Abstract
Rock-hosted subseafloor habitats are very challenging for life, and current knowledge about microorganisms inhabiting such lithic environments is still limited. This study explored the cultivable microbial diversity in anaerobic enrichment cultures from cores recovered during the International Ocean Discovery Program (IODP) Expedition 357 from the Atlantis Massif (Mid-Atlantic Ridge, 30°N). 16S rRNA gene survey of enrichment cultures grown at 10–25°C and pH 8.5 showed that Firmicutes and Proteobacteria were generally dominant. However, cultivable microbial diversity significantly differed depending on incubation at atmospheric pressure (0.1 MPa), or hydrostatic pressures (HP) mimicking the in situ pressure conditions (8.2 or 14.0 MPa). An original, strictly anaerobic bacterium designated 70B-AT was isolated from core M0070C-3R1 (1150 meter below sea level; 3.5 m below seafloor) only from cultures performed at 14.0 MPa. This strain named Petrocella atlantisensis is a novel species of a new genus within the newly described family Vallitaleaceae (order Clostridiales, phylum Firmicutes). It is a mesophilic, moderately halotolerant and piezophilic chemoorganotroph, able to grow by fermentation of carbohydrates and proteinaceous compounds. Its 3.5 Mb genome contains numerous genes for ABC transporters of sugars and amino acids, and pathways for fermentation of mono- and di-saccharides and amino acids were identified. Genes encoding multimeric [FeFe] hydrogenases and a Rnf complex form the basis to explain hydrogen and energy production in strain 70B-AT. This study outlines the importance of using hydrostatic pressure in culture experiments for isolation and characterization of autochthonous piezophilic microorganisms from subseafloor rocks.
Highlights
The subseafloor biosphere remains largely unexplored, estimated as a huge reservoir for prokaryotic life (Whitman et al, 1998; Orcutt et al, 2011; Kallmeyer et al, 2012)
Most of these microorganisms were isolated from sedimentary rock cores and included mostly aerobes or facultative anaerobes but a few strict anaerobes even though, passed the first tens centimeters depth below the seafloor, these environments are essentially anoxic (Toffin and Alain, 2014)
Our main goal was to cultivate anaerobic microorganisms inhabiting the rocky subseafloor associated with active serpentinization characterized by circulation of high pH fluids enriched in H2, CH4 and other organic molecules abiotically produced
Summary
The subseafloor biosphere remains largely unexplored, estimated as a huge reservoir for prokaryotic life (Whitman et al, 1998; Orcutt et al, 2011; Kallmeyer et al, 2012). Most of the enrichment tests performed on rocks recovered during ocean drilling programs were made at atmospheric pressure and failed to obtain microbial growth after first incubations or subcultures (Santelli et al, 2010; Hirayama et al, 2015) Both hydrostatic and lithostatic pressures in deepsea and deep subseafloor environments (increasing by about 10 and 30 MPa km−1, respectively (Schrenk et al, 2010)) have an impact on microbial growth, metabolism and physiology, on the cultivability of microorganisms (Bartlett et al, 2007; Lauro and Bartlett, 2007; Parkes et al, 2009; Takai, 2011; Picard and Daniel, 2013). HP incubation systems were used to cultivate and study physiology of microorganisms inhabiting the underexplored oceanic crustal biosphere
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