DNA from low-biodiversity fracture water collected at 2.8-kilometer depth in a South African gold mine was sequenced and assembled into a single, complete genome. This bacterium, Candidatus Desulforudis audaxviator , composes >99.9% of the microorganisms inhabiting the fluid phase of this particular fracture. Its genome indicates a motile, sporulating, sulfate-reducing, chemoautotrophic thermophile that can fix its own nitrogen and carbon by using machinery shared with archaea. Candidatus Desulforudis audaxviator is capable of an independent life-style well suited to long-term isolation from the photosphere deep within Earth's crust and offers an example of a natural ecosystem that appears to have its biological component entirely encoded within a single genome.

Bacterial nanowires are extracellular appendages that have been suggested as pathways for electron transport in phylogenetically diverse microorganisms, including dissimilatory metal-reducing bacteria and photosynthetic cyanobacteria. However, there has been no evidence presented to demonstrate electron transport along the length of bacterial nanowires. Here we report electron transport measurements along individually addressed bacterial nanowires derived from electron-acceptor–limited cultures of the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1. Transport along the bacterial nanowires was independently evaluated by two techniques: ( i ) nanofabricated electrodes patterned on top of individual nanowires, and ( ii ) conducting probe atomic force microscopy at various points along a single nanowire bridging a metallic electrode and the conductive atomic force microscopy tip. The S. oneidensis MR-1 nanowires were found to be electrically conductive along micrometer-length scales with electron transport rates up to 10 9 /s at 100 mV of applied bias and a measured resistivity on the order of 1 Ω·cm. Mutants deficient in genes for c -type decaheme cytochromes MtrC and OmcA produce appendages that are morphologically consistent with bacterial nanowires, but were found to be nonconductive. The measurements reported here allow for bacterial nanowires to serve as a viable microbial strategy for extracellular electron transport.

Banded iron formations (BIFs) are prominent sedimentary deposits of the Precambrian, but despite a century of endeavor, the mechanisms of their deposition are still unresolved. Interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation, in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy. On the basis of chemical analyses from BIF units of the 2.5 Ga Hamersley Group, Western Australia, we show here that even during periods of maximum iron precipitation, most, if not all, of the iron in BIFs could be precipitated by iron-oxidizing bacteria in cell densities considerably less than those found in modern Fe-rich aqueous environments. Those ancient microorganisms would also have been easily supported by the concentrations of nutrients (P) and trace metals (V, Mn, Co, Zn, and Mo) found within the same iron-rich bands. These calculations highlight the potential importance of early microbial activity on ancient metal cycling.

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1 2 It has long been suggested that hydrothermal systems might have provided habitats for the origin 3 and evolution of early life on Earth, and possibly other planets such as Mars. In this contribution 4 we show that most impact events that result in the formation of complex impact craters (i.e., >2-5 4 and >5-10 km diameter on Earth and Mars, respectively) are potentially capable of generating 6 a hydrothermal system. Consideration of the impact cratering record on Earth suggests that the 7 presence of an impact crater lake is critical for determining the longevity and size of the 8 hydrothermal system. We show that there are six main locations within and around impact 9 craters on Earth where impact-generated hydrothermal deposits can form: 1) crater-fill impact 10 melt rocks and melt-bearing breccias; 2) interior of central uplifts; 3) outer margin of central 11 uplifts; 4) impact ejecta deposits; 5) crater rim region; and 6) post-impact crater lake sediments. 12We suggest that these six locations are applicable to Mars as well. Evidence for impact-13 generated hydrothermal alteration ranges from discrete vugs and veins to pervasive alteration 14 depending on the setting and nature of the system. A variety of hydrothermal minerals have been 15 documented in terrestrial impact structures and these can be grouped into three broad categories: 16(1) hydrothermally-altered target-rock assemblages; (2) primary hydrothermal minerals 17 precipitated from solutions; and (3) secondary assemblages formed by the alteration of primary 18 hydrothermal minerals. Target lithology and the origin of the hydrothermal fluids strongly 19 influences the hydrothermal mineral assemblages formed in these post-impact hydrothermal 20systems. There is a growing body of evidence for impact-generated hydrothermal activity on 21 Mars; although further detailed studies using high-resolution imagery and multispectral 22 information are required. Such studies have only been done in detail for a handful of Martian 23 4 craters. The best example so far is from Toro Crater (Marzo et al., 2010). We also present new 1 evidence for impact-generated hydrothermal deposits within an unnamed ~32-km diameter crater 2 ~ 350 km away from Toro and within the larger Holden Crater. Synthesizing observations of 3 impact craters on Earth and Mars, we suggest that if there was life on Mars early in its history, 4 then hydrothermal deposits associated with impact craters may provide the best, and most 5 numerous, opportunities for finding preserved evidence for life on Mars. Moreover, 6hydrothermally altered and precipitated rocks can provide nutrients and habitats for life long 7 after hydrothermal activity has ceased. 8 5 1

