During the hotter months, Lake Erie becomes a great setting for cyanobacteria, also often known as blue-green algae, to grow rapidly. Under these conditions, algae can form large blooms that may release toxins which can be able to harming each wildlife and folks.
Researchers on the University of Michigan have now identified the organisms accountable for producing these toxins. Their work points to a particular sort of cyanobacteria, often known as Dolichospermum, because the source.
Harmful algal blooms, or hubs, can consist of many cyanobacterial species, each capable of manufacturing different toxins. Determining which species produce which species is essential for monitoring, predicting and managing bloom events.
Tracing the source of microcystin and suxitoxin
A big bloom in 2014 produced toxic microcystins and posed a serious threat to Toledo's drinking water supply. Earlier, in 2007, scientists detected traces of a highly potent toxin in Lake Erie, although its biological source remained unknown. Cixitoxin belongs to a gaggle of closely related neurotoxins which can be considered probably the most potent naturally occurring toxins.
“The biggest benefit of knowing which organisms produce toxins is that it helps us understand the conditions that cause toxins to be produced — that is, what conditions make those organisms successful,” said Gregory Dick, professor of Earth and Environmental Sciences and Ecology and Sustainability. “Such information can help guide policy and management, although we still have a long way to go in this regard.”
Use of DNA sequencing to discover toxin producers
To determine which organism is accountable for the cixitoxin, the UM team collected samples directly from the HABS as they appeared within the lake. Lead writer Paul Dan Yoel applied “shotgun” sequencing, a method that reads all of the DNA in a water sample. With these sequences, he reconstructed a complete genome after which searched that genome for the genes involved in making cixitoxin.
Their evaluation revealed several strains of dulcosperm living in Lake Erie. However, just some strains have the flexibility to supply cixitoxin. Although the rationale for this difference remains to be unclear, researchers began to look at environmental conditions that will affect toxin production.
Environmental clues in temperature and nutrient levels
The team collected samples from multiple sites across Lake Erie and measured how much of the cixitoxin-related gene appeared in each sample. They often detect higher levels of this gene in warmer water.
“It's interesting because we know that lakes are changing with climate change,” said Dan Yuell, a scientist at UM's Cooperative Institute for Great Lakes Research, or CIGLR. “With lakes warming, a big question is, how does that affect biological communities, including harmful cyanobacterial blooms?”
They also observed that the gene related to suxitoxin was less common in areas with high levels of ammonium. The team suspects that this pattern could also be related to a particular feature of Dolichospermum: the presence of a gene that shows it could possibly use nitrogen in the shape of dinitrogen, an abundant atmospheric gas. According to Dick, only a limited variety of organisms can use nitrogen in this kind, giving Dolichospermum a competitive advantage under certain conditions.
“One of the neat things about whole genomes is that you can see everything the organism does, at least theoretically,” said Dick, who can also be director of CIGLR. “You have the whole blueprint for what the organism can do, and we see the ability to get fixed nitrogen from water. It's just that getting it in the form of dinitrogen gas is like a superpower. A lot of organisms can't do that, and it makes them more competitive in those conditions.”
Monitoring long-term risks in a changing lake
According to the researchers, they've monitored cixitoxin within the lake for nine years, but that period is just too short to find out whether toxin levels will increase because the climate warms.
“But now that we know who's producing it, I think we can keep a better eye on these organisms and we can directly assess gene abundance over time,” Dick said. “We plan to continue to monitor the abundance of this organism, but it's too early to tell if it's becoming more abundant. It's just a correlation, but its correlation with temperature is concerning.”
Their study appears within the journal