“Toxic algae” fertilized since the Bronze Age

Humans have apparently been influencing the growth of potentially problematic blue-green algae for a long time. © marvod/iStock

It's a modern phenomenon, one might think - but now a study shows that humans have been promoting the growth of the notorious blue-green algae in water for thousands of years. This is shown by DNA traces of the tiny creatures in the sediments of a lake in Mecklenburg. Accordingly, the blue-green algae led a shadowy existence there for a long time until people settled in the region in the Bronze Age. Even then, they apparently provided a fertilizing effect in the lake, from which the potentially problematic algae could particularly benefit. According to the researchers, the basis of today's "poisonous algae blooms" goes far back in history.

Bathing forbidden! If blue-green algae multiply too much in lakes, the authorities have to intervene. Because certain representatives of these organisms, also known as cyanobacteria, form toxins that can be dangerous to humans. Sometimes there are even critical accumulations in the drinking water. In addition, blue-green algae blooms can also severely damage aquatic ecosystems. Man-made factors are considered to be the cause of the mass proliferation: global warming and, above all, nutrient inputs are particularly beneficial for the blue-green algae. Because these photosynthetic microbes have a special ability: They absorb atmospheric nitrogen and can use it as a nutrient. As a result, their growth is more likely to be limited by the lack of other nutritional elements – primarily phosphorus. With corresponding inputs into the water, blue-green algae can therefore multiply particularly rapidly at the expense of other algae.

Genetic insights into the history of blue-green algae

So far, however, there has been a knowledge gap in blue-green algae research: Little was known about what the original colonization of our lakes with these organisms looked like and how they have developed in the past to the present day. This was because cyanobacteria - unlike diatoms - leave no visible fossil traces in sediments. However, the researchers around Ebuka Nwosu from the German Research Center for Geosciences in Potsdam (GFZ) have now gained insights through genetic methods. "With modern techniques, DNA from different organisms can be detected in sediments, and this allows us to decipher the history of cyanobacteria," says Nwosu.

For their study, the researchers chose the "deep lake" in the Nossentiner-Schwinzer Heide nature park in Mecklenburg, because there was already a lot of information about this body of water. The test material was provided by an eleven meter long sediment core from the bottom of the lake. Its layers can be precisely dated - they go back from today to 11,000 years ago. In them, the researchers searched for genetic traces of the cyanobacteria, which could provide conclusions about their former population densities and biodiversity.

Developmental leap at the appearance of man

This initially showed that blue-green algae were already present in the lake 11,000 years ago – however, the number and diversity were very low. The researchers explain that this group of organisms originally did not play an important role in the ecosystem of the body of water. But about 4000 years ago there was a significant leap, which became apparent in the analysis results: the number and species diversity of blue-green algae increased significantly and, according to the genetic traces, potentially poisonous species also spread. Subsequently, the natural state before these first changes was never reached again.

But what caused the jump? It coincided exactly with the dating of the earliest known graves near the lake, the researchers report. The blue-green algae then flourished again with each further known colonization surge in the area of ​​the lake. "It is therefore obvious that the early cultures already had an influence on the nutrient balance of the lake," says Nwosu. In concrete terms, relevant nutrient inputs could have entered the lake through the ash from clearing by fire and the excrement of people and their livestock. The scientists explain that these then obviously benefited the blue-green algae in particular.

The study results also document how the increased nutrient inputs in the course of the industrialization of agriculture have significantly intensified the growth of blue-green algae over the past 150 years. "However, as our study shows, the prerequisites for this development were created much earlier than we previously assumed," emphasizes senior author Susanne Liebner from the GFZ. This could also have broader implications: "An awareness of the legacy of human influence, possibly stretching back several millennia, could be helpful for wise management of contemporary aquatic systems," the researchers write in conclusion.

Source: Helmholtz Center Potsdam – GFZ German Research Center for Geosciences, specialist article: Commun Biol, doi: 10.1038/s42003-023-04430-z

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