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The lithified oceanic crust, lower crust gabbros in particular, has remained largely unexplored by microbiologists. Recently, evidence for heterogeneously distributed viable and transcriptionally active autotrophic and heterotrophic microbial populations within low‐biomass communities was found down to 750 m below the seafloor at the Atlantis Bank Gabbro Massif, Indian Ocean. Here, we report on the diversity, activity and adaptations of fungal communities in the deep oceanic crust from ~10 to 780 mbsf by combining metabarcoding analyses with mid/high‐throughput culturing approaches. Metabarcoding along with culturing indicate a low diversity of viable fungi, mostly affiliated to ubiquitous (terrestrial and aquatic environments) taxa. Ecophysiological analyses coupled with metatranscriptomics point to viable and transcriptionally active fungal populations engaged in cell division, translation, protein modifications and other vital cellular processes. Transcript data suggest possible adaptations for surviving in the nutrient‐poor, lithified deep biosphere that include the recycling of organic matter. These active communities appear strongly influenced by the presence of cracks and veins in the rocks where fluids and resulting rock alteration create micro‐niches.

 

Our understanding of phosphorus (P) dynamics in the deep subseafloor environment remains limited. Here we investigate potential microbial P uptake mechanisms in oligotrophic marine sediments beneath the North Atlantic Gyre and their effects on the relative distribution of organic P compounds as a function of burial depth and changing redox conditions. We use metagenomic analyses to determine the presence of microbial functional genes pertaining to P uptake and metabolism, and solution 31 P nuclear magnetic resonance spectroscopy ( 31 P NMR) to characterize and quantify P substrates. Phosphorus compounds or compound classes identified with 31 P NMR include inorganic P compounds (orthophosphate, pyrophosphate, polyphosphate), phosphonates, orthophosphate monoesters (including inositol hexakisphosphate stereoisomers) and orthophosphate diesters (including DNA and phospholipid degradation products). Some of the genes identified include genes related to phosphate transport, phosphonate and polyphosphate metabolism, as well as phosphite uptake. Our findings suggest that the deep sedimentary biosphere may have adapted to take advantage of a wide array of P substrates and could play a role in the gradual breakdown of inositol and sugar phosphates, as well as reduced P compounds and polyphosphates. 

ABSTRACT International Ocean Discovery Program Expedition 360 drilled Hole U1473A at Atlantis Bank, an oceanic core complex on the Southwest Indian Ridge, with the aim of recovering representative samples of the lower oceanic crust. Recovered cores were primarily gabbro and olivine gabbro. These mineralogies may host serpentinization reactions that have the potential to support microbial life within the recovered rocks or at greater depths beneath Atlantis Bank. We quantified prokaryotic cells and analyzed microbial community composition for rock samples obtained from Hole U1473A and conducted nutrient addition experiments to assess if nutrient supply influences the composition of microbial communities. Microbial abundance was low (≤10 4 cells cm −3 ) but positively correlated with the presence of veins in rocks within some depth ranges. Due to the heterogeneous nature of the rocks downhole (alternating stretches of relatively unaltered gabbros and more significantly altered and fractured rocks), the strength of the positive correlations between rock characteristics and microbial abundances was weaker when all depths were considered. Microbial community diversity varied at each depth analyzed. Surprisingly, addition of simple organic acids, ammonium, phosphate, or ammonium plus phosphate in nutrient addition experiments did not affect microbial diversity or methane production in nutrient addition incubation cultures over 60 weeks. The work presented here from Site U1473A, which is representative of basement rock samples at ultraslow spreading ridges and the usually inaccessible lower oceanic crust, increases our understanding of microbial life present in this rarely studied environment and provides an analog for basement below ocean world systems such as Enceladus. IMPORTANCE The lower oceanic crust below the seafloor is one of the most poorly explored habitats on Earth. The rocks from the Southwest Indian Ridge (SWIR) are similar to rock environments on other ocean-bearing planets and moons. Studying this environment helps us increase our understanding of life in other subsurface rocky environments in our solar system that we do not yet have the capability to access. During an expedition to the SWIR, we drilled 780 m into lower oceanic crust and collected over 50 rock samples to count the number of resident microbes and determine who they are. We also selected some of these rocks for an experiment where we provided them with different nutrients to explore energy and carbon sources preferred for growth. We found that the number of resident microbes and community structure varied with depth. Additionally, added nutrients did not shape the microbial diversity in a predictable manner.