Blue-green algae (cyanobacteria) can produce toxins and deplete lakes of oxygen when they die. Because they feed on phosphorus, billions of dollars have been spent around the world to reduce phosphorus loads and prevent the growth of these cyanobacteria. However, as the total number of bacteria decreases, the remaining bacteria have more of another important nutrient available: nitrogen. And higher concentrations of nitrogen help certain cyanobacteria to produce a toxin which protects them against damage resulting from oxidation. This means that reducing phosphorus is actually advantageous to the toxic cyanobacteria strains, which in turn can lead to an increase in the toxins in a lake.
Researchers from Technische Universität Berlin (TU Berlin) have now described this correlation for the first time in a paper for the interdisciplinary journal Science. Using an agent-based model, they simulate how cyanobacteria behave in Lake Erie, which borders the USA and Canada. They call for a shift in thinking about water management and adopting an approach that not only reduces phosphorus but also nitrogen loading in bodies of water.
Summer is just around the corner and with it blue-green algal blooming in the lakes. Cyanobacteria, which multiply en masse, can be dangerous not only for free-roaming dogs, but also for humans, especially children. In fact, this risk led the Berlin State Office for Health and Social Affairs to impose a swimming ban at Lake Tegel this past year . In August 2019, three dogs died in Bavaria after bathing in contaminated water. Several dogs also died following similar events at Lake Tegel in May 2017 . And in August 2014, the entire metropolitan city of Toledo, Ohio in the USA was affected: Half a million people were unable to drink, wash their hands, or shower using tap water for three days . The reason? Contaminated water from nearby Lake Erie. A type of blue-green algae, Microcystis, had produced particularly high levels of the liver toxin microcystin (MC) there. Even prior to in-depth scientific research, the bacteria was well-known for its “fast death factor” toxicity.
“While microcystin is a strong toxin for humans and animals, it’s highly beneficial to cyanobacteria,” says Professor Dr. Ferdi Hellweger, head of the Chair of Water Quality Engineering at TU Berlin’s Institute of Environmental Technology. The toxic microcystin can occupy certain sites on the enzymes important for life processes in the bacteria. In doing so, it shields the bacteria from aggressive hydrogen peroxide (H2O2), which could otherwise attack these binding sites, oxidize the enzymes and render them useless. "Hydrogen peroxide is found everywhere in nature, including as a by-product of photosynthesis," explains Hellweger. As such, producing MC is an important protective mechanism for the bacteria. Yet, there are some bacteria strains which produce a lot of MC and others which 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,” says Hellweger. Because phosphorus is a nutrient that is only available to a limited extent in nature for the bacteria, efforts until now have previously focused on cutting down on the use of phosphates as fertilizers in agriculture and reducing the phosphorus content of wastewater by means of tertiary treatment of wastewater to slow down the growth of blue-green algae even in larger bodies of water such as Lake Erie. “Less phosphorus in water reduces the number of blue-green algae and thus also the toxin level. That was generally the rule of thumb for water management,” continues Hellweger. However, the actual natural processes are more complex than this. “Less blue-green algae means they also have to compete less for other nutrients, the most of important of which is nitrogen. And nitrogen, similar to phosphorus, is also only available in limited amounts. And, as it happens, it is an important element of the MC molecule.” In other words: Those strains of bacteria which produce considerable amounts of MC can now do so more easily as well as more easily multiply than before, because the MC also protects them from the harmful hydrogen peroxide.
As a result, although phosphorus reduction leads to less blue-green algae overall, it leads to more toxin-producing blue-green algae in relation – so much more that the toxin level in the lake can also increase in absolute terms. “This discovery represents a real turning point for the management of water bodies. If we want to reduce the toxins produced by blue-green algae, we not only have to reduce the amount of phosphorus but also nitrogen in lakes, which is likewise used in large quantities as fertilizer in agriculture,” explains Ferdi Hellweger. Since this would involve significant additional expense, virtually all lake health or restoration programs are now under scrutiny – including that for Lake Erie, where the U.S. and Canada have pledged to reduce phosphorus by 40 percent, which at $40 million per year has a significant cost to U.S. agriculture alone , .
In addition to their findings, the method used by the researchers is revolutionary. For the first time ever, they used agent-based simulation to illustrate this blue-green algae’s behavior. Each blue-green alga is represented on the computer as an individual, behaving slightly differently depending on its assumed "life history" and membership of a particular bacterial phylum. "A blue-green alga that was frequently at the water surface, for example, will have been particularly exposed to light and thus hydrogen peroxide. This increases the likelihood that it will make full use of its MC production capacities," says Hellweger.
Every cyanobacterium in the simulation is assigned a simplified but nevertheless complex control loop, which includes a number of elements such as the mechanism of action of the most important gene for the production of MC and the process of oxidation by hydrogen peroxide. “These processes are made more complex by a number of factors such as the bacteria producing enzymes which can break down hydrogen peroxide, thus protecting them from oxidization too,” says Hellweger. In addition, light plays an important role as it can activate the gene needed to produce MC. This mechanism also contributes to the fact that less biomass leads to more toxins because more light can penetrate to deeper depths and stimulate production.
The researchers used the blue-green alga Microsystis and Lake Erie as the model organism and environment for their simulation. To precisely model the processes there, they conducted extensive literature research and evaluated 103 studies with 708 experiments dating back to 1958. Scientists at the University of Tennessee in Knoxville, USA also conducted their own laboratory experiments to assist with building the model. Researchers at the University of Michigan in Ann Arbor, USA took field measurements at Toledo’s drinking water tapping point at Lake Erie.
Other cyanobacteria produce other toxins; the blue-green algae at Lake Tegel, for example, produce the neurotoxin “anatoxin-a.” It is still unclear, though, how this impacts the bacteria. However, the scientists believe that their method of agent-based simulation based on known biological mechanisms may also be useful for the management of other blue-green algae systems. “We hope our publication will lead many other research groups to consider our method, reproduce it, and apply it to other cases of blue-green algal blooms,” explains Ferdi Hellweger.
The project received generous support from the United States’ National Oceanic and Atmospheric Administration (NOAA).
 Bathing ban at Lake Tegel
 Blue-green algae poisoning of dogs in Bavaria and Blue-green algae poisoning of dogs in Berlin
 Drinking water crisis in Toledo (USA) 2014
 US action plan against blue-green algae growth in Lake Erie
 Cost estimate for the reduction of phosphorus in Lake Erie (US agriculture only)
Study by Prof. Hellweger et al. in Science