Diversity and Composition of Methanotroph Communities in Caves.

Methane oxidizing microorganisms (methanotrophs) are ubiquitous in the environment and represent a major sink for the greenhouse gas methane (CH4). Recent studies have demonstrated methanotrophs are abundant and contribute to CH4 dynamics in caves. However, very little is known about what controls the distribution and abundance of methanotrophs in subterranean ecosystems. Here, we report a survey of soils collected from > 20 caves in North America to elucidate the factors shaping cave methanotroph communities. Using 16S rRNA sequencing, we recovered methanotrophs from nearly all (98%) of the samples, including cave sites where CH4 concentrations were at or below detection limits (≤0.3 ppmv). We identified a core methanotroph community among caves comprised of high-affinity methanotrophs. Although associated with local-scale mineralogy, methanotroph composition did not systematically vary between the entrances and interior of caves, where CH4 concentrations varied. We also observed methanotrophs are able to disperse readily between cave systems showing these organisms have low barriers to dispersal. Lastly, the relative abundance of methanotrophs was positively correlated with cave-air CH4 concentrations, suggesting these microorganisms contribute to CH4 flux in subterranean ecosystems. IMPORTANCE Recent observations have shown the atmospheric greenhouse gas methane (CH4) is consumed by microorganisms (methanotrophs) in caves at rates comparable to CH4 oxidation in surface soils. Caves are abundant in karst landscapes that comprise 14% of Earth's land surface area, and therefore may represent a potentially important, but overlooked, CH4 sink. We sampled cave soils to gain a better understand the community composition and structure of cave methanotrophs. Our results show the members of the USC-γ clade are dominant in cave communities and can easily disperse through the environment, methanotroph relative abundance was correlated with local scale mineralogy of soils, and the relative abundance of methanotrophs was positively correlated with CH4 concentrations in cave air.

Assessment of major ions and trace elements in groundwater supplied to the Monterrey metropolitan area, Nuevo León, Mexico.

The Monterrey metropolitan area (MMA) is the third greatest urban area and the second largest economic city of Mexico. More than four million people living in this megacity use groundwater for drinking, industrial and household purposes. Thus, major ion and trace element content were assessed in order to investigate the main hydrochemical properties of groundwater and determine if groundwater of the area poses a threat to the MMA population. Hierarchical cluster analysis using all the groundwater chemical data showed five groups of water. The first two groups were classified as recharge waters (Ca-HCO3) coming from the foothills of mountain belts. The third group was also of Ca-HCO3 water type flowing through lutites and limestones. Transition zone waters of group four (Ca-HCO3-SO4) flow through the valley of Monterrey, whereas discharge waters of group 5 (Ca-SO4) were found toward the north and northeast of the MMA. Principal component analysis performed in groundwater data indicates four principal components (PCs). PC1 included major ions Si, Co, Se, and Zn, suggesting that these are derived by rock weathering. Other trace elements such as As, Mo, Mn, and U are coupled in PC2 because they show redox-sensitive properties. PC3 indicates that Pb and Cu could be the less mobile elements in groundwater. Although groundwater supplied to MMA showed a high-quality, high mineralized waters of group 5 have NO3- concentrations higher than the maximum value proposed by international guidelines and SO42-, NO3-, and total dissolved solid concentrations higher than the maximum levels allowed by the Mexican normative.