An ionic limit to life in the deep subsurface
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The physical and chemical factors that can limit or prevent microbial growth in the deep subsurface are not well defined. Brines from an evaporite sequence were sampled in the Boulby Mine, United Kingdom between 800 and 1300 m depth. Ionic, hydrogen and oxygen isotopic composition were used to identify two brine sources, an aquifer situated in strata overlying the mine, and another ambiguous source distinct from the regional groundwater. The ability of the brines to support microbial replication was tested with culturing experiments using a diversity of inocula. The examined brines were found to be permissive for growth, except one. Testing this brine’s physicochemical properties showed it to have low water activity and to be chaotropic, which we attribute to the high concentration of magnesium and chloride ions. Metagenomic sequencing of the brines that supported growth showed their microbial communities to be similar to each other and comparable to those found in other hypersaline environments. These data show that solutions high in dissolved ions can shape the microbial diversity of the continental deep subsurface biosphere. Furthermore, under certain circumstances, complex brines can establish a hard limit to microbial replication in the deep biosphere, highlighting the potential for subsurface uninhabitable aqueous environments at depths far shallower than a geothermally-defined limit to life.
Payler , S J , Biddle , J F , Sherwood Lollar , B , Fox-Powell , M G , Edwards , T , Ngwenya , B T , Paling , S M & Cockell , C S 2019 , ' An ionic limit to life in the deep subsurface ' , Frontiers in Microbiology , vol. 10 , 00426 . https://doi.org/10.3389/fmicb.2019.00426
Frontiers in Microbiology
Copyright © 2019 Payler, Biddle, Sherwood Lollar, Fox-Powell, Edwards, Ngwenya, Paling and Cockell. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
DescriptionSP was supported by EPSRC. We would also like to acknowledge the Deep Carbon Observatory for partly funding the metagenome sequencing and Natural Sciences and Engineering Council of Canada (NSERC) funding to BS Lollar for funding the fluid isotope work (Discovery Grant No. 453949).
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