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In the warm summertime waters of Lake Erie, cyanobacteria, or blue-green algae, can proliferate out of control, creating algal blooms that produce toxins at a rate that can harm wildlife and human health.

Now, University of Michigan researchers have identified the organism responsible for producing the toxins: a type of cyanobacteria called Dolichospermum. Their findings are in the journal Environmental Science & Technology.

Harmful algal blooms, or HABs, can be composed of different types of cyanobacteria, which can produce different types of toxins. Knowing which cyanobacteria produces which toxins can help scientists track and respond to harmful algal blooms.

A bloom in 2014 produced a toxin called microcystin, which threatened Toledo's drinking water supply. In 2007, scientists first detected evidence of a potent toxin called saxitoxin in Lake Erie, but they weren't sure which organism produced it. Saxitoxins, a group of closely related neurotoxins, are among the most potent natural toxins known.

"The main advantage of knowing which organism produces the toxin is that it helps us understand the conditions that cause toxin production—that is, what conditions make those organisms successful," said Gregory Dick, professor of Earth and environmental sciences and of environment and sustainability. "Such information can help guide policy and management, though we're still a long way from that in this case."

To identify the cyanobacteria, U-M researchers took samples from HABs as they occurred in the lake. Then, first author Paul Den Uyl used what's called "shotgun" sequencing. This method sequences all of the DNA in a given sample of lakewater. Den Uyl used these DNA sequences to piece together a whole genome sequence, then searched within that sequence for genes that encode the toxin saxitoxin.

The researchers found that there were several strains of Dolichospermum in the lake, but only some of them produce saxitoxin. While they aren't sure why some produce the toxin and others don't, the researchers tried to parse the environmental factors that promote saxitoxin production.

To do this, they took samples from different locations on the lake throughout each season and quantified how much of the gene associated with saxitoxin was present in each sample. The researchers tended to find more of the gene in warm water.

"That is interesting because we do know that the lakes are changing with climate change," said Den Uyl, a scientist at U-M's Cooperative Institute for Great Lakes Research, or CIGLR. "With the warming of the lakes, one of the big questions is, how is that going to change the biological communities, including harmful cyanobacterial blooms?"

The researchers also found that the gene associated with saxitoxin production was less likely to be in areas that had higher concentrations of ammonium. They think this may be because Dolichospermum has an unusual adaptation: It has a gene that suggests it can use nitrogen in the form of dinitrogen, which is abundant in the atmosphere—not something many organisms can do, according to Dick.

"One of the neat things about having the whole genome is you can see everything the organism can do, at least theoretically," said Dick, who is also director of CIGLR.

"You have the whole blueprint for what the organism can do, and we do see the capability of obtaining fixed nitrogen from the water. It's just that getting it in the form of dinitrogen gas is kind of a superpower. Not a lot of organisms can do that, and it makes them more competitive under those conditions."

The researchers say they have been tracking saxitoxin in the lake for nine years, but that doesn't provide enough data to tell whether saxitoxin production will increase as the lakes warm.

"But now that we know who's producing it, I think we can keep a better watch on these organisms and we can also directly assess the gene abundance over time," Dick said. "We plan to continue monitoring the abundance of this organism, but it's too early to tell if it's becoming more abundant. It's just a correlation, but that correlation with temperature is concerning."