The sediment cores studied retrace the sedimentary history of the two lakes over the last 10,000 years which is the geological period known as the Holocene. How is this possible? When the organisms in a lake die, they sink to the bottom and form a layer of sediment which will itself be covered with a new layer and so on and so forth for millennia. Each of these layers presents particularities of composition according to what lived in the lake at a given period, a life which is directly linked to its ecosystem and therefore to the climate. These residues, which are the footprints of our past, are paleolimnological markers (paleo, ancient/prehistoric; limnology, the scientific study of lakes or other bodies of freshwater). After taking a sample of core sediment, the sample is segmented into layers which are then dated by means of the carbon-14 isotope. Therefore, it becomes possible to date the evolution of the composition of the lake. 

The sediment cores collected within the framework of this study were analysed by researchers from the British Antarctic Survey in Cambridge, the University of Ghent and the University of Liege. Researchers from the University of Liege focused on attempting to detect proof of earlier presence of cyanobacteria. How did they do this? By looking for cyanobacterial DNA sequences. “These studies are quite recent. In the present case, a feasibility study was required to determine if the DNA of fossil cyanobacteria was interesting to add to the list of tools traditionally used to study our past. And the results were encouraging, even though we now need to measure those factors that could bias our results”, explains the researcher.

Sequencing and other methodologies

In order to observe the evolution of cyanobacteria over the last few thousand years, the researchers used three methodologies. First, observation by light microscopy. “Except for the more recent communities, this technique was not very efficient because cyanobacteria do not fossilize easily in the long term except under certain conditions but we did this in the interest of being thorough”. At the same time, this study had another objective (see “The importance of a multi-methodology” below).

A second method involved the study of fossil pigments by High Performance Liquid Chromatography (HPLC)(performed at the University of Ghent). “The sediment cores contain photosynthetic pigments which differ according to the communities (mosses, diatoms, cyanobacteria…). The HPLC method makes it possible to extract and separate these pigments. The pigments then only need to be compared with reference pigments (carotenoids, chlorophyll…) to connect them with the communities that synthesized them”.