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To understand the origin and early evolution of life it is crucial to establish characteristics of the primordial environment that facilitated the emergence and evolution of life. One importantenvironmental factor is the pH of the primordial environment. Here, we assessed the pH-dependent thermal stabilities of previously reconstructed ancestral nucleoside diphosphate kinases and ribosomal protein uS8s. The selected proteins were likely to be present in ancient organisms such as the last common ancestor of bacteria and that of archaea. We also assessed the thermal stability of homologous proteins from extant acidophilic, neutralophilic, and alkaliphilic microorganisms as a function of pH. Our results indicate that the reconstructed ancestral proteins are more akin to those of extant alkaliphilic bacteria, which display greater stability under alkaline conditions. These findings suggest that the common ancestors of bacterial and archaeal species thrived in an alkaline environment. Moreover, we demonstrate the reconstruction method employed in this study is a valuable technique for generating alkali-tolerant proteins that can be used in a variety of biotechnological and environmental applications.

With future long-term space exploration and life detection missions on Mars, understanding the microbial survival beyond Earth as well as the identification of past life traces on other planetary bodies becomes increasingly important. The series of the Tanpopo space mission experiments target a long-term exposure (one to three years) of microorganisms on the KIBO Module of the International Space Station (ISS) in the low Earth orbit (LEO) (Kawaguchi et al., 2020; Ott et al., 2020). In the search for possible past and/or present microbial life on Mars, metallophilic archaeal species are of a special interest due to their inherent extraordinary characteristics. Chemolithotrophic archaea (e.g., from the order Sulfolobales) employ a number of ancient metabolic pathways to extract energy from diverse inorganic electron donors and acceptors. Metallosphaera sedula, an iron- and sulfur-oxidizing chemolithotrophic archaeon, which flourishes under hot and acidic conditions (optimal growth at 74°C and pH 2.0), was cultivated on genuine extraterrestrial minerals (Milojevic et al., 2019; Milojevic et al., 2021) as well as synthetic Martian materials (Kölbl et al., 2017). In all cases, M. sedula cells were able to utilize given mineral materials as the sole energy source for cellular growth and proliferation. During the growth of M. sedula cells on these materials, a natural mineral impregnation and encrustation of microbial cells was achieved, followed by their preservation under the conditions of desiccation (Kölbl et al. 2020). Our studies indicate that this archaeon, when impregnated and encrusted with minerals, withstand long-term desiccation and can be even recovered from the dried samples to the liquid cultures (Kölbl et al., 2020). The achieved preservation of desiccated M. sedula cells facilitated our further survivability studies with this desiccated microorganism under simulated Mars-like environmental conditions and during the Tanpopo-4 space exposure experiment.Consequently, M. sedula cells grown on various mineral materials were exposed at the Astrobiology Space Simulation facilities (DLR, Cologne) to a simulated Martian environment for one month, to ensure their suitability for a long-term experiment on the ISS within the Tanpopo-4 mission. Recovery of cells after exposure to a simulated Martian environment shows an insignificant impairment in cellular growth compared to ground control cells.A set of desiccated M. sedula cells grown on Martian breccia NWA 7034 (Milojevic et al., 2021) has already been launched for 1 year outside the ISS to Mars-like conditions in frames of the Tanpopo-4 mission (2022-2023). Upon retrieval of cells from the ISS, we will evaluate survivability of this chemolithoautotroph protected by various mineral materials in Mars-like environment. Furthermore, high-resolution electron microscopy and (nano)-spectroscopy techniques will be applied to identify preservable biosignatures of chemolithotrophic life on Martian materials.Resolving the interface of microbial interactions with Mars minerals under the influence of destructive Martian environmental constrains can bring us closer to identify traces of unicellular life for current and future Mars exploration missions. We will also reveal a role of mineral encapsulation as a natural shielding crust for cell protection during exposure to simulated Martian conditions.References:Kawaguchi Y. et al., Front. Microbiol. 11:2050. (2020) doi: 10.3389/fmicb.2020.02050Kölbl D. et al., Front. Microbiol. 8, 1918 (2017) doi: 10.3389/fmicb.2017.01918Kölbl D. et al., Front. Astron. Space Sci. 7:41 (2020) doi: 10.3389/fspas.2020.00041Milojevic T. et al., Sci. Rep. 9, 18028 (2019). doi: 10.1038/s41598-019-54482-7Milojevic T. et al., Commun. Earth Environ. 2, 39 (2021) doi: 10.1038/s43247-021-00105-xOtt E. et al., Microbiome 8, 150 (2020) doi: 10.1186/s40168-020-00927-5

Deinococcus aetherius ST0316 is a radioresistant bacterium that possess proficient DNA repair capacity. Here, we report the complete genome sequence of D. aetherius , which was obtained by hybrid assembly using short- and long-read sequencing. This sequence will be important information for elucidating the unique DNA repair mechanism of Deinococcus bacteria.

This paper presents a review of the space exploration for life signature search with a special focus on the fluorescence microscope we developed for the life signature search on Mars and in other sites. Considering where, what, and how to search for life signature is essential. Life signature search exploration can be performed on the Mars surface and underground, on Venus’ cloud, moon, asteroids, icy bodies (e.g., moons of Jupiter and Saturn), and so on. It is a useful strategy to consider the targeted characteristics that may be similar to those of terrestrial microorganisms, which are microorganisms with uniform spherical or rod structures with approximately 1 μm diameter surrounded by a membrane having a metabolic activity and mainly made of carbon-based molecules. These characteristics can be analyzed by using a fluorescence microscope and a combination of fluorescence pigments with specific staining characteristics to distinguish the microorganism characteristics. Section 1 introduces the space exploration for life signature search. Section 2 reviews the scientific instruments and achievements of past and ongoing Mars exploration missions closely related to astrobiology. Section 3 presents the search targets and analysis of astrobiology. Section 4 discusses the extraterrestrial life exploration methods that use a microscope together with other methods (based on mass spectrometry, morphology, detection of growth, movement, and death, etc. for microscopic and macroscopic organism). Section 5 expounds on the life signature detection fluorescence microscope, for which we have manufactured a bread board model and tested for extraterrestrial life exploration.

The detection and analysis of extraterrestrial life are important issues of space science. Mars is among the most important planets to explore for extraterrestrial life, owing both to its physical properties and to its ancient and present environments as revealed by previous exploration missions. In this paper, we present a comparative study of methods for detecting extraterrestrial life and life-related substances. To this end, we have classified and summarized the characteristics targeted for the detection of extraterrestrial life in solar system exploration mission and the methods used to evaluate them. A summary table is presented. We conclude that at this moment (i) there is no realistic single detection method capable of concluding the discovery of extraterrestrial life, (ii) no single method has an advantage over the others in all respects, and (iii) there is no single method capable of distinguishing extraterrestrial life from terrestrial life. Therefore, a combination of complementary methods is essential. We emphasize the importance of endeavoring to detect extraterrestrial life without overlooking possible alien life forms, even at the cost of tolerating false positives. Summaries of both the targets and the detection methods should be updated continuously, and comparative studies of both should be pursued. Although this study assumes Mars to be a model site for the primary environment for life searches, both the targets and detection methods described herein will also be useful for searching for extraterrestrial life in any celestial environment and for the initial inspection of returned samples.