The Cuatro Cié negas Basin (CCB) in the central part of the Chihuahan desert (Coahuila, Mexico) hosts a wide diversity of microorganisms contained within springs thought to be geomorphological relics of an ancient sea. A major question remaining to be answered is whether bacteria from CCB are ancient marine bacteria that adapted to an oligotrophic system poor in NaCl, rich in sulfates, and with extremely low phosphorus levels (<0.3 M). Here, we report the complete genome sequence of Bacillus coahuilensis, a sporulating bacterium isolated from the water column of a desiccation lagoon in CCB. At 3.35 Megabases this is the smallest genome sequenced to date of a Bacillus species and provides insights into the origin, evolution, and adaptation of B. coahuilensis to the CCB environment. We propose that the size and complexity of the B. coahuilensis genome reflects the adaptation of an ancient marine bacterium to a novel environment, providing support to a ''marine isolation origin hypothesis'' that is consistent with the geology of CCB. This genomic adaptation includes the acquisition through horizontal gene transfer of genes involved in phosphorous utilization efficiency and adaptation to high-light environments. The B. coahuilensis genome sequence also revealed important ecological features of the bacterial community in CCB and offers opportunities for a unique glimpse of a microbe-dominated world last seen in the Precambrian.evolution ͉ genomic adaptation ͉ horizontal gene transfer ͉ phosphorus stress ͉ sulfolipids T he Cuatro Ciénegas Basin (CCB) is located in a valley Ϸ740 m above sea level in the state of Coahuila, Mexico, that measures Ϸ30 km by 40 km and is surrounded by high mountains (Ͼ3,000 m) (Fig. 1). CCB is an enclosed evaporitic basin that receives Ϸ150 mm of annual precipitation. Despite the dry climate of the valley, the CCB harbors an extensive system of springs, streams, and pools (1). The CCB ecosystem is not only characterized by a high endemism of macrooganisms and biodiversity of microorganisms (1, 2), but also by extremely oligotrophic waters that are unable to sustain algal growth, making microbial mats the base of the food web (3). In particular, phosphorus (P) levels in CCB appear to be rather low, because they were below the level of detection of several methods used (0.3 M) and the extremely high biomass C:P and N:P ratios (Ͼ100 by moles) previously reported for CCB stromatolites (3, 4). Unlike the present sea, the Churince spring water is poor in NaCl and carbonates, but it is rich in sulfates, magnesium, and calcium (4). Characterization of the microbiological diversity by sequencing 16S rRNA genes revealed that nearly half of the phylotypes from the CCB were closely related to bacteria from marine environments (2). Bacillus coahuilensis is a free-living, spore-forming bacteria isolated from the water column of a shallow desiccation lagoon in the Churince system at CCB (4) (Fig. 1 A and B). A molecular phylogenetic analysis of 16S rRNA sequences indicates that B. coahuilensis is closely ...

At the desert oasis of Cuatro Ciénegas in Coahuila, México, more than 300 oligotrophic pools can be found and a large number of endemic species of plants and animals. The most divergent taxa of diatoms, snail and fishes are located in the Churince hydrological system, where we analyzed the local diversification of cultivable Firmicutes and Actinobacteria. The Churince hydrological system is surrounded by gypsum dunes and has a strong gradient for salinity, temperature, pH and dissolved oxygen. In August 2003, surface water samples were taken in 10 sites along the Churince system together with the respective environmental measurements. 417 thermo-resistant bacteria were isolated and DNA was extracted to obtain their BOX-PCR fingerprints, revealing 55 different patterns. In order to identify similarities and differences in the diversity of the various sampling sites, an Ordination Analysis was applied using Principal Component Analysis. This analysis showed that conductivity is the environmental factor that explains the distribution of most of the microbial diversity. Phylogenetic reconstruction from their 16S rRNA sequences was performed for a sample of 150 isolates. Only 17 sequences had a 100% match in the Gene Bank (NCBI), representing 10 well known cosmopolitan taxa. The rest of the sequences cluster in 22 clades for Firmicutes and another 22 clades for Actinobacteria, supporting the idea of high diversity and differentiation for this site.