Oceanic crust comprises the largest hydrogeologic reservoir on Earth, containing fluids in thermodynamic disequilibrium with the basaltic crust. Little is known about microbial ecosystems that inhabit this vast realm and exploit chemically favorable conditions for metabolic activities. Crustal samples recovered from ocean drilling operations are often compromised for microbiological assays, hampering efforts to resolve the extent and functioning of a subsurface biosphere. We report results from the first in situ experimental observatory systems that have been used to study subseafloor life. Experiments deployed for 4 years in young (3.5 Ma) basaltic crust on the eastern flank of the Juan de Fuca Ridge record a dynamic, post-drilling response of crustal microbial ecosystems to changing physical and chemical conditions. Twisted stalks exhibiting a biogenic iron oxyhydroxide signature coated the surface of mineral substrates in the observatories; these are biosignatures indicating colonization by iron oxidizing bacteria during an initial phase of cool, oxic, iron-rich conditions following observatory installation. Following thermal and chemical recovery to warmer, reducing conditions, the in situ microbial structure in the observatory shifted, becoming representative of natural conditions in regional crustal fluids. Firmicutes, metabolic potential of which is unknown but may involve N or S cycling, dominated the post-rebound bacterial community. The archaeal community exhibited an extremely low diversity. Our experiment documented in situ conditions within a natural hydrological system that can pervade over millennia, exemplifying the power of observatory experiments for exploring the subsurface basaltic biosphere, the largest but most poorly understood biotope on Earth.
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We acknowledge the assistance of S Fakra, M Marcus, S Bennett, and A Turner with synchrotron analysis at the Advanced Light Source, and A Thompson at the Center for Electron Microscopy and Micro-Analysis for assistance with SEM measurements, and T Thompsen for assistance with cloning. We also thank M Malone for providing samples of the pipe dopes. This work was supported by funding from the NSF Ocean Drilling Program (OCE-0737300 to KJE, OCE-0400471 to KB, OCE-0400462 to CGW and OCE-0727952 to AF), the Gordon and Betty Moore Foundation (No. 1609 to KJE and CGW), and the Office of the Vice President for Research, University of Minnesota (BMT). The Advanced Light Source is supported by the Office of Science, Basic Energy Sciences, Division of Materials Science of the US Department of Energy under contract No. DE-AC02-05CH11231.
- deep biosphere
- oceanic crust