DNA reveals unique microorganisms evolved at the poles

Microorganism communities at the bottom of polar lakes have evolved independently of other regions, influenced by the particular geological, biological, and climatic history of their regions. The uniqueness of microbial communities highlights the urgent need to include these life forms in management plans for polar regions.

An international research team of scientists led by biologists from Ghent University, Belgium, and Professor Dominic Hodgson from the British Antarctic Survey, sampled and sequenced the biodiversity and evolutionary history of microorganisms in more than 200 Arctic, sub-Antarctic and Antarctic lakes.

The new study shows striking differences in the composition of microbial communities between polar regions, with lakes at the North and South Poles dominated by different groups. Their work constitutes the first large-scale DNA study of these unique microbiomes and was published in the journal Scientists progress.

Polar regions are characterized by extreme living conditions, such as very low temperatures and low availability of water and nutrients. Lakes in these harsh environments are oases of biodiversity and productivity where microscopic organisms dominate life.

It is assumed that microorganisms, due to their astronomical numbers and small cell sizes, moved unhindered across the Earth and colonized all suitable habitats. This hypothesis has now been tested using DNA to compare community composition in more than 200 Arctic, sub-Antarctic and Antarctic lakes to determine whether they have the same evolutionary history.

Professor Dominic Hodgson leads the British Antarctic Survey’s ice sheets and climate change team and is a co-author of the study. He says: “This research is the culmination of many years of sampling hard-to-reach microscopic communities and applying the latest advances in DNA. »

“It is a game-changer to find that microbial life at the poles has been shaped by climatic and biological factors specific to individual regions and is not, as previously thought, part of the same global evolutionary trajectory.”

Antarctica has been gradually covered by an ice sheet since the end of the Eocene (34 million years ago), leading to the extinction of most groups of plants and animals. It is also very isolated in the southern hemisphere, which has prevented these groups from recolonizing the continent. The Arctic landmasses, on the other hand, are geographically very connected and were only covered by ice sheets during glacial maxima since 3.2 million years ago.

In animals and plants, this has led to strong differences in the evolutionary history of species and in their geographic distribution at the two poles. However, microorganisms were thought to move freely on Earth.

The new study reveals different groups of microorganisms in lakes at the North and South Poles. Heterotrophic bacteria (microorganisms that feed on organic carbon) and grazing-resistant algae appear to be relatively more abundant in the Arctic, while cyanobacteria (microorganisms that obtain energy from photosynthesis) and green algae are more dominant in Antarctic lakes. Species diversity is also lower in Antarctica.

Analyzes showed that the formation of ice sheets during historic glacial periods led to the disappearance of several groups of microbial organisms from Antarctica. The team was able to demonstrate that the groups that survived could evolve in isolation. Thus, a common ancestor species gave rise to several subspecies and varieties whose distribution is limited to Antarctica.

Even at deep evolutionary levels, it appears that many genetic lineages are ancient and unique to Antarctica. This shows that movement between the poles over long periods is rather limited and that the Southern Ocean also constitutes a major barrier for microorganisms.

The unique character of the microbial communities of these polar lakes constitutes a strong argument in favor of better protection of these habitats against human influence, in particular by increasing the tourist exploitation of Antarctica.