Cyanobacterium Found in Clean Lakes Shows Little Toxic Potential

Gloeotrichia echinulata is a type of cyanobacterium that receives scrutiny since it tends to bloom in clear lakes, forming thick mats of visible green or white puff balls in otherwise clean water. Because of the slew of toxic compounds they can produce, blooms of cyanobacteria (historically called “blue-green algae”) are of growing global concern. Yet, a new study by researchers at Bigelow Laboratory for Ocean Sciences and Oregon State University suggests this particular species might not deserve its maligned reputation.

The team assembled the first complete genomes of Gloeotrichia echinulata, comparing DNA of samples from both West Coast and Northeast lakes. They found no evidence of the machinery required to produce the toxins commonly associated with cyanobacteria, according to the new study recently published in Harmful Algae.

Gloeotrichia can still pose a nuisance, though: the samples possessed genes for producing geosmin, a compound that can affect the taste of water and warrant additional treatment. Yet, the researchers found striking genetic similarities across all the samples, despite the vast distances between them. That suggests this species has predictable properties, which is useful for public health and drinking water management.

“It may be producing yet-to-be-characterized toxins or molecules we don’t understand,” said Research Scientist Robin Sleith, the study’s co-lead author. “But for all the toxins that science has done the hard work of characterizing, and that have the most acute human health hazards, there were no obvious pathways.”

Cyanobacteria blooms are increasing worldwide in response to changing conditions and can pose significant risks by altering food webs, depleting lakes of oxygen, and producing toxins that are harmful to people.

Gloeotrichia has become a particular frustration in the Northeast where it has a wide distribution and often blooms — very visibly — in popular recreational lakes. At least one prior study has also suggested that the cyanobacterium is capable of producing microcystin, one of the most prevalent cyanotoxins that is known to cause serious liver damage.

“There’s always some suspicion about cyanobacteria just because so many of them produce toxins, and there's such a huge suite of compounds you have to worry about,” said Senior Research Scientist Peter Countway, one of the study’s co-authors. “Prime swimming season here in New England is August, which also happens to be when they’re most abundant.”

Yet, while working with a sample from New Hampshire’s famed Lake Winnipesaukee, Sleith said he just could not find the genes for producing microcystin. That led him to connect with Theo Dreher, an emeritus professor at Oregon State University and the study’s other co-lead, who was working on a study about toxin production on the West Coast. They decided to collaborate, ultimately pulling together a detailed comparative analysis with several complete and partial genomes from lakes in Maine, New Hampshire, California, Oregon, and Washington.

To pull together the Northeast samples, the Bigelow Laboratory team relied on Oxford Nanopore sequencing, an emerging molecular tool that is able to sequence incredibly long strands or fragments of DNA without sacrificing accuracy. That enabled them to do deep-dives into what genes were present, and how they were assembled in each sample.

In addition to finding genes responsible for producing geosmin — and none responsible for major toxin synthesis — they found genes across all the samples that could enable Gloeotrichia to grow on top of lake sediment. That provides insight into how the species might thrive in low-nutrient lakes.

The findings highlight the value and potential of emerging molecular approaches for this kind of comparative genomic analysis. The authors caution, however, that Gloeotrichia can bloom alongside species that are toxigenic, and there may be toxin-producing strains they have not found. That points to the need for ongoing monitoring of lakes, especially those that are important for drinking water or recreation.

“There’s a whole consortium of cyanobacteria,” Sleith said. “Even if you don’t have to worry about Gloeotrichia, it’s valuable to understand who the other players are and what all the risks are. Ongoing monitoring with these molecular approaches can help answer those questions.”

Photo: Gloeotrichia echinulata (Credit: Peter Countway).