An examination of DNA on sediments from a lake in Germany has shown humans have influenced the growth of blue-green algal blooms in lakes for millennia

In recent years, there have been increasing reports of toxic blue-green algae blooms in summer, caused by climate warming and increased nutrient inputs. But humans have not only had an influence on the development of blue-green algae in modern times, but since the Bronze Age around 2000 BC.

This is the result of a study by researchers from the German Research Centre for Geosciences GFZ and colleagues.

Since some blue-green algae, also known as cyanobacteria, leave no visible fossil traces in sediments due to their small size, little is known about how they evolved in our lakes over centuries and millennia. Using DNA from sediments, the researchers have now been able to decipher for the first time the history of blue-green algae over the last 11,000 years in the sediments of a lake in Mecklenburg, Germany.

See also: Genome-based taxonomy to better manage algal blooms

mass blooms

Blue-green algae have proliferated in many bodies of water in recent decades and the causes are man-made: increasing nutrient inputs; and global warming.

Some blue-green algae species are toxic, so that mass blooms in bathing waters can even be hazardous to health. They can cause allergies if they come into contact with the skin or – in the case of small wounds – infections and – if they get into drinking water, can even cause liver cancer in extreme cases.

Cyanobacteria, as blue-green algae are known, are among the oldest known organisms that carry out photosynthesis. Today’s land plants have ultimately inherited their ability to produce oxygen and fix carbon dioxide from cyanobacteria.

Cyanobacteria also have another ability that distinguishes them from other algae; they can absorb nitrogen from the atmosphere and use it as a nutrient. This ability is one reason for the proliferation of blue-green algae at the expense of other aquatic organisms, which are increasingly being displaced.

But unlike diatoms, for example, most blue-green algae do not leave fossil traces in the sediment to allow extensive taxonomic differentiation as distinct species. As a result, it is poorly known how blue-green algae in our lakes have evolved over past millennia.

Lead author of the study, Ebuka Nwosu of the GFZ German Research Centre for Geosciences, said: “However, new methods now make it possible to detect DNA from various organisms in sediments, and this allows us to decipher the history of these cyanobacteria.”

The work, which involved other partners, was carried out as part of Nwosu’s doctoral thesis funded by the German Federal Foundation for the Environment (DBU). It was supervised by Susanne Liebner, GFZ working group leader in the Geomicrobiology Section and professor at the University of Potsdam, and Achim Brauer, head of the GFZ Climate Dynamics and Landscape Development Section and professor at the University of Potsdam.

bronze-age burial

For this study, the researchers selected sediments from a lake in the Nossentiner-Schwinzer Heide Nature Park in Mecklenburg-Western Pomerania, Germany.

Brauer explained the choice of lake for the study: “We know Lake Tiefer See very well and have dated its sediments very precisely because it has been part of the TERENO lake monitoring programme at the German Research Center for Geosciences for many years.”

For the study, sedimentary cyanobacterial DNA was determined on an 11m sediment core, and then both the number and composition of blue-green algae species and their diversity were analysed.

In this way, the researchers were able to show that blue-green algae were already present in the oldest samples studied, 11,000 years ago, shortly after the lake was formed. However, the number and diversity of species were very low and blue-green algae probably did not play a special role in the lake’s ecosystem.

This did not change for many millennia. Only with the appearance of the first Bronze Age burial finds near the lake from around 2000 BC did the number and species communities of blue-green algae increase significantly.

Nwosu said: “This suggests that even early cultures had an impact on the nutrient balance of the lake through agricultural activities.”

Subsequently, the natural state prior to these initial changes was never regained. On the contrary, with each subsequent settlement phase, and preferentially during warmer climatic conditions, blue-green algae and their species communities continued to proliferate in Lake Tiefer See.

Since industrial agriculture started with greatly increased nitrogen inputs, this development has accelerated even more.

Liebner explained: “However, the conditions for these developments were created much earlier than we previously assumed.”

She added that the nutrient-induced increase in blue-green algae will probably be further favoured by the increasingly warmer summers in the future.

The study is published in Communications Biology.

Image 1: TERENO Monitoring Station on Lake Tiefer See, Germany (weather station, water probes, sediment traps). © A Brauer.

Image 2: Lake Tiefer See in the Nossentiner/Schwinzer Heide Nature Park; Mecklenburg-Western Pomerania (Germany); view from the north. © A Brauer/ GFZ.