Heterotrophic Growth Dominates in the Most Extremotolerant Extremophile Cultures.

Due to their ability to withstand "extreme" conditions, Earth's extremophilic organisms can constrain habitability windows for other planetary systems. However, there are many other considerations to microbial growth requirements beyond environmental extremes, such as nutrient availability. Here, we conduct a literature review of the most extremotolerant extremophiles in culture, since working with cultured organisms allows environmental and nutrient variables to be constrained with a high level of specificity. We generated a database that includes the isolation environment, carbon source(s) used, and growth preferences across temperature, pressure, salinity, and pH extremes. We found that the "most extreme" conditions were primarily sustained by heterotrophs, except for hyperthermophiles. These results highlight the importance of considering organic carbon availability when using extremophiles for habitability constraints. We also interrogated polyextreme potential across temperature, pressure, salinity, and pH conditions. Our findings suggest that the investigation of growth tolerance rather than growth optimum may reveal wider habitability parameters. Overall, these results highlight the potential polyextremes, environments, nutrient requirements, and additional analyses that could improve the application of cultured investigations to astrobiology questions.

Transient surface ocean oxygenation recorded in the ∼2.66-Ga Jeerinah Formation, Australia.

Many paleoredox proxies indicate low-level and dynamic incipient oxygenation of Earth's surface environments during the Neoarchean (2.8-2.5 Ga) before the Great Oxidation Event (GOE) at ∼2.4 Ga. The mode, tempo, and scale of these redox changes are poorly understood, because data from various locations and ages suggest both protracted and transient oxygenation. Here, we present bulk rock and kerogen-bound nitrogen isotope ratios as well as bulk rock selenium abundances and isotope ratios from drill cores sampled at high stratigraphic resolution through the Jeerinah Formation (∼2.66 Ga; Fortescue Group, Western Australia) to test for changes in the redox state of the surface environment. We find that both shallow and deep depositional facies in the Jeerinah Formation display episodes of positive primary δ15N values ranging from +4 to +6‰, recording aerobic nitrogen cycling that requires free O2 in the upper water column. Moderate selenium enrichments up to 5.4 ppm in the near-shore core may indicate coincident oxidative weathering of sulfide minerals on land, although not to the extent seen in the younger Mt. McRae Shale that records a well-documented "whiff" of atmospheric oxygen at 2.5 Ga. Unlike the Mt. McRae Shale, Jeerinah selenium isotopes do not show a significant excursion concurrent with the positive δ15N values. Our data are thus most parsimoniously interpreted as evidence for transient surface ocean oxygenation lasting less than 50 My, extending over hundreds of kilometers, and occurring well before the GOE. The nitrogen isotope data clearly record nitrification and denitrification, providing the oldest firm evidence for these microbial metabolisms.