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Uncovering a wider toxin cocktail
While municipalities and federal agencies primarily monitor U.S. waterways for microcystins—toxins generated by harmful algal blooms in Lake Erie—a recent study conducted by the University of Michigan has revealed that these blooms produce a far broader spectrum of potentially toxic compounds than previously understood.
The study’s researchers identified these compounds, termed bioactive cyanopeptides, which can interact with one another in ways that may enhance their toxicity over the course of the season. The research underscores the need to accurately characterize these compounds, evaluate their toxicity, and assess their interactions.
"A lot of people are aware of these algal toxins, but the big picture is that these harmful algal blooms are expanding with climate change, and they're a real threat to recreation, drinking water and ecosystems," stated senior author Gregory Dick, a professor of Earth and environmental sciences and of environment and sustainability.
"What our paper demonstrates for the first time is that in western Lake Erie, a complex mixture of these various compounds exists, with discernible seasonal patterns indicating when these compounds appear."
The findings are detailed in The ISME Journal, resulting from a collaborative effort that included scientists from U-M's Cooperative Institute for Great Lakes Research and Great Lakes Center for Freshwaters and Human Health, alongside contributions from NOAA and USGS.
Tracking blooms across seasons
To map the compounds produced within Lake Erie, the researchers analyzed samples collected monthly from four NOAA Great Lakes Environmental Research Laboratory stations in western Lake Erie, spanning from May to October, from 2016 to 2022.
Utilizing a method to detect microbial DNA, the researchers identified the bacteria present in the samples. Concurrently, they measured the occurrence of microbially derived compounds, including known cyanotoxins like microcystins. This approach allowed the team to correlate specific bacteria with the compounds they generate.
Notably, the researchers identified the well-known microcystin toxin attributed to the cyanobacteria Microcystis. However, they also discovered several additional compounds that had not been previously characterized in Lake Erie and are not included in conventional monitoring protocols.
"Microcystin is merely the tip of the iceberg concerning the compounds that Microcystis and other cyanobacteria can produce," remarked Dick, who also directs the Cooperative Institute for Great Lakes Research.
Three phases of harmful blooms
During her analysis, Lauren Hart, a U-M graduate student and the lead author of the study, identified three distinct phases of algal blooms, commencing in early spring as rainfall and runoff transport nitrogen into the lake.
Microcystin is predominant in the initial phase of blooms. Subsequently, as nitrogen levels diminish in the lake, other microbial communities take over, transforming the remaining nitrogen into forms that facilitate the production of additional molecules as the season progresses. The latter two phases yield cyanopeptides such as anabaenopeptins and aeruginosins, followed by aerucyclamides.
How toxin mixtures affect human cells
Hart also investigated how these compounds may interact, potentially escalating or diminishing their toxicity. This inquiry was prompted by her observations that microcystins and anabaenopeptins were simultaneously present in Lake Erie. In her study, published in Environmental Toxicology, she examined various combinations and doses of microcystins and anabaenopeptins on three distinct human cell lines from the lung, liver, and kidney.
"I aimed to explore, 'What does this imply for human health?'" she stated. "We discovered that anabaenopeptins are as toxic as some of the most harmful microcystin congeners, yet this compound is not included in our monitoring efforts."
Hart and her colleagues also observed that when microcystin and anabaenopeptin were combined, their effects were amplified. While findings from cell line studies do not directly translate to effects on organisms, the potential health risks to humans and animals merit further examination and highlight the necessity for additional research.
Rethinking what we monitor for
"In our initial study, we characterized the molecules present in this 'forbidden soup' and noted their co-occurrences,” Hart explained. “Then we evaluated the implications of these co-occurrences and established that not only do these compounds exist, but they also pose a concern."
"It is crucial to reassess the current monitoring practices, questioning what we are monitoring, the rationale behind it, and ensuring that risk management models for large lakes encompass the broader scope of potential threats."
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