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The permanent ice cover of Lake Vida (Antarctica) encapsulates an extreme cryogenic brine ecosystem (−13°C; salinity, 200). This aphotic ecosystem is anoxic and consists of a slightly acidic (pH 6.2) sodium chloride-dominated brine. Expeditions in 2005 and 2010 were conducted to investigate the biogeochemistry of Lake Vida's brine system. A phylogenetically diverse and metabolically active Bacteria dominated microbial assemblage was observed in the brine. These bacteria live under very high levels of reduced metals, ammonia, molecular hydrogen (H 2 ), and dissolved organic carbon, as well as high concentrations of oxidized species of nitrogen (i.e., supersaturated nitrous oxide and ∼1 mmol·L −1 nitrate) and sulfur (as sulfate). The existence of this system, with active biota, and a suite of reduced as well as oxidized compounds, is unusual given the millennial scale of its isolation from external sources of energy. The geochemistry of the brine suggests that abiotic brine-rock reactions may occur in this system and that the rich sources of dissolved electron acceptors prevent sulfate reduction and methanogenesis from being energetically favorable. The discovery of this ecosystem and the in situ biotic and abiotic processes occurring at low temperature provides a tractable system to study habitability of isolated terrestrial cryoenvironments (e.g., permafrost cryopegs and subglacial ecosystems), and is a potential analog for habitats on other icy worlds where water-rock reactions may cooccur with saline deposits and subsurface oceans.astrobiology | geomicrobiology | microbial ecology | extreme environment T he observation of microbes surviving and growing in a variety of icy systems on Earth has expanded our understanding of how life pervades, functions, and persists under challenging conditions (e.g., refs. 1-3). Studies of the physical characteristics, the geochemical properties, and microbes in ice (triple point junctions, brine channels, gas bubbles) have also changed our perceptions of the environments that may contain traces of, or even sustain, life beyond Earth [e.g., Mars (4), Europa (5), and Enceladus (6)].Solute depression of ice crystal formation or solar radiation melting of water ice are key processes that provide liquid waterthe key solvent that makes life possible-within icy systems. Microbial communities in these conditions are often sustained by a supply of energy that ultimately derives from photosynthesis (present or past). The understanding of ecosystems based on energy sources other than the Sun comes mainly from realms where hydrothermal processes have provided reduced compounds necessary to fuel chemosynthetically driven ecosystems. Methane derived from thermogenic or biogenic sources can also support microbial communities in deep sea (7) and high arctic cold saline seeps (8). More recently, discoveries of life and associated processes in deep terrestrial subsurface ecosystems (9) provide compelling evidence of subsurface life that in some cases is fueled by nonphotosynthetic processes. Ou...

The timing and causes of the transition to an icehouse climate in the Late Ordovician are controversial. Results of an integrated ␦ 13 C and sequence stratigraphic analysis in Nevada show that in the Late Ordovician Chatfieldian Stage (mid-Caradoc) a positive ␦ 13 C excursion in the upper part of the Copenhagen Formation was closely followed by a regressive event evidenced within the prominent Eureka Quartzite. The Chatfieldian ␦ 13 C excursion is known globally and interpreted to record enhanced organic carbon burial, which lowered atmospheric pCO 2 to levels near the threshold for ice buildup in the Ordovician greenhouse climate. The subsequent regressive event in central Nevada, previously interpreted as part of a regional tectonic adjustment, is here attributed in part to sea-level drawdown from the initiation of continental glaciation on Gondwana. This drop in sea level-which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of pCO 2 by suppressing shelf-carbonate production-signals the transition to a Late Ordovician icehouse climate ϳ10 m.y. before the widespread Hirnantian glacial maximum at the end of the Ordovician.

