Seyler, L. M. | bioRxiv Science

bioRxiv2020

Time-series transcriptomics from cold, oxic subseafloor crustal fluids reveals a motile, mixotrophic microbial community

The rock-hosted oceanic crustal aquifer is one of the largest habitable volumes on Earth, and it harbors a reservoir of microbial life that influences global-scale biogeochemical cycles. Here, we use time series metagenomic and metatranscriptomic data from a low-temperature, ridge flank environment that is representative of the majority of global hydrothermal fluid circulation in the ocean to reconstruct microbial metabolic potential, transcript abundance, and community dynamics. The data suggest that the microbial community in this subseafloor habitat is motile, chiefly heterotrophic or mixotrophic, and capable of using alternative electron acceptors such as nitrate and thiosulfate, in addition to oxygen. Anaerobic processes are most abundant in subseafloor horizons deepest in the aquifer, furthest from connectivity with the deep ocean, and there was little overlap in the active microbial populations between sampling horizons. Together, our results indicate the microbial community in the North Pond aquifer plays an important role in the oxidation of organic carbon within the crust, and is also metabolically flexible, with the ability to switch from autotrophy to heterotrophy, as well as function under low oxygen conditions. This work highlights the heterogeneity of microbial life in the subseafloor aquifer and provides new insights into biogeochemical cycling in ocean crust.

microbiology↗

bioRxiv2019

Carbon Assimilation Strategies in Ultrabasic Groundwater: Clues from the Integrated Study of a Serpentinization-Influenced Aquifer

Serpentinization is a low-temperature metamorphic process by which ultramafic rock chemically reacts with water. These reactions provide energy and materials that may be harnessed by chemosynthetic microbial communities at hydrothermal springs and in the subsurface. However, the biogeochemistry of microbial populations that inhabit these environments are understudied and are complicated by overlapping biotic and abiotic processes. We applied metagenomics, metatranscriptomics, and untargeted metabolomics techniques to environmental samples taken from the Coast Range Ophiolite Microbial Observatory (CROMO), a subsurface observatory consisting of twelve wells drilled into the ultramafic and serpentinite melange of the Coast Range Ophiolite in California. Using a combination of DNA and RNA sequence data and mass spectrometry data, we determined that several carbon assimilation strategies, including the Calvin-Benson-Bassham cycle, the reverse tricarboxylic acid cycle, the reductive acetyl-CoA pathway, and methylotrophy are used by the microbial communities inhabiting the serpentinite-hosted aquifer. Our data also suggests that the microbial inhabitants of CROMO use products of the serpentinization process, including methane and formate, as carbon sources in a hyperalkaline environment where dissolved inorganic carbon is unavailable.\n\nImportanceThis study describes the metabolic pathways by which microbial communities in a serpentinite-influenced aquifer may produce biomass from the products of serpentinization. Serpentinization is a widespread geochemical process, taking place over large regions of the seafloor, particularly in slow-spreading mid ocean ridge and subduction zone environments. The serpentinization process is implicated in the origin of life on Earth and as a possible environment for the discovery of life on other worlds in our solar system. Because of the difficulty in delineating abiotic and biotic processes in these environments, major questions remain related to microbial contributions to the carbon cycle and physiological adaptation to serpentinite habitats. This research explores multiple mechanisms of carbon assimilation in serpentinite-hosted microbial communities.

microbiology↗