In this comprehensive review, Acidithiobacillus ferrooxidans, an acidophilic bacterium, has been thoroughly examined as a plausible analogue for microbial life in Venus's lower cloud layer. Given its ability to adapt to extreme conditions, including low pH environments and metal-rich settings, Acidithiobacillus ferrooxidans is considered a promising candidate for studying life analogues in Venus's clouds. This article comprehensively analyses the bacterium's distinctive phenotypic and genotypic features, investigating its metabolic pathways, adaptive strategies and potential ecological niche within Venusian cloud ecosystems. After careful consideration of the environmental parameters characterizing Venus, the unidentified UV absorber in its clouds, and the prospects for microbial life, this review underscores the imperative nature of future Venus missions and the pivotal role that Acidithiobacillus ferrooxidans may play in exploring the possible habitability of Venus and advancing astrobiological research.

Discoveries of transient liquid water in the Martian polar caps and the presence of liquid lakes and subsurface oceans in icy satellites have increased the interest of scientists in the capabilities of terrestrial extremophiles to grow and remain metabolically active in these extreme environments. The principal goal of this research is to understand the metabolic capacity of the anaerobic psychrophile, Desulfotalea psychrophila, cultured at subfreezing temperatures in media containing various concentrations of sulphate minerals. In this regard, our experiments focused on the detection of D. psychrophila survival and active metabolism, employing a biochamber that can recreate Martian temperatures. Using standard bacteriological methods for determining growth, combined with molecular and enzymatic determination of sulphate reduction, we have found that D. psychrophila is capable to carry out biological processes at temperatures down to −5°C, at concentrations that range from 0.35 to 18 wt% of MgSO4, 0.1 wt% of CaSO4 and 10 to 14 wt% of FeSO4 in which the highest sulphate concentration gradually returned the biosynthetic rate to basal limits, and the lowest temperature decreased bacterial cell division. These chemical salts, whose ions are classified as chaotropes, are known to act by maintaining water molecules in liquid state at subfreezing temperatures and by altering the stability of cellular components. This ‘chaotropic effect’ could potentially benefit the microbial metabolic activity up to a concentration in which cellular viability is jeopardized. Consequently, our hypothesis is directed towards the detection of metabolic activity as an indirect measurement of the potential influence of these ions in the flexibility/functionality of biological structures that at cold temperatures are highly rigid, compact and partially/non-functional due to water freezing. Studies of this type of microorganism are critical considering the possibility of survival and colonization of psychrophilic sulphate reducers in other planets and icy satellites.

The Cyanidiophyceae are a class of unicellular red algae that live in environments at the extremes of temperature, pH, salt, and heavy metal concentrations. These photosynthetic organisms have been able to occupy these rather unusual niches by acquiring a variety of genes from extremophilic prokaryotes and/or viruses via the process of horizontal gene transfer (HGT). Elucidating how the proteins encoded by these genes integrate into existing metabolic systems is crucial to understanding and evaluating the significance of HGT in eukaryotes and its role in conferring adaptive traits. This article addresses the fundamentals of HGT in model red algae, its function as a driver of evolution across the web of life with a focus on eukaryotes, and summarizes ongoing work and the future directions of this field.

Mars is considered to be one of the most favourable places in the Solar System to search for past and present life. In the past Mars was warmer and wetter, so terrestrial halophiles can be regarded as analogues of hypothetical ancient Martian halophiles. In this study we used microorganisms from unique Altai region (Russia) to estimate the capability of terrestrial bacteria and archaea to survive at low temperatures and high concentration of salts and metals, similar to the Martian environment. The current report demonstrates that both halophilic archaea and halotolerant bacteria from saline lakes of the Altai region may be considered as analogues of ancient Martian organisms, since they are able to withstand conditions that hypothetically existed in subsurface layers of the early Mars (low temperatures, salt solutions with a high content of NaCl) with only slight decrease in viability. We also found that the studied microorganisms can use some organic substances found in meteorites. We consider that transfer of unicellular halophiles from Earth to Mars was possible, and, moreover, they could successfully survive and grow on early Mars. Adjusting our growth media to the chemical composition of the lakes, from which the studied strains were isolated, resulted in significant increase in survival and growth rates. Certain strains could survive several freeze–thaw cycles at −70 °C typical for Martian nights.

