CHARACTERIZATION OF MICROBIAL COMMUNITIES IN DEEP HYPERSALINE AQUIFERS

In both aquatic and terrestrial systems, microbial communities are omnipresent and face various pressures. While halophilic organisms surviving extremely high salinity levels have been studied, what about bacterial and archaeal organisms having to deal not only with hyper-salinity, but also hyper-depths and their accompanying stress factors (pressure, absence of oxygen, etc.)? Given the variations in the environment generated by the increase in depth, a decrease in microbial diversity is expected in deeper salt rock formations, particularly in underground brines.

To verify these hypotheses, samples of brine and rocks of varying depths (from 660 to 1240 m depth) of six halite-rich previously drilled wells from Gaspé and Bécancour (both in Quebec) were obtained through partnership with the drilling company Junex Inc. These samples thus gave us a glimpse of microbial diversity of communities living in the rock matrices, and in the groundwater. After extracting DNA, the microbial diversity was analyzed via archaeal and bacterial 16S rRNA Illlumina sequencing.

Bacterial endolithic taxa, especially Xanthomonas and Pseudomonas, dominated rock samples. Only in brines could the presence of archaea be observed, although minimal. Surprisingly, close to no known halophilic microbes were found. Regression analysis shows the absence of correlation between depth and diversity (Shannon index), both in bacteria and in archaea. Also, clusters representing same geological layers can be found with close levels of diversity, put forward by a CCA multivariate analysis. However, Gaspé samples appear to possess low levels of diversity variation, and diverge from Bécancour wells, considering their remoteness in the CCA. Taken as a whole, these results demonstrate that endolithic microbial community diversity is strongly linked to the geological surroundings of the microbes. This demonstrates that microbial surveys could help identify unknown geological layers.