Projected phosphorus reductions will make Lake Erie more toxic

According to a new modeling study, reducing nutrient phosphorus levels to control harmful algal blooms in places like Lake Erie actually benefits toxic strains of cyanobacteria, which can lead to increased toxins in the body. water.

Researchers from the Technische Universität Berlin (TU Berlin) detail their findings in an article published online in the interdisciplinary journal Science. Two scientists from the University of Michigan (UM) are among the co-authors.

“The big step here was to integrate our understanding of bloom microbiology into predictive models,” said Gregory Dick, UM environmental microbiologist and co-author of the study. “The results suggest that biologically informed models are able to reproduce emergent properties of blooms that are not predicted by traditional models.”

Cyanobacteria, also known as blue-green algae, can produce toxins and deplete oxygen lakes when they die. Phosphorus is an important nutrient for these algae, and efforts are underway around the world to reduce phosphorus levels and inhibit the growth of cyanobacteria.

However, as the total number of cyanobacteria decreases, the remaining cyanobacteria have more of another important nutrient available: nitrogen. And higher concentrations of nitrogen help some cyanobacteria produce a toxin that protects them from damage resulting from oxidation.

Using an agent-based model, the researchers simulated the behavior of cyanobacteria in Lake Erie. They call for a change in thinking about water management and adopting an approach that not only reduces the loading of phosphorus but also nitrogen into water bodies.

Cyanobacteria can be dangerous to pets and humans. In August 2014, nearly half a million people in the Toledo region were without tap water for nearly three days due to contaminated drinking water. A type of blue-green algae, Microcystis, had produced particularly high levels of microcystin (MC), a liver toxin, in Lake Erie.

“Although microcystin is a potent toxin for humans and animals, it is very beneficial for cyanobacteria,” said Ferdi Hellweger, chair of water quality engineering at the Institute of Environmental Technology in the TU Berlin and main author of the scientific article.

Microcystin can occupy certain sites on enzymes important for the life processes of the bacteria. In doing so, it protects the bacteria from harsh hydrogen peroxide, which could otherwise attack these binding sites, oxidize the enzymes and render them useless.

“Hydrogen peroxide is found everywhere in nature, including as a byproduct of photosynthesis,” Hellweger said. As such, microcystin production is an important protective mechanism for bacteria. However, there are bacterial strains that produce a lot of microcystin, and others that produce very little or none at all.

“This diversity among bacterial strains is precisely what is responsible for the phenomenon that a reduction in phosphorus can lead to an increase in MC production,” Hellweger said.

Since phosphorus is a nutrient that is only available to bacteria in nature to a limited extent, efforts have so far focused on reducing the use of phosphates as fertilizers in agriculture and reducing the the phosphorus content of wastewater using tertiary wastewater treatment to slow the growth of blue-green algae, even in large bodies of water like Lake Erie.

The United States and Canada have pledged to reduce the amount of phosphorus entering Lake Erie by 40%.

“Less phosphorus in the water reduces the number of blue-green algae and therefore also the toxin level. This was generally the rule of thumb for water management,” Hellweger said. However, the actual natural processes are more complex than that, he said.

“Less blue-green algae means they also have to compete less for other nutrients, the most important of which is nitrogen. And nitrogen, like phosphorus, is also only available in limited amounts. And, as it happens, it’s an important part of the MC molecule,” Hellweger said.

In other words: strains of bacteria that produce considerable amounts of microcystin can now do so more easily because microcystin also protects them from harmful hydrogen peroxide.

“This study supports the idea that phosphorus reduction will successfully reduce the overall abundance of cyanobacteria, consistent with current policy goals,” said UM’s Dick, professor in the Department of Earth and Environmental Sciences and director from the Cooperative Institute for Great Lakes Research at the School of Environment and Sustainability.

“However, it also suggests that reducing phosphorus will lead to an increase in the abundance of the subset of cyanobacteria capable of producing toxins, which will lead to more toxins overall,” Dick said.

The other UM co-author is Derek Smith, who earned a Ph.D. from UM’s Department of Earth and Environmental Sciences and was a postdoctoral researcher in Dick’s lab. UM researchers took field measurements at the Toledo drinking water intake into Lake Erie, then used environmental genomics techniques to quantify the proportion of toxin-producing and non-toxin-producing Microcystis cells in Lake Erie blooms.

“Previous experiments have suggested that managing harmful cyanobacterial blooms requires reducing phosphorus and nitrogen pollution in lakes. Now, a model based on current knowledge of toxic cyanobacterial biology suggests the same. said Smith.

“Reducing phosphorus inputs alone may be insufficient to manage harmful cyanobacterial blooms,” he said.

In the new modeling study, the researchers for the first time used agent-based simulation to illustrate the behavior of this blue-green algae. Each blue-green algae is represented on the computer as an individual, behaving slightly differently depending on its supposed life history.

A blue-green algae that was frequently on the surface of the water, for example, will have been particularly exposed to the sun and therefore to oxygenated water. This increases the likelihood that she will fully utilize her microcystin-producing abilities.

Sunlight can also activate the gene needed to produce microcystin. This mechanism contributes to the fact that less biomass leads to more toxins because more light can penetrate to greater depths and stimulate production.

The researchers used the blue-green algae Microcystis and Lake Erie as the model organism and environment for their simulation. To precisely model the processes there, they conducted extensive literature research and assessed 103 studies with 708 experiments dating back to 1958.

Scientists from the University of Tennessee at Knoxville also conducted their own lab experiments to help build the model. The study was supported by the National Oceanic and Atmospheric Administration, the National Institute of Environmental Health Sciences and the National Science Foundation.

– This press release originally appeared on the University of Michigan website