Camille Thomas , Alexander Francke, Hendrik Vogel, Bernd Wagner, Daniel Ariztegui 

Understanding the response of geo- and biosystems to past climatic disturbance is primordial to assess the short to long terms effects of current global change. Lacustrine sediments are commonly used to investigate the impact of climatic change on biogeochemical cycling. In these sediments, subsurface microbial communities play a primordial role in nutrient, organic matter and elemental cycling, but they also can affect the sedimentary record and overprint the original paleoenvironmental signal. Subsurface microbial communities have therefore been investigated to assess the potential connection between microbial diversity and environmental change. Lake Ohrid (North Macedonia, Albania) is the oldest lake in Europe and has been the target of a scientific deep drilling in 2013. The upper 447 m of the 584-m-long sedimentary drill core record obtained from the central part of the lake (DEEP site) is composed of clayey to silty-clayey lithologies differing substantially in terms of carbonate and organic matter content between glacials and interglacials. We investigated the microbial diversity in the retrieved sediment using 16S rRNA gene sequences along the upper ca. 200 m of the DEEP site record spanning ca. 515 ka to assess whether subsurface microbial communities were following a similar trend.
Results show that Atribacteria, Betaproteobacteria, Bathyarchaeota and to a lower extent Dehalococcoidia phyla structured the community but their occurrence appears to be independent from each other. Atribacteria and Bathyarchaeota together with Dehalococcoidia are commonly encountered in deep lacustrine and marine sediments. Their metabolic versatility is adapted to low energy environments where they can realize the fermentation of various substrates (sugars, propionate and amino acids). The generation of H2 from Atribacteria and other fermenters can be used by Dehalococcoidia and Bathyarchaeota for acetogenesis, and even for chemolithoautrophic processes suggested at greater depths. Betaproteobacteria-associated sequences were often co-occurring with cyanobacterial sequences that suggest preservation of ancient DNA from the water column or catchment, down to at least 340 ka. In particular, fossil DNA from Cyanobacteria in dry glacial intervals may be the relict of ancient blooms of N-fixing clades in periods of nitrogen depletion.
We compared the richness and diversity of all phylotypes with environmental parameters measured in corresponding intervals to test for the relationship between paleoenvironmental conditions, climatic modes and the subsurface biosphere. We found no significant relationship between any phylotype and measured environmental parameters, nor with sediment age or climate patterns. Our preliminary results support a weak recording of early diagenetic processes and their actors by bulk prokaryotic sedimentary DNA in Lake Ohrid, which might suggest dominant turnover and replacement by specialized low-energy clades of the deep biosphere.