Liquid water on Mars might be created by deliquescence of hygroscopic salts or by permafrost melts, both potentially forming saturated brines. Freezing point depression allows these heavy brines to remain liquid in the near-surface environment for extended periods, perhaps as eutectic solutions, at the lowest temperatures and highest salt concentrations where ices and precipitates do not form. Perchlorate and chlorate salts and iron sulphate form brines with low eutectic temperatures and may persist under Mars near-surface conditions, but are chemically harsh at high concentrations and were expected to be incompatible with life, while brines of common sulphate salts on Mars may be more suitable for microbial growth. Microbial growth in saturated brines also may be relevant beyond Mars, to the oceans of Ceres, Enceladus, Europa and Pluto. We have previously shown strong growth of salinotolerant bacteria in media containing 2M MgSO4 heptahydrate (~50% w/v) at 25°C. Here we extend those observations to bacterial isolates from Basque Lake, BC and Hot Lake, WA, that grow well in saturated MgSO4 medium (67%) at 25°C and in 50% MgSO4 medium at 4°C (56% would be saturated). Psychrotolerant, salinotolerant microbes isolated from Basque Lake soils included Halomonas and Marinococcus, which were identified by 16S rRNA gene sequencing and characterized phenetically. Eutectic liquid medium constituted by 43% MgSO4 at −4°C supported copious growth of these psychrotolerant Halomonas isolates, among others. Bacterial isolates also grew well at the eutectic for K chlorate (3% at −3°C). Survival and growth in eutectic solutions increases the possibility that microbes contaminating spacecraft pose a contamination risk to Mars. The cold brines of sulphate and (per)chlorate salts that may form at times on Mars through deliquescence or permafrost melt have now been demonstrated to be suitable microbial habitats, should appropriate nutrients be available and dormant cells become vegetative.

Astrobiology seeks to understand the limits of life and to determine the physiology of organisms in order to better assess the habitability of other worlds. To successfully achieve these goals we require microorganisms from environments on Earth that approximate to extraterrestrial environments in terms of physical and/or chemical conditions. The most challenging of these environments with respect to sample collection, isolation and cultivation of microorganisms are anoxic environments. In this paper, an approach to this challenge was implemented within the European Union's MASE (Mars Analogues for Space Exploration) project. In this review paper, we aim to provide a set of methods for future field work and sampling campaigns. A number of anoxic environment based on characteristics that make them analogous to past and present locations on Mars were selected. They included anoxic sulphur-rich springs (Germany), the salt-rich Boulby Mine (UK), a lake in a basaltic context (Iceland), acidic sediments in the Rio Tinto (Spain), glacier samples (Austria) and permafrost samples (Russia and Canada). Samples were collected under strict anoxic conditions to be used for cultivation and genomic community analysis. Using the samples, a culturing approach was implemented to enrich anaerobic organisms using a defined medium that would allow for organisms to be grown under identical conditions in future physiological comparisons. Anaerobic microorganisms were isolated and deposited with the DSMZ (Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH) culture collection to make them available to other scientists. In MASE, the selected organisms are studied with respect to survival and growth under Mars relevant stresses. They are artificially fossilized and the resulting biosignatures studied and used to investigate the efficacy of life detection instrumentation for planetary missions. Some of the organisms belong to genera with medical and environmental importance such as Yersinia spp., illustrating how astrobiology field research can be used to increase the availability of microbial isolates for applied terrestrial purposes.

The Phoenix lander at Mars polar cap found appreciable levels of (per)chlorate salts, a mixture of perchlorate and chlorate salts of Ca, Fe, Mg and Na at levels of ~0.6% in regolith. These salts are highly hygroscopic and can form saturated brines through deliquescence, likely producing aqueous solutions with very low freezing points on Mars. To support planetary protection efforts, we have measured bacterial growth tolerance to (per)chlorate salts. Existing bacterial isolates from the Great Salt Plains of Oklahoma (NaCl-rich) and Hot Lake in Washington (MgSO4-rich) were tested in high concentrations of Mg, K and Na salts of chlorate and perchlorate. Strong growth was observed with nearly all of these salinotolerant isolates at 1% (~0.1 M) (per)chlorate salts, similar to concentrations observed in bulk soils on Mars. Growth in perchlorate salts was observed at concentrations of at least 10% (~1.0 M). Greater tolerance was observed for chlorate salts, where growth was observed to 2.75 M (>25%). Tolerance to K salts was greatest, followed by Mg salts and then Na salts. Tolerances varied among isolates, even among those within the same phylogenetic clade. Tolerant bacteria included genera that also are found in spacecraft assembly facilities. Substantial microbial tolerance to (per)chlorate salts is a concern for planetary protection since tolerant microbes contaminating spacecraft would have a greater chance for survival and proliferation, despite the harsh chemical conditions found near the surface of Mars.

