Humans have caused toxic algae blooms since the Bronze Age

Recently, there has been an increasing number of reports of toxic algae – also known as blue-green algae or cyanobacteria – in many lakes around the world, mainly due to global warming and rises in nutrient inputs. By examining DNA from sediments of a lake in Germany, a team of scientists from the GFZ German Research Center for Geosciences has discovered that humans have not only had an influence on the development of blue-green algae since modern times, but already since the Bronze Age (approximately 2,000 years BCE). 

Blue-green algae are among the oldest organisms capable of photosynthesis and are considered some of the main ancestors of today’s land plants, which have inherited from cyanobacteria the ability to produce oxygen and sequester carbon dioxide. These algae have proliferated in many bodies of water in recent decades, mainly due to anthropogenic factors such as increased nutrient output and climate change. Since some of them are toxic, mass blooms in bathing waters can be dangerous, causing allergies if they come into contact with our skin, infections in case of small wounds, or even liver cancer if they get into drinking water. Moreover, since – unlike other types of algae – they can absorb nitrogen from the atmosphere and use it as a nutrient, they can frequently outcompete and displace other aquatic organisms.

To better understand the evolution of these algae over the past millennia, the scientists collected sediments from the Lake Tiefer See in Mecklenburg-Western Pomerania in Germany and inferred the number, composition, and diversity of blue-green algae species by analyzing DNA from various organisms found in the sediments.

The analysis revealed that cyanobacteria were already present in the oldest samples examined – dating 11,000 years ago, shortly after the lake was formed – although the number and diversity of species were very low and, thus, they probably did not play a major role in the lake’s early ecosystems. However, with the appearance of the first Bronze Age burial finds near the lake around 2,000 BCE, the number and species communities of these algae increased significantly. “This suggests that even early cultures had an impact on the nutrient balance of the lake through agricultural activities,” said study lead author Ebuka Nwosu, an expert in Geomicrobiology at GFZ.

Afterwards, with each subsequent settlement phase – and mainly during warmer climatic conditions – blue-green algae continued to proliferate in Lake Tiefer See. Moreover, since industrial agriculture started with greatly increased nutrient inputs, this development accelerated even more and will most likely be further favored by the increasingly warmer future climatic conditions.

“Our study provides evidence that humans began to locally impact lake ecology much earlier than previously assumed. Consequently, managing aquatic systems today requires awareness of the legacy of human influence dating back potentially several millennia,” the authors concluded.

The study is published in the journal Nature Communications Biology.

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By Andrei Ionescu, Earth.com Staff Writer

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