Microbiology and the limits to life in deep salts

Abstract

Deep subsurface evaporites are common terrestrial deep subsurface environments found globally. These deposits are known to host communities of halophilic organisms, some of which have been suggested to be millions of years old. The discovery of evaporite minerals on Mars has led to these environments becoming of interest to astrobiology, particularly because the subsurface of Mars represents the best chance of finding more clement conditions conducive to life. Despite this interest, deep subsurface evaporites remain poorly understood and we have little insight into how different salts shape the Earth’s biosphere, much of which is underground. This thesis addresses several knowledge gaps present in the literature by sampling a selection of brine seeps and rock salt samples taken from Boulby Potash Mine, UK. The origin and evolution of the brines is determined with geochemical techniques, showing the majority to have been sourced from an aquifer above where they were intersected in the mine. These brines appear to have taken a variety of pathways through the subsurface leading to the presence of a range of different ions dissolved within them. The majority are Na/Cl dominated, whilst one is K/Cl dominated. One brine appears to have a different origin and probably interacted with dolomite becoming very concentrated in Mg. This variety in brine origins and migration pathways has impacted the habitability of the brines. Physicochemical measurements for chaotropicity, water activity and ionic strength, combined with culturing experiments suggest brines from the Sherwood Sandstone were habitable, but the brine from a distinct unknown source was uninhabitable. DNA was successfully extracted from three of the habitable brines and their metagenomes sequenced. These revealed communities largely functionally and phylogenetically similar to surface near saturation brines, indicating that the structure of the communities present in saturated Na/Cl brines are controlled almost exclusively by these ions rather than any other environmental difference between the surface and subsurface. Organisms were also taken from these brines and culturing experiments carried out to determine if any carbon sources were present in ancient salt that might promote growth in the absence of other carbon sources. Controls showed that the geochemical changes to the growth media induced by solving the salts, particularly sylvinite, were responsible for the increases in growth observed, indicating certain salt minerals effectively fertilise the growth of halophiles. Culturing on hydrocarbon seeps collected in the mine suggested they may provide a carbon source periodically to some organisms within the deposit. Work was done to show the presence of dissimilatory sulphate and iron reducing halophiles. Overall this significantly advances our understanding of how salts shape the Earth’s biosphere, particularly its deep subsurface component, and what functional capabilities life has to persist in these environments. This work provides a new window on the potential habitability of deep subsurface extraterrestrial environments and how we might go about investigating these environments for habitable conditions.