Halophilic Archaea are known to tolerate multiple extreme conditions on Earth and have been proposed as models for astrobiology. In order to assess the importance of cold-adaptation of these microorganisms in surviving stratospheric conditions, we launched live, liquid cultures of two species, the mesophilic model Halobacterium sp. NRC-1 and the cold-adapted Antarctic isolate Halorubrum lacusprofundi ATCC 49239, on helium balloons. After return to Earth, the cold-adapted species showed nearly complete survival while the mesophilic species exhibited slightly reduced viability. Parallel studies found that the cold-adapted species was also better able to survive freezing and thawing in the laboratory. Genome-wide transcriptomic analysis was used to compare the two haloarchaea at optimum growth temperatures versus low temperatures supporting growth. The cold-adapted species displayed perturbation of a majority of genes upon cold temperature exposure, divided evenly between up-regulated and down-regulated genes, while the mesophile exhibited perturbation of only a fifth of its genes, with nearly two-thirds being down-regulated. These results underscore the importance of genetic responses of H. lacusprofundi to cold temperature for enhanced survival in the stratosphere.

The mean air temperature of the Icelandic interior is below 10 °C. However, we have previously observed 16S rDNA sequences associated with thermophilic lineages in Icelandic basalts. Measurements of the temperatures of igneous rocks in Iceland showed that solar insolation of these low albedo substrates achieved a peak surface temperature of 44.5 °C. We isolated seven thermophilic Geobacillus species from basalt with optimal growth temperatures of ~65 °C. The minimum growth temperature of these organisms was ~36 °C, suggesting that they could be active in the rock environment. Basalt dissolution rates at 40 °C were increased in the presence of one of the isolates compared to abiotic controls, showing its potential to be involved in active biogeochemistry at environmental temperatures. These data raise the possibility of transient active thermophilic growth in macroclimatically cold rocky environments, implying that the biogeographical distribution of active thermophiles might be greater than previously understood. These data show that temperatures measured or predicted over large scales on a planet are not in themselves adequate to assess niches available to extremophiles at micron scales.

Tests on cyanobacteria communities embedded in cryptobiotic crusts collected in hot and cold deserts on Earth were performed under Mars-like conditions. The simulations were realized as a survey, to find the best samples for future research. During the tests organisms have to resist Mars-like conditions such as atmospheric composition, pressure, variable humidity (saturated and dry conditions) and partly strong UV irradiation. Organisms were tested within their original habitat inside the crust. Nearly half of the cryptobiotic samples from various sites showed survival of a substantial part of their coexisting organisms. The survival in general depended more on the nature of the original habitat and type of the sample than on the different conditions they were exposed to. The best survival was observed in samples from United Arab Emirates (Jebel Ali, 25 km SW of Dubai town) and from Western Australia (near the South edge of Lake Barley), by taxa: Tolypothrix byssoidea, Gloeocapsopsis pleurocapsoides, Nostoc microscopicum, Leptolyngbya or Symploca sp. At both places in salty desert areas members of the Chenopodiaceae family dominated among the higher plants and in the cryptobiotic crust cyanobacterial taxa Tolypothrix was dominant. These organisms were all living in salty locations with dry conditions most of the year. Among them Tolypothrix, Gloeocapsopsis and Symploca sp. were tested in Mars simulation chambers for the first time. The results suggest that extremophiles should be tested with taken into account the context of their original microenvironment, and also the importance to analyse communities of microbes beside single organisms.