Cytosine methylation at CpG dinucleotides is a critical epigenetic modification of mammalian genomes. CpG binding protein (CGBP) exhibits a unique DNA-binding specificity for unmethylated CpG motifs and is essential for early murine development. Embryonic stem cell lines deficient for CGBP were generated to further examine CGBP function. CGBP ؊/؊ cells are viable but show an increased rate of apoptosis and are unable to achieve in vitro differentiation following removal of leukemia inhibitory factor from the growth media. Instead, CGBP ؊/؊ embryonic stem cells remain undifferentiated as revealed by persistent expression of the pluripotent markers Oct4 and alkaline phosphatase. CGBP ؊/؊ cells exhibit a 60 to 80% decrease in global cytosine methylation, including hypo-methylation of repetitive elements, single-copy genes, and imprinted genes. Total DNA methyltransferase activity is reduced by 30 to 60% in CGBP ؊/؊ cells, and expression of the maintenance DNA methyltransferase 1 protein is similarly reduced. However, de novo DNA methyltransferase activity is normal. Nearly all aspects of the pleiotropic CGBP ؊/؊ phenotype are rescued by introduction of a CGBP expression vector. Hence, CGBP is essential for normal epigenetic modification of the genome by cytosine methylation and for cellular differentiation, consistent with the requirement for CGBP during early mammalian development.The CpG dinucleotide is an important regulatory component of mammalian genomes. The cytosine of this dinucleotide serves as the target for methylation by DNA methyltransferase (Dnmt) enzymes, which functions as a critical epigenetic modification of DNA. Methylated DNA is correlated with heterochromatin and transcriptionally inactive genes, while actively expressed genes are generally hypomethylated (58). Cytosine methylation may also represent a defense mechanism to silence parasitic repetitive DNA elements present in mammalian genomes (72,78). In addition, cytosine methylation is involved in the processes of genomic imprinting, in which paternal and maternal alleles of a gene exhibit distinct patterns of cytosine methylation and expression (65), and X chromosome inactivation, in which one X chromosome in each cell of a female becomes irreversibly inactivated during early development (52). The CpG dinucleotide is underrepresented in mammalian genomes (5 to 10% of the expected frequency), presumably due to the propensity of 5-methylcytosine to undergo spontaneous deamination to form thymine (8). Approximately 50% of human and mouse genes reside near unmethylated CpG islands, which contain the statistically expected frequency of CpG dinucleotides.Global cytosine methylation patterns inherited from gametes are erased during early embryogenesis (morula), followed by a wave of de novo DNA methylation in the blastocyst upon implantation (44). Dnmt3a and Dnmt3b are de novo methyltransferases that preferentially recognize unmethylated CpG motifs (49), while Dnmt1 is a maintenance methyltransferase that recognizes hemimethylated DNA (5), the imme...

A rise in atmospheric O 2 has been linked to the Cambrian explosion of life. For the plankton and animal radiation that began some 40 million yr later and continued through much of the Ordovician (Great Ordovician Biodiversification Event), the search for an environmental trigger(s) has remained elusive. Here we present a carbon and sulfur isotope mass balance model for the latest Cambrian time interval spanning the globally recognized Steptoean Positive Carbon Isotope Excursion (SPICE) that indicates a major increase in atmospheric O 2 . We estimate that this organic carbon and pyrite burial event added approximately 19 × 10 18 moles of O 2 to the atmosphere (i.e., equal to change from an initial starting point for O 2 between 10–18% to a peak of 20–28% O 2 ) beginning at approximately 500 million years. We further report on new paired carbon isotope results from carbonate and organic matter through the SPICE in North America, Australia, and China that reveal an approximately 2‰ increase in biological fractionation, also consistent with a major increase in atmospheric O 2 . The SPICE is followed by an increase in plankton diversity that may relate to changes in macro- and micronutrient abundances in increasingly oxic marine environments, representing a critical initial step in the trophic chain. Ecologically diverse plankton groups could provide new food sources for an animal biota expanding into progressively more ventilated marine habitats during the Ordovician, ultimately establishing complex ecosystems that are a hallmark of the Great Ordovician Biodiversification Event.