Current state of athalassohaline deep‐sea hypersaline anoxic basin research—recommendations for future work and relevance to astrobiology

Abstract

Deep-sea hypersaline anoxic basins (DHABs) are uniquely stratified polyextreme environments generally found in enclosed seas. These environments select for elusive and widely uncharacterized microbes that may be living below the currently recognized window of life on Earth. Still, there is strong evidence of highly specialized active microbial communities in the Kryos, Discovery, and Hephaestus basins located in the Eastern Mediterranean Sea; the only known athalassohaline DHABs. Life is further constrained in these DHABs as near-saturated concentrations of magnesium chloride significantly reduces water activity (a w ) and exerts extreme chaotropic stress, the tendency of a solution to disorder biomolecules. In this review, we provide an overview of microbial adaptations to polyextremes focusing primarily on chaotropicity, summarize current evidence of microbial life within athalassohaline DHABs and describe the difficulties of life detection approaches and sampling within these environments. We also reveal inconsistent measurements of chaotropic activity in the literature highlighting the need for a new methodology. Finally, we generate recommendations for future investigations and discuss the importance of athalassohaline DHAB research to help inform extraterrestrial life detection missions.

A Young Scientist’s Perspective on the Future of Ocean Worlds Research

“…Kevin Hand and Robert Carlson from NASA's Jet Propulsion Laboratory conducted studies exposing NaCl to conditions known to exist on Europa, notably radiation, that resulted in a discoloration similar to what has been observed on the surface of that moon (Hand and Carlson, 2015). I found this result intriguing, as the majority of DHAB environments are NaCl dominated (Fisher et al, 2021). Given the uncertainty of ionic compositions within icy moons, studying the My work entailed coordinating and encouraging conversations between Earth and planetary scientists to develop synergistic research questions to advance our understanding of ocean worlds.…”

“…stress, permanent anoxia, and elevated hydrostatic pressure makes them one of the most extreme and least explored environments on Earth. Despite these conditions, life still finds a way to survive (Fisher et al, 2021). The varied chemical conditions present in these environments serve as an ideal testing ground for understanding the suite of conditions that may be present on icy moons (Antunes et al, 2020).…”

Astrobiology of life on Earth

“…Therefore, the apparent limitation of the chaotropicityquantification technique (Cray et al, 2013) at high molar concentrations of MgCl 2 is in reality a biologically pertinent indication that there is no further chaotropic activity. The authors of Fisher et al (2021) were also involved in a study by Klempay et al (2021), which analysed Archaea and Bacteria and water chemistry, in salterns hosting a range of evaporated seawater from San Diego Bay (southern California, USA). The Klempay et al study has implications for interactions of chaotropicity and water activity in relation to biophysical constraints on cellular activity, an issue pertinent to diverse microbial systems (Hallsworth et al, 2007;Alves et al, 2015;Cray et al, 2015).…”

“…The authors of Fisher et al . (2021) were also involved in a study by Klempay et al . (2021), which analysed Archaea and Bacteria and water chemistry, in salterns hosting a range of evaporated seawater from San Diego Bay (southern California, USA).…”

“…In a comprehensive review of athalassohaline deep‐sea hypersaline anoxic brines, Fisher et al . (2021) point out that the art of extrapolating chaotropicity values can be imprecise. We agree with this finding and believe that this is because, beyond specific concentrations of highly chaotropic salts, the chaotropicity limit has been surpassed in terms of both biological tolerance of the cell and biophysical tolerance of some biomacromolecules.…”