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In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets—Biosignatures—Habitability markers—Photosynthesis—Planetary surfaces—Atmospheres—Spectroscopy—Cryptic biospheres—False positives. Astrobiology 18, 663–708.

The Rehai and Ruidian geothermal fields, located in Tengchong County, Yunnan Province, China, host a variety of geochemically distinct hot springs. In this study, we report a comprehensive, cultivation-independent census of microbial communities in 37 samples collected from these geothermal fields, encompassing sites ranging in temperature from 55.1 to 93.6°C, in pH from 2.5 to 9.4, and in mineralogy from silicates in Rehai to carbonates in Ruidian. Richness was low in all samples, with 21–123 species-level OTUs detected. The bacterial phylum Aquificae or archaeal phylum Crenarchaeota were dominant in Rehai samples, yet the dominant taxa within those phyla depended on temperature, pH, and geochemistry. Rehai springs with low pH (2.5–2.6), high temperature (85.1–89.1°C), and high sulfur contents favored the crenarchaeal order Sulfolobales, whereas those with low pH (2.6–4.8) and cooler temperature (55.1–64.5°C) favored the Aquificae genus Hydrogenobaculum. Rehai springs with neutral-alkaline pH (7.2–9.4) and high temperature (>80°C) with high concentrations of silica and salt ions (Na, K, and Cl) favored the Aquificae genus Hydrogenobacter and crenarchaeal orders Desulfurococcales and Thermoproteales. Desulfurococcales and Thermoproteales became predominant in springs with pH much higher than the optimum and even the maximum pH known for these orders. Ruidian water samples harbored a single Aquificae genus Hydrogenobacter, whereas microbial communities in Ruidian sediment samples were more diverse at the phylum level and distinctly different from those in Rehai and Ruidian water samples, with a higher abundance of uncultivated lineages, close relatives of the ammonia-oxidizing archaeon “Candidatus Nitrosocaldus yellowstonii”, and candidate division O1aA90 and OP1. These differences between Ruidian sediments and Rehai samples were likely caused by temperature, pH, and sediment mineralogy. The results of this study significantly expand the current understanding of the microbiology in Tengchong hot springs and provide a basis for comparison with other geothermal systems around the world.

We report here a test of the hypothesis that the extent of organic matter preservation in continental margin sediments is controlled by the average period accumulating particles reside in oxic porewater immediately beneath the water/sediment interface. Oxygen penetration depths, organic element compositions, and mineral surface areas were determined for 16 sediment cores collected along an offshore transect across the Washington continental shelf, slope, and adjacent Cascadia Basin. Individual amino acid, sugar, and pollen distributions were analyzed for a 11 to 12 cm horizon from each core, and 14 C-based sediment accumulation rates and stable carbon isotope compositions were determined from depth profiles within a subset of six cores from representative sites. Sediment accumulation rates decreased, and dissolved O 2 penetration depths increased offshore along the sampling transect. As a result, oxygen exposure times (OET) increased seaward from decades (mid-shelf and upper slope) to more than a thousand years (outer Cascadia Basin). Organic contents and compositions were essentially constant within individual sediment cores but varied consistently with location. In particular, organic carbon/surface area ratios decreased progressively offshore and with increasing OET. Three independent compositional parameters demonstrated that the remnant organic matter in farther offshore sediments is more degraded. Both concentration and compositional patterns indicated that sedimentary organic matter exhibits a distinct and reproducible ''oxic effect.'' OET helps integrate and explain organic matter preservation in accumulating continental margin sediments and hence provides a useful tool for assessing transfer of organic matter from the biosphere to the geosphere.

