Fluctuations in cyanobacterial populations could help us to better understand previous and current climate variations. However, these organisms do not easily fossilize and generally do not resist the passing of time very well. Nonetheless, the extraction and sequencing of their DNA present in layers of sediment could satisfy the curiosity of paleolimnologists and biologists. This new method, developed by researchers in Liege, was applied to sediment cores taken from the bottom of two lakes in the Antarctic Peninsula. The method has made it possible to attest to the presence of bacteria in the two lakes for more than 3,000 years and to demonstrate variations in their diversity at certain periods. It remains to be established whether the cause of the variations in diversity of these DNA sequences is of climatic origin or if it is due to poor or good conservation processes according to the species in question.

Given what they leave behind after their passage on Earth, cyanobacteria seem to be very discreet. Yet the role they play in the development of life is anything but banal. They produce their energy by photosynthesis and have populated the Earth for more than 2.45 billion years. It is partly thanks to them that our atmosphere contains the oxygen we are able to breathe. (See article Cyanobacteria found near the South Pole)

But what exactly is so discreet about cyanobacteria? Contrary to other microscopic organisms like diatoms, they do not fossilize well in unconsolidated sediments and therefore are generally not preserved for a long time. “With the help of microscopy, we find a sheath or remains of a specimen belonging to a more resistant community, but mostly, we don’t find anything, even in places which satisfy the optimal conditions for harboring large communities of bacteria”, explains Annick Wilmotte, a research associate at the FNRS in the Centre for Protein Engineering at the Department of Life Sciences of the University of Liege

However, as they are dependent on nutrients, sunshine and water in order to develop, cyanobacteria also respond to climate variations. There is therefore no reason why they should not have something to tell us. During research published in the Journal of Paleolimnology(1), Annick Wilmotte and Rafael Fernández-Carazo, who was then working at the Department of Life Sciences of the University of Liege, initiated a promising methodology to do justice to this microbial community secretive under the light microscope.

Towards a new paleolimnological marker

The work of the two researchers was part of a wider analysis of sediment cores taken from the bottom of two lakes in the Antarctic Peninsula, Beak 1 and Beak 2, both of which are on the same island. With depths of 26 meters for Beak 1 and 4 meters for Beak 2, the lakes are frozen for 8 months of the year, reaching temperatures of between 4 and 12 degrees Celsius during the austral summer. However, photosynthetic communities of bacteria thrive there, as there are no other living organisms to affect their development. Cyanobacteria develop better in Lake Beak 2 which is shallower and sunnier.

(1) Rafael Fernández-Carazo, Elie Verleyen, Dominique A. Hodgson, Stephen J. Roberts, Krzysztof Waleron, Wim Vyverman, Annick Wilmotte, Late Holocene changes in Cyanobacterial community structure in maritime Antarctic lakes, Journal of Paleolimnology (Springer), published online: 12 march 2013