers collected samples from harmful algal blooms as they formed in the lake. First author Paul Den Uyl utilized a technique known as “shotgun” sequencing, which sequences all DNA present in a particular lakewater sample. Den Uyl then assembled a complete genome sequence and searched for genes related to the production of saxitoxin within that sequence.

The findings revealed multiple strains of Dolichospermum in the lake, though only a subset of these strains produced saxitoxin. Although the researchers were uncertain as to why some strains generate the toxin while others do not, they endeavored to investigate the environmental variables that might encourage saxitoxin production.

To this end, they sampled various locations in the lake across different seasons and measured the prevalence of the gene linked to saxitoxin in each sample. A notable trend emerged, showing that warmer waters tended to have higher gene counts.

“This is fascinating because we are aware that climate change is impacting the lakes,” noted Den Uyl, a scientist at U-M’s Cooperative Institute for Great Lakes Research, or CIGLR. “As the lakes warm, a significant question arises: how will this affect the biological communities, including harmful cyanobacterial blooms?”

The team also observed that the gene associated with saxitoxin production was less frequently detected in areas with elevated ammonium levels. They hypothesize that this could stem from a unique adaptation in Dolichospermum: it possesses a gene indicating its ability to utilize nitrogen in the form of dinitrogen, which is plentiful in the atmosphere—a rarity among organisms, according to Dick.

“One of the intriguing aspects of having access to the complete genome is that it allows us to see all the potential capabilities of the organism, at least in theory,” remarked Dick, who also directs CIGLR. “You have a comprehensive blueprint of the organism’s potential, and we can observe the capacity to obtain fixed nitrogen from the water. However, acquiring it in the form of dinitrogen gas is a remarkable ability; very few organisms can achieve this, enhancing their competitiveness under such conditions.”

The researchers indicate that they have monitored saxitoxin in the lake for nine years, but acknowledged that this duration is insufficient to determine whether saxitoxin production will escalate as temperatures rise.

“Now that we have identified the organism responsible, I believe we can more effectively monitor these species and directly evaluate gene abundance over time,” Dick stated. “We intend to continue observing the prevalence of this organism, yet it’s premature to ascertain if it is becoming more prevalent. While there is a correlation, the connection with temperature raises concerns.”

Their research findings are published in the journal Environmental Science & Technology.

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