Astrobiology is a relatively recent scientific field that seeks to understand the origin and dynamics of life in the Universe. Several hypotheses have been proposed to explain life in the cosmic context throughout human history, but only now, technology has allowed many of them to be tested. Laboratory experiments have been able to show how chemical elements essential to life, such as carbon, nitrogen, oxygen and hydrogen combine in biologically important compounds. Interestingly, these compounds are ubiquitous. How these compounds were combined to the point of originating cells and complex organisms is still to be unveiled by science. However, our 4.5 billion years old Solar system appeared in a 10 billion years old Universe. Thus, simple cells such as micro-organisms may have had time to form in planets older than ours or in other suitable places in the Universe. One hypothesis related to the appearance of life on Earth is called panspermia, which predicts that microbial life could have been formed in the Universe billions of years ago, travelling between planets, and inseminating units of life that could have become more complex in habitable planets such as Earth. A project designed to test the viability of extremophile micro-organisms exposed to simulated extraterrestrial environments is in progress at the Carlos Chagas Filho Institute of Biophysics (UFRJ, Brazil) to test whether microbial life could withstand inhospitable environments. Radiation-resistant (known or novel ones) micro-organisms collected from extreme terrestrial environments have been exposed (at synchrotron accelerators) to intense radiation sources simulating Solar radiation, capable of emitting radiation in a few hours equivalent to many years of accumulated doses. The results obtained in these experiments reveal an interesting possibility of the existence of microbial life beyond Earth.

We present a summary of a three-year academic research proposal drafted during the Sao Paulo Advanced School of Astrobiology (SPASA) to prepare for upcoming observations of tidally locked planets orbiting M-dwarf stars. The primary experimental goal of the suggested research is to expose extremophiles from analogue environments to a modified space simulation chamber reproducing the environmental parameters of a tidally locked planet in the habitable zone of a late-type star. Here we focus on a description of the astronomical analysis used to define the parameters for this climate simulation.

Extremophiles are organisms adapted to grow at extreme ranges of environmental variables, such as high or low temperatures, acid or alkaline medium, high salt concentration, high pressures and so forth. Most extremophiles are micro-organisms that belong to the Archaea and Bacteria domains, and are widely spread across the world, which include the polar regions, volcanoes, deserts, deep oceanic sediments, hydrothermal vents, hypersaline lakes, acid and alkaline water bodies, and other extreme environments considered hostile to human life. Despite the tropical climate, Brazil has a wide range of ecosystems which include some permanent or seasonally extreme environments. For example, the Cerrado is a biome with very low soil pH with high Al+3 concentration, the mangroves in the Brazilian coast are anaerobic and saline, Pantanal has thousands of alkaline–saline lakes, the Caatinga arid and hot soils and the deep sea sediments in the Brazilian ocean shelf. These environments harbour extremophilic organisms that, coupled with the high natural biodiversity in Brazil, could be explored for different purposes. However, only a few projects in Brazil intended to study the extremophiles. In the frame of astrobiology, for example, these organisms could provide important models for defining the limits of life and hypothesize about life outside Earth. Brazilian microbiologists have, however, studied the extremophilic micro-organisms inhabiting non-Brazilian environments, such as the Antarctic continent. The experience and previous results obtained from the Brazilian Antarctic Program (PROANTAR) provide important results that are directly related to astrobiology. This article is a brief synopsis of the Brazilian experience in researching extremophiles, indicating the most important results related to astrobiology and some future perspectives in this area.

The survival strategies of extremophilic organisms in terrestrially stressed locations and habitats are critically dependent on the production of protective chemicals in response to desiccation, low wavelength radiation insolation, temperature and the availability of nutrients. The adaptation of life to these harsh prevailing conditions involves the control of the substratal geology; the interaction between the rock and the organisms is critical and the biological modification of the geological matrix plays a very significant role in the overall survival strategy. Identification of these biological and biogeological chemical molecular signatures in the geological record is necessary for the recognition of the presence of extinct or extant life in terrestrial and extraterrestrial scenarios. Raman spectroscopic techniques have been identified as valuable instrumentation for the detection of life extra-terrestrially because of the use of non-invasive laser-based excitation of organic and inorganic molecules, and molecular ions with high discrimination characteristics; the interactions effected between biological organisms and their environments are detectable through the molecular entities produced at the interfaces, for which the vibrational spectroscopic band signatures are unique. A very important attribute of Raman spectroscopy is the acquisition of molecular experimental data non-destructively without the need for chemical or mechanical pre-treatment of the specimen; this has been a major factor in the proposal for the adoption of Raman instrumentation on robotic landers and rovers for planetary exploration, particularly for the forthcoming European Space Agency (ESA)/National Aeronautics and Space Administration (NASA) ExoMars mission. In this paper, the merits of using Raman spectroscopy for the recognition of key molecular biosignatures from several terrestrial extremophile specimens will be illustrated. The data and specimens used in this presentation have been acquired from Arctic and Antarctic cold deserts and a meteorite crater, from which it will be possible to assess spectral data relevant for the detection of extra-terrestrial extremophilic life signatures.

