The Cuatro Ciénegas Basin (CCB) is located in the Chihuahuan desert in the Mexican state of Coahuila; it has been characterized as a site with high biological diversity despite its extreme oligotrophic conditions. It has the greatest number of endemic species in North America, containing abundant living microbialites (including stromatolites and microbial mats) and diverse microbial communities. With the hypothesis that this high biodiversity and the geographic structure should be reflected in the virome, the viral communities in 11 different locations of three drainage systems, Churince, La Becerra, and Pozas Rojas, and in the intestinal contents of 3 different fish species, were analyzed for both eukaryotic and prokaryotic RNA and DNA viruses using next-generation sequencing methods. Double-stranded DNA (dsDNA) virus families were the most abundant (72.5% of reads), followed by single-stranded DNA (ssDNA) viruses (2.9%) and ssRNA and dsRNA virus families (0.5%). Thirteen families had dsDNA genomes, five had ssDNA, three had dsRNA, and 16 had ssRNA. A highly diverse viral community was found, with an ample range of hosts and a strong geographical structure, with very even distributions and signals of endemicity in the phylogenetic trees from several different virus families. The majority of viruses found were bacteriophages but eukaryotic viruses were also frequent, and the large diversity of viruses related to algae were a surprise, since algae are not evident in the previously analyzed aquatic systems of this ecosystem. Animal viruses were also frequently found, showing the large diversity of aquatic animals in this oasis, where plants, protozoa, and archaea are rare. In this study, we tested whether the high biodiversity and geographic structure of CCB is reflected in its virome. CCB is an extraordinarily biodiverse oasis in the Chihuahuan desert, where a previous virome study suggested that viruses had followed the marine ancestry of the marine bacteria and, as a result of their long isolation, became endemic to the site. In this study, which includes a larger sequencing coverage and water samples from other sites within the valley, we confirmed the high virus biodiversity and uniqueness as well as the strong biogeographical diversification of the CCB. In addition, we also analyzed fish intestinal contents, finding that each fish species eats different prey and, as a result, presents different viral compositions even if they coexist in the same pond. These facts highlight the high and novel virus diversity of CCB and its "lost world" status.

Mexico City has a population of 9 million inhabitants and was settled on a lakebed with high seismic potential. It is currently embedded in the Mexico City Metropolitan Area, which encompasses 22 million inhabitants and which was self-sufficient in water in the past, but currently extracts 70% from the regional aquifers and imports about 30% of the water required by this megacity. Groundwater represents its main water source but its water quality is increasingly threatened. The purpose of this study was to determine the water quality in areas related to seismic fractures, which may increase the vulnerability of water provision, and to identify specific zones that could be affected. Official water quality data from the period 2002 to 2017 was analyzed and compared to recent data taken in wells in the city after the September 2017 earthquake. Physicochemical parameters were determined and compared to the existing data. Statistical and temporal analyses were performed in order to understand the evolution and spatial distribution of water quality. The results show that free chlorine was below the limits according to the Mexican regulatory framework, while the presence of fecal coliforms, aluminum, ammonia, iron, and manganese exceeded the standards. The presence of arsenic, boron, and chrome was detected in some areas. Clusters show specific parameters that increase with time: turbidity, sulfates, nitrates, arsenic, manganese, lead, and iron. These tendencies could imply the deterioration of groundwater quality and a potential effect on the health of the exposed population. Spatially, vulnerability was observed in Iztapalapa, Tláhuac, Xochimilco, and Coyoacán. Wells coincide spatially with some of the geological damaged areas from the earthquakes in Iztapalapa and Xochimilco. In addition to water quantity, water quality represents a challenge for the urban future, since water disinfection systems are limited to treating the diversity of compounds detected. The water quality-monitoring program must be changed to improve its capacities within a framework of sustainable water management in different regions of the city, and based on the season, considering the potential exposure to the city's population. This represents an opportunity to propose a strategic plan for the groundwater system in order to improve conditions toward a more equitable and sustainable pathway for Mexico City.

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