Microbiology of a NaCl stalactite 'salticle' in Triassic halite.

Stephen A. Kelly,Gill Plunkett,Brendan F. Gilmore,Thomas P. Thompson,Jason Hopps,Alastair Ruffell,Timofey Skvortsov,John E. Hallsworth

doi:10.1111/1462-2920.15524

Abstract

Large regions of Earth's surface are underlain by salt deposits that evaporated from ancient oceans and are populated by extreme halophilic microbes. Some of these halophiles may have been preserved over geological timescales within hypersaline fluid inclusions, but ingresses of water and/or anthropogenic activities can lead to the formation of alternative habitats, including NaCl stalactites or other speleothems. While the microbiology of ancient evaporites has been well studied, the ecology of these recently formed structures is less-well understood. Here, the microbiology of a NaCl stalactite ('salticle') in a Triassic halite mine is characterized. The specific aims were to determine the presence of fluid inclusions, determine the microbial structure of the salticle compared with a nearby brine-pool and surficial soil, and characterize the ecophysiological capabilities of this unique ecosystem. The salticle contained fluid inclusions, and their microbiome was composed of Euryarchaetota, Proteobacteria, and Actinobacteria, with Haloarchaea in greater abundance than brine-pool or soil microbiomes. The salticle metagenome exhibited a greater abundance of genes involved in osmoregulation, anaerobic respiration, UV resistance, oxidative stress, and stress-protein synthesis relative to the soil microbiome. We discuss the potential astrobiological implications of salticles as enclosed salt-saturated habitats that are protected from ionizing radiation and have a stable water activity.

Highlights

Well-perserved, unaltered evaporite deposits are distributed throughout much of the Earth and are known from Proterozoic until modern times (Sankaranarayanan et al, 2011)
Some of these halophiles may have been preserved over geological timescales within hypersaline fluid inclusions, but ingresses of water and/or anthropogenic activities can lead to the formation of alternative habitats, including NaCl stalactites or other speleothems
A series of insightful studies have elucidated aspects of the microbiology of ancient halite deposits (Jaakkola et al, 2016; Megaw et al, 2020), the geology of salt-based speleothems (Filippi et al, 2011), the microbial ecology of hypersaline fluid inclusions (Schubert et al, 2009; Benison, 2019), and the precipitation of minerals in anthropogenically perturbed subsurface environments (Borgonie et al, 2015)

Summary

Euryarchaetota, Proteobacteria, and Actinobacteria, with Haloarchaea in greater abundance than brinepool or soil microbiomes. The salticle metagenome exhibited a greater abundance of genes involved in osmoregulation, anaerobic respiration, UV resistance, oxidative stress, and stress-protein synthesis relative to the soil microbiome. We discuss the potential astrobiological implications of salticles as enclosed salt-saturated habitats that are protected from ionizing radiation and have a stable water activity

Introduction

Results and discussion

Experimental procedures

Conclusion