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Microbial iron reduction is a widespread and ancient metabolism on Earth, and may plausibly support microbial life on Mars and beyond. Yet, the extreme limits of this metabolism are yet to be defined. To investigate this, we surveyed the recorded limits to microbial iron reduction in a wide range of characterized iron-reducing microorganisms (n  = 141), with a focus on pH and temperature. We then calculated Gibbs free energy of common microbially mediated iron reduction reactions across the pH–temperature habitability space to identify thermodynamic limits. Comparing predicted and observed limits, we show that microbial iron reduction is generally reported at extremes of pH or temperature alone, but not when these extremes are combined (with the exception of a small number of acidophilic hyperthermophiles). These patterns leave thermodynamically favourable combinations of pH and temperature apparently unoccupied. The empty spaces could be explained by experimental bias, but they could also be explained by energetic and biochemical limits to iron reduction at combined extremes. Our data allow for a review of our current understanding of the limits to microbial iron reduction at extremes and provide a basis to test more general hypotheses about the extent to which biochemistry establishes the limits to life. [ABSTRACT FROM AUTHOR]

Four facultative anaerobic and two obligate anaerobic bacteria were isolated from extreme environments (deep subsurface halite mine, sulfidic anoxic spring, mineral-rich river) in the frame MASE (Mars Analogues for Space Exploration) project. The isolates were investigated under anoxic conditions for their survivability after desiccation up to 6 months and their tolerance to ionizing radiation up to 3000 Gy. The results indicated that tolerances to both stresses are strain-specific features. Yersinia intermedia MASE-LG-1 showed a high desiccation tolerance but its radiation tolerance was very low. The most radiation-tolerant strains were Buttiauxella sp. MASE-IM-9 and Halanaerobium sp. MASE-BB-1. In both cases, cultivable cells were detectable after an exposure to 3 kGy of ionizing radiation, but cells only survived desiccation for 90 and 30 days, respectively. Although a correlation between desiccation and ionizing radiation resistance has been hypothesized for some aerobic microorganisms, our data showed that there was no correlation between tolerance to desiccation and ionizing radiation, suggesting that the physiological basis of both forms of tolerances is not necessarily linked. In addition, these results indicated that facultative and obligate anaerobic organisms living in extreme environments possess varied species-specific tolerances to extremes. [ABSTRACT FROM AUTHOR]

The surface of the early Earth was probably subjected to a higher flux of ultraviolet (UV) radiation than today. UV radiation is known to severely damage DNA and other key molecules of life. Using a liquid culture and a rock analogue system, we investigated the interplay of protective and deleterious effects of iron oxides under UV radiation on the viability of the model organism, Bacillus subtilis. In the presence of hydrogen peroxide, there exists a fine balance between iron oxide's protective effects against this radiation and its deleterious effects caused by Photo-Fenton reactions. The maximum damage was caused by a concentration of hematite of ~1 mg/mL. Concentrations above this confer increasing protection by physical blockage of the UV radiation, concentrations below this cause less effective UV radiation blockage, but also a correspondingly less effective Photo-Fenton reaction, providing an overall advantage. These results show that on anoxic worlds, surface habitability under a high UV flux leaves life precariously poised between the beneficial and deleterious effects of iron oxides. These results have relevance to the Archean Earth, but also the habitability of the Martian surface, where high levels of UV radiation in combination with iron oxides and hydrogen peroxide can be found. [ABSTRACT FROM AUTHOR]

We have applied an image-based approach combining epifluorescence microscopy, electron microscopy and nanoscale secondary ion mass spectrometry (nano SIMS) with stable isotope probing to examine directly the characteristic cellular features involved in the expression of the cold phenotype in the Antarctic bacterium Clostridium psychrophilum exposed to a temperature range from +5 to −15 °C under anoxic conditions. We observed dramatic morphological changes depending on temperature. At temperatures below −10 °C, cell division was inhibited and consequently filamentous growth predominated. Bacterial cells appeared surrounded by a remarkably thick cell wall and a capsule formed of long exopolysaccharide fibres. Moreover, bacteria were entirely embedded within a dense extracellular matrix, suggesting a role both in cryo-protection and in the cycling of nutrients and genetic material. Strings of extracellular DNA, transient cell membrane permeability and release of membrane vesicles were observed that suggest that evolution via transfer of genetic material may be especially active under frozen conditions. While at −5 °C, the bacterial population was metabolically healthy, at temperatures below −10 °C, most cells showed no sign of active metabolism or the metabolic flux was extremely slowed down; instead of being consumed, carbon was accumulated and stored in intracellular granules as in preparation for a long-term survival. [ABSTRACT FROM AUTHOR]

Silicate weathering improves soils by releasing bioessential nutrients from the bedrock to the soil ecosystem. However, whether bacteria are capable of inhabiting subsurface critical zones (zone of active rock weathering), and their role therein, are unknown. Next-generation sequencing and community fingerprinting permitted us to characterize communities from an Icelandic critical zone environment. Communities were compared with respect to physico-chemical properties of the environment to determine the factors influencing bacterial diversity. We showed that land coverage influenced critical zone communities. Analysis of tree-covered site ( TCS) soils exhibited high cell densities ( TCS = 2.25 × 107 g−1), whereas lichen- and moss-covered sites ( LMS) had lower cell densities ( LMS = 1.06 × 107 cells g−1), thought to be a result of the organic carbon produced by the trees. Differences in the bacterial community were observed from the abundance of 16S rRNA gene sequences affiliated with Acidobacteria and Proteobacteria, with TCS possessing higher abundances of Proteobacteria [no of sequences: LMS = 1526 (±497); TCS = 2214 (±531)], specifically Alpha- and Betaproteobacteria, and lower Acidobacteria numbers [no of sequences: LMS = 1244 (±338); TCS = 598 (±140)]. Diversity indices and 16S rRNA gene rarefaction showed that communities from TCS soils had lower α-diversity than sites without, indicative of specialized communities at sites with root-forming plants. [ABSTRACT FROM AUTHOR]

The article discusses studies which investigated microbial metabolisms on planet Mars. Topics explored include the potential implications of the hydrated mineralogical assemblages observed on the planet's surface, the availability of chemical compounds on the planet such as iron and sulphur which may contribute to microbial metabolism, and the way reduction-oxidation processes allow microorganisms to produce energy.

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The article focuses on the Boulby International Subsurface Astrobiology Laboratory (BISAL) in Saltburn-on-the-Sea in North Yorkshire, England. Topics discussed include BISAL's concentration on in-situ deep subsurface astrobiology and space technology development, its location in the Boulby potash mine, and the study of environments with analogues to the planet Mars. Also mentioned are capabilities to be provided for the study of the Earth's biosphere and the testing of space technology.

Charles S Cockell reviews the eventful history and exciting future of astrobiology: the collaboration between astronomers and biologists. [ABSTRACT FROM AUTHOR]

Charles Cockell and colleagues consider what it takes to establish and maintain an experiment that lasts for decades – or even for centuries. [ABSTRACT FROM AUTHOR]

Kian Raafat, Jordan Burnett, Thomas Chapman and Charles S Cockell ask: what's different about mining off Earth?Asteroids and planets are potentially valuable mineral resources, but finding and exploiting them will be a challenge. Kian Raafat, Jordan Burnett, Thomas Chapman and Charles S Cockell ask: what's different about mining off Earth? [ABSTRACT FROM AUTHOR]

Meeting report Ian Crawford and Charles Cockell report on an RAS Specialist Discussion Meeting held on Friday 14 May 2010. [ABSTRACT FROM AUTHOR]