While the cells of cyanobacteria do not resist fossilization very well, the theory which led to the development of a third method was that part of their DNA remained present in the layers of sediment. By being able to extract and amplify these sequences, the researchers were able to observe the presence and evolution of the communities of cyanobacteria over the last few centuries. This method was fruitful because the researchers were able to find the DNA sequences of these bacteria stretching back over the last three thousand years. “By finding cyanobacterial DNA in the successive layers of sediment, we were certain that they had populated the lake at a given time. This DNA could not have appeared out of nowhere”.

A study made possible thanks to PCR

Observation by microscope and the extraction of pigments are two studies described as “traditional”, but which do not in themselves make it possible to study every aspect. The idea of the two biologists was therefore to search for DNA residues, to amplify their DNA coding sequences for a taxonomic marker (16S ribosomal RNA  or 16S rRNA) and to isolate the latter by DGGE. By comparison with reference sequences in databases, it was possible to determine which taxonomic group these sequences belonged to. According to their reciprocal similarities, these sequences were then classified into Operational Taxonomic Units (OTUs). Overall, a certain diversity of DNA sequences was discovered.