Molecular adaptations specific to extreme halophilic archaea could promote high perchlorate tolerance.

Perchlorate is a strong chaotropic agent that causes macromolecule denaturation, DNA damage, and oxidative stress. However, perchlorate deliquescence is thought to promote the formation of liquid salt brines, even at hyper-arid and cold environments, such as the Martian regolith. For that reason, the detection of high levels of perchlorate at different locations on the Martian surface led to hypotheses about the existence of Martian microenvironments compatible with life, especially with those organisms tolerant to hyper-salinity and perchlorate. Extreme halophilic archaea have been proposed as the best candidates to inhabit those environments not only due to their high tolerance to salinity and perchlorate, but also because of their resistance to a wide variety of stress conditions. Since specific perchlorate responses remain largely unknown, in this work, we have analyzed the molecular mechanisms of perchlorate tolerance exhibited by the model extreme halophilic archaeon Haloferax volcanii using a transcriptomic approach. We report that perchlorate produced transcriptional effects opposite to those of salinity, and we propose that the "salt-in" strategy could promote high perchlorate tolerance in extreme halophilic archaea due to the intracellular accumulation of KCl, which may shield the chaotropic activity of perchlorate. This natural adaptation would be enhanced by changes in other stress responses like DNA repair, refolding and turnover of damaged proteins, removal of oxidative species, and tRNA modifications, among others. These results may help to understand how life could survive on Mars, now or in the past, and highlight the importance of extreme halophiles in the development of in situ resource utilization systems.IMPORTANCEPerchlorate is a toxic chlorinated compound that promotes the formation of liquid salt brines, even at hyper-arid and cold environments. For the past two decades, different probes have reported high levels of perchlorate salts at multiple locations on the Martian surface, which could facilitate the presence of potentially habitable environments by specific microorganisms capable of tolerating both hyper-salinity and high perchlorate concentrations. Therefore, the significance of this research was to investigate the molecular mechanisms for perchlorate tolerance using the extreme haloarchaeon Haloferax volcanii as a model organism. This analysis leads to the identification of critical genes and pathways involved in perchlorate tolerance and supports that certain molecular adaptations specific to extreme haloarchaea may be responsible for the high levels of perchlorate tolerance exhibited by these microorganisms, serving as a valuable resource for Mars exploration.