Even by focussing only on cyanobacteria, a multi-methodology can prove to be revealing. “For example, the DNA of one of the species is present in each layer of sediment (OTU 16ST58, compare table below, Editor’s note). In reality, these are small Synechococcus, unicellular cyanobacteria with a diameter of about one micron. A possible theory is that their DNA is better protected during fossilization. This could be the reason why we systematically find them”.

This hypothesis can be illustrated by another observation. Currently, the bottom of Lake Beak 2 is covered with a carpet of cyanobacteria which we do not find in the sediment studied. Therefore, they have been observed during sample-taking, but once they are dead, there is no longer any trace of them (except OTU 16ST44 at 5 and 10 cm). There is not even any DNA. We need to understand the reason for this. In the meantime, these anomalies constitute a limitation in the methodology. But the methodology presents another advantage which is a greater specification of the diversity of the bacterial communities. “For the cyanobacteria that are still alive, we are lucky to be able to combine DNA analysis and microscopy. What we noticed on analyzing 16S rRNA, is that cyanobacteria with the same morphology hide a large genetic diversity. It is important to note these differences because there seems to be several processes which occur at the same time. The fossilizations may differ according to the taxonomic group and these groups are differentiated from each other by characteristics such as the presence or non-presence of spores which are better at resisting external aggressions, or thicker and more protective cell walls”, concludes the researcher.

A promising development

All things considered, this feasibility study is encouraging. The past presence of cyanobacteria in the two lakes could be proved by analysis of DNA fossils. It remains to be seen to what extent the cause of the variation in these DNA samples is simply of climatic origin, or if it is due to better or worse conservation faculties according to the species.  An observation by using several tools will make it possible to determine these points. 

Whatever the case may be, DNA fossil analysis is still only in its infancy and may well become a first-rate paleolimnological marker over the coming years. This will make it possible to study new aspects of past variations of our planet, notably by the observation of non-fossilized species. In addition to this, DNA sequencing techniques are making constant and ground-breaking progress and they make it possible to observe the residue of past and present genetic codes more reliably and more easily. These are technological advances which allow for a better understanding of climate variations on our planet.