Physiological Studies of Alkaliphilic Anaerobic Organotrophs in a Serpentinizing Subsurface Habitat

Abstract

Microbial habitats in serpentinizing ultramafic rocks represent one of the largest, yet least understood portions of the biosphere, with potentially major consequences for global biogeochemical cycles. Serpentinization is a process where ultramafic rock is uplifted to Earth's surface through tectonic activity, where it reacts with water to create a highly reducing environment rich in hydrogen, methane, and small abiogenic organic compounds. These compounds may serve as important sources of nutrients and energy to sustain microbial metabolism. While serpentinites can provide the energy to sustain microbial communities, the conditions created by serpentinization can be harsh and challenge the limits of microbial physiology in terms of extreme pH (>11), and availability of terminal electron acceptors. Ongoing culture-independent studies by our research group have identified bacterial taxa related to the order Clostridiales as important components of the highest pH, most reducing subsurface habitats. The goal of this project was to systematically analyze the nutrients necessary to sustain life in a serpentinizing subsurface habitat located on McLaughlin Natural Reserve near Lower Lake, CA. The research consisted of creating microcosm enrichment cultures to evaluate the distribution and environmental controls on Clostridiales species within a subsurface microbial observatory in serpentinizing rocks. Subsequently, the cellular abundance of a novel isolate from these experiments was measured in relation to organic carbon utilization, and utilization of iron and sulfur as terminal electron acceptors. We have also conducted a cursory analysis of annotated biochemical pathways in the genomes of two isolates in relation to respiration and metabolism of carbohydrates. From these experiments we have shown that Clostridiales may play important roles in the biogeochemistry of carbon, iron, and sulfur in serpentinite environments. The research also provides further insight into habitability in these extreme environments and sources of nutrients and energy. Knowledge on the subsurface metabolic capabilities gained through this research has implications for the development of future biotechnologies related to bioremediation and microbial fuel cells.