It seems paradoxical at first: reducing phosphorus could ultimately pollute water bodies more, according to a study. The selective reduction of this fertilizer can therefore give particularly toxic blue-green algae a growth advantage, which enrich lakes with dangerous toxins. The researchers explain that in order to counteract the effect, the nitrogen input into water bodies must also be reduced at the same time. According to them, a rethinking of the management of inland waters is therefore required.
Bathing forbidden! If blue-green algae multiply too much in lakes, the authorities have to intervene because there is a danger to humans and animals. Because the organisms, also known as cyanobacteria, form liver-damaging and carcinogenic toxins that accumulate in lakes. Bathers can absorb these substances in critical quantities and the contaminated water also poses a great danger to dogs. For this reason, bathing bans are imposed in some German bodies of water in summer. North America is also affected by the summer algal blooms – there the consequences are even more drastic: In August 2014, half a million people in the US state of Ohio were not allowed to use tap water for drinking or washing for three days. Because the blue-green algae species Microcystis had contaminated nearby Lake Erie with the strong toxin microcystin.
How can blue-green algae blooms be curbed?
In addition to this direct danger to humans, blue-green algae blooms can also severely damage aquatic ecosystems. For this reason, measures have already been taken in many places to remove the basis for the problem. The focus was on the over-fertilization of the water bodies by inputs from agriculture. In concrete terms, it was previously assumed that high levels of the plant nutrient phosphorus in particular lead to the explosive proliferation of cyanobacteria. For this reason, considerable effort was sometimes made to keep this substance away from water bodies. “Less phosphorus in the water reduces the mass of blue-green algae and thus also the amount of poison – that was the simple formula for water management,” says Ferdi Hellweger from the Technical University of Berlin.
However, the processes in nature are often much more complex than it first appears. That is why Hellweger and his international colleagues have now looked more closely at the role of phosphorus and other factors in the development of blue-green algae. They used the blue-green algae species Microcystis as the model organism for their computer simulations and Lake Erie as the model body of water. The basis of the data was an extensive collection of information on the biology of the cyanobacteria and their development processes.
As the researchers emphasize, the toxin plays a special role for the photosynthetic microbes: “Microcystin is a strong poison for humans and animals, but it has a great advantage for cyanobacteria,” explains Hellweger. Because it can protect enzymes in them from aggressive hydrogen peroxide (H2O2), which, among other things, is a by-product of photosynthesis. However, the researchers report that there is a considerable range in the production of microcystin: There are cyanobacteria strains that form a large amount of this substance and others that use it little or not at all.
Nitrogen inputs must also be reduced
It is precisely this diversity among bacterial strains that is an important factor in connection with the availability of nutrients, the scientists explain. According to their model simulations, although a reduced supply of phosphorus actually leads to fewer cyanobacteria overall, the composition of the strains shifts in favor of the particularly problematic ones. “When there are fewer blue-green algae, there is less competition for the other nutrients, the most important of which is nitrogen, which is also limited. And nitrogen, in turn, is an important building block for the microcystin molecule,” explains Hellweger. This means: The bacterial strains that produce a lot of microcystin get comparatively favorable development conditions and can spread disproportionately fast.
According to the scientists’ simulation, this effect is significant: a phosphorus reduction leads to so many more toxin-producing cyanobacteria that the amount of toxin increases despite the overall lower total biomass. “This finding could mean a turning point for the management of water bodies,” says Hellweger. “Because if you want to reduce the toxins of blue-green algae, you not only have to reduce the input of phosphorus into the lakes, but also of nitrogen, which is also used in large quantities as fertilizer in agriculture”.
The scientists say that this could pose a considerable challenge to programs to keep water bodies healthy or clean them up. However, according to them, the connections should now be examined more closely, because so far the results relate to the blue-green algae species Microcystis and Lake Erie. But other species with different toxins can also cause problems in water bodies. “We hope that as a result of our publication, many other research groups will now study our method, reproduce it and apply it to other cases of blue-green algae growth,” says Hellweger.
Source: Technical University of Berlin, specialist article: Science, doi: 10.1126/science.abm6791