Divergence and Phylogeny of Firmicutes from the Cuatro Cienegas Basin, Mexico: A Window to an Ancient Ocean

The Cuatro Cienegas Basin (CCB) has been identified as a center of endemism for many life-forms. Nearly half the bacterial species found in the spring systems have their closest relatives in the ocean. This raises the question of whether the high diversity observed today is the product of an adaptive radiation similar to that of the Galapagos Islands or whether the bacterial groups are survivors of an ancient sea, which would be of interest for astrobiology. To help answer this question, we focused on Firmicutes from Cuatro Cienegas (mainly Bacillus and Exiguobacterium). We reconstructed the phylogenetic relationships of Firmicutes with 28 housekeeping genes and dated the resulting tree using geological events as calibration points. Our results show that marine Bacillus diverged from other Bacillus strains 838 Ma, while Bacillus from Cuatro Cienegas have divergence dates that range from 770 to 202 Ma. The members of Exiguobacterium from the CCB conform to a much younger group that diverged from the Andes strain 60 Ma and from the one in Yellowstone 183 Ma. Therefore, the diversity of Firmicutes in Cuatro Cienegas is not the product of a recent radiation but the product of the isolation of lineages from an ancient ocean. Hence, Cuatro Cienegas is not a Galapagos Archipelago for bacteria but is more like an astrobiological time machine in which bacterial lineages survived in an oligotrophic environment that may be very similar to that of the Precambrian.