Humankind's innate curiosity makes us wonder whether life is or was present on other planetary bodies such as Mars. The EuroGeoMars 2009 campaign was organized at the Mars Desert Research Station (MDRS) to perform multidisciplinary astrobiology research. MDRS in southeast Utah is situated in a cold arid desert with mineralogy and erosion processes comparable to those on Mars. Insight into the microbial community composition of this terrestrial Mars analogue provides essential information for the search for life on Mars: including sampling and life detection methodology optimization and what kind of organisms to expect. Soil samples were collected from different locations. Culture-independent molecular analyses directed at ribosomal RNA genes revealed the presence of all three domains of life (Archaea, Bacteria and Eukarya), but these were not detected in all samples. Spiking experiments revealed that this appears to relate to low DNA recovery, due to adsorption or degradation. Bacteria were most frequently detected and showed high alpha- and beta-diversity. Members of the Actinobacteria, Proteobacteria, Bacteroidetes and Gemmatimonadetes phyla were found in the majority of samples. Archaea alpha- and beta-diversity was very low. For Eukarya, a diverse range of organisms was identified, such as fungi, green algae and several phyla of Protozoa. Phylogenetic analysis revealed an extraordinary variety of putative extremophiles, mainly Bacteria but also Archaea and Eukarya. These comprised radioresistant, endolithic, chasmolithic, xerophilic, hypolithic, thermophilic, thermoacidophilic, psychrophilic, halophilic, haloalkaliphilic and alkaliphilic micro-organisms. Overall, our data revealed large difference in occurrence and diversity over short distances, indicating the need for high-sampling frequency at similar sites. DNA extraction methods need to be optimized to improve extraction efficiencies.

Mars simulation studies have in the past mainly investigated the effect of the simulation conditions such as UV radiation, low pressure and temperature on pure cultures and much has been learnt about the survival potential of sporeformers such as Bacillus subtilis. However, this approach has limitations as the studies only investigate the properties of a very limited number of microorganisms. In this paper we propose that Mars simulations should be carried out with complex microbial communities of Martian analogues such as permafrost or the deep biosphere. We also propose that samples from these environments should be studied by a number of complementary methods and claim that these methods in combination can provide a comprehensive picture of how imposed Martian conditions affect the microbial community and in particular the survival of its constituents – microbes as well as biological material in general. As an interesting consequence this approach can lead to the isolation of bacteria, which are more recalcitrant to the imposed Martian conditions than the pure cultures that have previously been studied.

The Antarctic psychrophilic ciliate Euplotes focardii manifests a dramatic reduction in the activation of its hsp70 gene in response to a heat-shock, while oxidative and chemical stresses activate the transcription of this gene to appreciable extents. To investigate the genetic causes of this eccentric behaviour of E. focardii in the hsp70 gene transcription activation, we carried out a comparative structural analysis of this gene between E. focardii and another Antarctic Euplotes, E. nobilii, which manifests a psychrotrophic behaviour and an inducible thermal response. No substantial difference was detected in the organization of the hsp70 5' promoter region, both species bearing canonical regulatory cis-acting elements deputed to bind transcriptional trans-activating factors. Adenine-rich elements favouring mRNA degradation were instead detected in the hsp70 3' regulatory region of E. nobilii, but not in that of E. focardii. These observations lend further support to the hypothesis that the causes of the Euplotes focardii unresponsiveness to thermal stress resides in some structural, or functional modifications of transcriptional trans-activating factors.