Planococcus halocryophilus strain Or1, isolated from high Arctic permafrost, grows and divides at À 15 1C, the lowest temperature demonstrated to date, and is metabolically active at À 25 1C in frozen permafrost microcosms. To understand how P. halocryophilus Or1 remains active under the subzero and osmotically dynamic conditions that characterize its native permafrost habitat, we investigated the genome, cell physiology and transcriptomes of growth at À 15 1C and 18% NaCl compared with optimal (25 1C) temperatures. Subzero growth coincides with unusual cell envelope features of encrustations surrounding cells, while the cytoplasmic membrane is significantly remodeled favouring a higher ratio of saturated to branched fatty acids. Analyses of the 3.4 Mbp genome revealed that a suite of cold and osmotic-specific adaptive mechanisms are present as well as an amino acid distribution favouring increased flexibility of proteins. Genomic redundancy within 17% of the genome could enable P. halocryophilus Or1 to exploit isozyme exchange to maintain growth under stress, including multiple copies of osmolyte uptake genes (Opu and Pro genes). Isozyme exchange was observed between the transcriptome data sets, with selective upregulation of multi-copy genes involved in cell division, fatty acid synthesis, solute binding, oxidative stress response and transcriptional regulation. The combination of protein flexibility, resource efficiency, genomic plasticity and synergistic adaptation likely compensate against osmotic and cold stresses. These results suggest that non-spore forming P. halocryophilus Or1 is specifically suited for active growth in its Arctic permafrost habitat (ambient temp. B À 16 1C), indicating that such cryoenvironments harbor a more active microbial ecosystem than previously thought.

Carbon dioxide (CO2) is sequestered through the weathering and subsequent mineralization of the chrysotile mine tailings at Clinton Creek, Yukon Territory, and Cassiar, British Columbia, Canada. Accelerated weathering is attributed to a dramatic increase in surface area, which occurs during the milling of ore. We provide a detailed account of the natural process of carbon trapping and storage as it occurs at Clinton Creek and Cassiar, including mineralogy, modes of occurrence, methods of formation for carbonate alteration, light stable isotope geochemistry, and radiocarbon analysis. Powder X-ray diffraction data were used to identify weathering products as the hydrated magnesium carbonate minerals nesquehonite, and less commonly lansfordite [MgCO3⋅5H2O]. Textural relationships suggest that carbonate precipitates formed in situ after milling and deposition of tailings. Samples of efflorescent nesquehonite are characterized by δ 13 C values between 6.52 and 14.36 per mil, δ 18 O values between 20.93 and 26.62 per mil, and F 14 C values (fraction of modern carbon) between 1.072 and 1.114, values which are consistent with temperature-dependent fractionation of modern atmospheric CO2 during mineralization. Samples of dypingite ± hydromagnesite collected from within 0.2 m of the tailings surface give δ 13 C values between -1.51 and +10.02 per mil, δ 18 O values between +17.53 and +28.40 per mil, and F 14 C values between 1.026 and 1.146, which suggests precipitation from modern atmospheric CO2 in a soil-like environment. Field observations and isotopic data suggest that hydrated magnesium carbonate minerals formed in two environments. Nesquehonite formed in an evaporative environment on the surface of tailings piles, and dypingite and hydromagnesite formed in the subsurface environment with characteristics similar to soil carbonate. In both cases, these minerals †

Anaerobic oxidation of methane (AOM) is a major biological process that reduces global methane emission to the atmosphere. Anaerobic methanotrophic archaea (ANME) mediate this process through the coupling of methane oxidation to different electron acceptors, or in concert with a syntrophic bacterial partner. Recently, ANME belonging to the archaeal family Methanoperedenaceae (formerly known as ANME-2d) were shown to be capable of AOM coupled to nitrate and iron reduction. Here, a freshwater sediment bioreactor fed with methane and Mn(IV) oxides (birnessite) resulted in a microbial community dominated by two novel members of the Methanoperedenaceae, with biochemical profiling of the system demonstrating Mn(IV)-dependent AOM. Genomic and transcriptomic analyses revealed the expression of key genes involved in methane oxidation and several shared multiheme c-type cytochromes (MHCs) that were differentially expressed, indicating the likely use of different extracellular electron transfer pathways. We propose the names "Candidatus Methanoperedens manganicus" and "Candidatus Methanoperedens manganireducens" for the two newly described Methanoperedenaceae species. This study demonstrates the ability of members of the Methanoperedenaceae to couple AOM to the reduction of Mn (IV) oxides, which suggests their potential role in linking methane and manganese cycling in the environment. Etymology "Candidatus Methanoperedens manganicus" sp. nov "Candidatus Methanoperedens manganireducens" sp. nov "Candidatus Methanoperedens manganicus": man.