Physiological and Phylogenetic Studies of the Biogeography of Alkaliphilic Heterotrophic Bacteria from Serpentinizing Habitats

Abstract

Serpentinization occurs when ultramafic rocks containing the mineral olivine react with water to produce highly reducing conditions, which are commonly coincident with high concentrations of hydrogen. Hydrogen provides an energy source for microbial metabolism and in combination with mineral catalysts can lead to the abiogenic synthesis of small organic compounds, such as methane. Serpentinite habitats contain abundant potential electron donors, but microorganisms presumably have limited accessibility to electron acceptors. A consequence of serpentinization is the production of highly alkaline (pH 10-12), highly reducing fluids. The stress exerted on microorganisms in such ecosystems can make fundamental cellular processes such as maintaining a proton motive force or stabilizing RNA a difficult task and limit microbial diversity in these environments. It is reputed that the microbial communities in these extreme habitats are able to adjust to the ultrabasic conditions through biochemical and metabolic adaptations. However, it remains to be discovered how microorganisms have physiologically adapted to the serpentinite environment. Taxonomic characterization of novel isolates from different serpentinite habitats allows us to examine microbial biodiversity and assess whether certain microorganisms are found only at a particular site or whether their occurrence is widespread and correlated with specific environmental conditions. Comparisons of microbial diversity of the numerous serpentine sites our laboratory group is working on in the United States, Canada, and Italy to previously published studies provides insight into the biogeography of alkaliphiles in serpentinizing habitats at a global scale. Detailed physiological studies focused upon a high-resolution environmental data set from the Coast Range Ophiolite Microbial Observatory (CROMO) site in California have been used to relate physiological data from a subset of new isolates to environmental characteristics of the environments from which they were sampled. Alkaliphilic microorganisms were isolated and grown aerobically from samples collected from three different serpentinite habits located in California, Italy, and Canada between 2009 and 2013. Total genomic DNA from the isolates was used to sequence their 16S rRNA gene to establish the taxa present in each serpentinite habitat as well as in "background" soils and water. Culture-dependent analyses on a subset of isolates investigated the physiology of the isolated alkaliphilic heterotrophs. This research contributes to our understanding of microbial life in ultrabasic habitats associated with serpentinization, explores the biodiversity of the subsurface microorganisms, and characterizes their physiological adaptations.