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Benthic carbon oxidation rates and carbon burial rates have been determined for two contrasting continental margins and their sum has been used as an estimate of carbon rain rate to the sediments. On the Washington State margin, where the water column is oxic, rain rates at 100 m were about 15 to 20 mmoles C m Ϫ2 d Ϫ1 and they decreased with increasing water depth to values of near 3 mmoles C m Ϫ2 d Ϫ1 at 1,000 m. The rain rate estimates, C R , were described by a power function, C R ϭ C R 100m (z/100) Ϫ␣ , with an attenuation coefficient, ␣, of 0.93 (C R 100m ϭ 16.2, r 2 ϭ 0.89). This attenuation rate is similar to numerous others previously reported for various oceanic areas. In contrast, off northwest Mexico, where the water column is oxygen deficient between 180 and 700 m, rain rates at 100 m were considerably less, about 7.5 mmoles C m Ϫ2 d Ϫ1 , but rain rates at 1,000 m were similar to those off Washington. Thus, the attenuation coefficient for the Mexican margin was significantly lower, ␣ ϭ 0.36 (C R 100m ϭ 7.4, r 2 ϭ 0.77). Off Mexico, the rain rates estimated from sedimentary parameters were corroborated by values determined directly from sediment-trap deployments. The generally smaller rain rates off Mexico are probably due to the lower primary production, hence lower initial supply. The lower attenuation rate, however, is hypothesized to result from a decreased oxidation rate of the sinking flux within the oxygen-deficient zone relative to a more typical oxic water column.The transition from the last glacial to the present interglacial was accompanied by an 85 ppm increase in the CO 2 content of the atmosphere. It is currently thought that much of this change was due to movement of CO 2 between the deep ocean and the atmosphere, although the exact mechanism is still under debate (see Broecker and Henderson 1998 for discussion). Three important carbon cycle processes that determine CO 2 uptake from the atmosphere and subsequent transfer and storage in the ocean interior are (1) photosynthetic carbon fixation in the euphotic zone, (2) transfer of some fraction of this fixed carbon out of the euphotic zone as export production, and (3) permanent carbon burial in marine sediments. As the export flux falls through the oceans, it is biologically oxidized such that the sinking flux of organic matter decreases progressively with increasing depth leaving only a small fraction available for burial. The manner in which the export flux, or carbon rain rate, decreases with depth is important for two reasons. First, it determines the quantity and time scale at which carbon is sequestered as CO 2 in the deep ocean, i.e., the shallower the regeneration depth the shorter, in general, the sequestration time. Second, greater attenuation rates result in proportionally less of the initial carbon flux reaching the sediments, which has attendant implications for benthic ecology and 1 Current address: Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901. Acknow...

Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth's surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solarto-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plantmicrobe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term. (4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes. (5) Biology shapes the topography of the Critical Zone. (6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws. (7) Rising global temperatures will increase carbon losses from the Critical Zone. (8) Rising atmospheric P CO2 will increase rates and extents of mineral weathering in soils. (9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering. (10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer Ó 2010 Blackwell Publishing Ltd 1 Geobiology (2011) DOI: 10.1111DOI: 10. /j.1472DOI: 10. -4669.2010 timescales. (12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

We used electrospray‐ionization mass spectrometry (ESI‐MS) to characterize, at the compound level, dissolved organic matter (DOM) composition and bioavailability in two streams. There was considerable consistency in the composition of the DOM between the two streams (unit mass resolution): ≫70% of the masses detected occurred in both streams. Approximately 40–50% of the bulk dissolved organic carbon in the stream water was bioavailable during a 12‐d microbial decomposition experiment. ESI‐MS compound level analysis identified which masses were used, which were not, and their patterns of utilization. In both streams, ~40% of the masses decreased in concentration, ~55% did not change, and ≪5% increased. Despite the complex system (≫1,500 DOM compounds and a natural consortia of bacteria), there was a high degree of similarity in which masses were used and the amount of each mass used between replicate flasks for a stream. There was also good agreement between the two streams in which masses were used and the amount of each mass used. This suggests that the selection by the microbial consortia of organic compounds in the complex and heretofore largely uncharacterized DOM pool is repeatable and, therefore, ultimately predictable.