Green algae in Lake Erie may be hiding toxic, deadly compounds

Every summer, Lake Erie transforms. Warm, still waters near the surface invite the growth of tiny, plant-like organisms.

To the naked eye, these blooms of cyanobacteria – commonly known as blue-green algae – seem like nothing more than a green scum. But beneath the surface lies a silent, potent threat. Some of these blooms release toxins that pose risks to fish, birds, and even humans.

In recent years, harmful algal blooms, or HABs, have become more common in Lake Erie. The toxins they release can contaminate drinking water, disrupt ecosystems, and impact public health.

One such bloom in 2014 forced officials to shut down Toledo’s water supply due to contamination by a toxin called microcystin. But another toxin – saxitoxin – has remained a mystery for years. Scientists detected it in 2007 but could not confirm which organism was producing it.

Now, researchers at the University of Michigan have finally solved that puzzle.

Identifying the toxin’s source

The research focused on answering a long-standing question: Which toxic cyanobacteria are responsible for saxitoxin in Lake Erie? The answer, they discovered, is Dolichospermum, a genus of cyanobacteria that can survive and thrive in nutrient-rich freshwater systems.

Harmful algal blooms, or HABs, form from various cyanobacteria, each capable of making different toxins. Identifying which type makes which toxin helps scientists monitor and respond to these dangerous blooms more effectively.

Saxitoxins are among the most dangerous natural toxins known. Even at very low levels, they can damage the nervous system. Understanding their biological origin is crucial for managing risks associated with drinking water and wildlife exposure.

Gregory Dick, professor of earth and environmental sciences and of environment and sustainability, highlighted the value of this discovery.

“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. Such information can help guide policy and management, though we’re still a long way from that in this case,” said Dick.

Toxic water samples from Lake Erie

To trace the source, the researchers collected toxic water samples from Lake Erie during bloom events. These samples held a complex mix of DNA from many organisms.

Lead author Paul Den Uyl applied a method called shotgun sequencing. Instead of isolating individual species, this method captures all the DNA in a sample and reassembles it into complete genomic sequences.

Using this data, Den Uyl was able to reconstruct full genomes from the bloom samples. Within one of these genomes, he identified the genetic signature of saxitoxin. It confirmed that some strains of Dolichospermum carry the genes necessary to produce the toxin.

This discovery came with a twist. Not all Dolichospermum strains produce saxitoxin. Some have the gene, and others do not. What influences this genetic variation remains unclear, but researchers turned their attention to the lake’s environmental conditions to find answers.

Role of temperature and nutrients

The team studied samples from across Lake Erie, covering different seasons and locations. They measured how much of the saxitoxin gene was present in each sample and looked for patterns linked to environmental factors.

The team found a consistent trend: warmer water tended to contain higher amounts of the gene. This raises urgent questions in the context of climate change.

“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?”

But temperature wasn’t the only factor. The researchers also looked at nutrient concentrations in the toxic lake water, particularly ammonium. They found that the gene appeared less frequently in areas with high ammonium levels.

A unique nitrogen advantage

This pattern may be linked to a special feature of Dolichospermum. While most organisms cannot access the nitrogen found in the atmosphere, Dolichospermum appears to have that ability.

The team found a gene in its genome that allows it to use nitrogen in the form of dinitrogen gas – a rare trait in aquatic ecosystems.

“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.”

This ability gives Dolichospermum a clear advantage, especially in low-nitrogen environments. It may also explain why this organism is capable of thriving in parts of the lake where other algae struggle.

Future toxic blooms in Lake Erie

Although the team has been tracking saxitoxin in Lake Erie for nine years, that’s not enough time to predict long-term changes. Climate conditions, nutrient runoff, and microbial competition all influence the lake’s ecology. Still, the discovery provides a better foundation for future monitoring and research.

“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.”

As lake temperatures continue to rise, harmful algal blooms may become more frequent or more toxic. Understanding the organisms involved, their genetic capabilities, and the environmental triggers for toxin production is now more important than ever.

The findings from this study were published in the journal Environmental Science & Technology. They offer a critical piece of the puzzle in managing one of North America’s most valuable freshwater resources.

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