Potential role for microbial ureolysis in the rapid formation of carbonate tufa mounds.

Modern carbonate tufa towers in the alkaline (~pH 9.5) Big Soda Lake (BSL), Nevada, exhibit rapid precipitation rates (exceeding 3 cm/year) and host diverse microbial communities. Geochemical indicators reveal that carbonate precipitation is, in part, promoted by the mixing of calcium-rich groundwater and carbonate-rich lake water, such that a microbial role for carbonate precipitation is unknown. Here, we characterize the BSL microbial communities and evaluate their potential effects on carbonate precipitation that may influence fast carbonate precipitation rates of the active tufa mounds of BSL. Small subunit rRNA gene surveys indicate a diverse microbial community living endolithically, in interior voids, and on tufa surfaces. Metagenomic DNA sequencing shows that genes associated with metabolisms that are capable of increasing carbonate saturation (e.g., photosynthesis, ureolysis, and bicarbonate transport) are abundant. Enzyme activity assays revealed that urease and carbonic anhydrase, two microbial enzymes that promote carbonate precipitation, are active in situ in BSL tufa biofilms, and urease also increased calcium carbonate precipitation rates in laboratory incubation analyses. We propose that, although BSL tufas form partially as a result of water mixing, tufa-inhabiting microbiota promote rapid carbonate authigenesis via ureolysis, and potentially via bicarbonate dehydration and CO2 outgassing by carbonic anhydrase. Microbially induced calcium carbonate precipitation in BSL tufas may generate signatures preserved in the carbonate microfabric, such as stromatolitic layers, which could serve as models for developing potential biosignatures on Earth and elsewhere.

Microscale Biosignatures and Abiotic Mineral Authigenesis in Little Hot Creek, California.

Hot spring environments can create physical and chemical gradients favorable for unique microbial life. They can also include authigenic mineral precipitates that may preserve signs of biological activity on Earth and possibly other planets. The abiogenic or biogenic origins of such precipitates can be difficult to discern, therefore a better understanding of mineral formation processes is critical for the accurate interpretation of biosignatures from hot springs. Little Hot Creek (LHC) is a hot spring complex located in the Long Valley Caldera, California, that contains mineral precipitates composed of a carbonate base (largely submerged) topped by amorphous silica (largely emergent). The precipitates occur in close association with microbial mats and biofilms. Geological, geochemical, and microbiological data are consistent with mineral formation via degassing and evaporation rather than direct microbial involvement. However, the microfabric of the silica portion is stromatolitic in nature (i.e., wavy and finely laminated), suggesting that abiogenic mineralization has the potential to preserve textural biosignatures. Although geochemical and petrographic evidence suggests the calcite base was precipitated via abiogenic processes, endolithic microbial communities modified the structure of the calcite crystals, producing a textural biosignature. Our results reveal that even when mineral precipitation is largely abiogenic, the potential to preserve biosignatures in hot spring settings is high. The features found in the LHC structures may provide insight into the biogenicity of ancient Earth and extraterrestrial rocks.

2D/3D Microanalysis by Energy Dispersive X-ray Absorption Spectroscopy Tomography.

X-ray spectroscopic techniques have proven to be particularly useful in elucidating the molecular and electronic structural information of chemically heterogeneous and complex micro- and nano-structured materials. However, spatially resolved chemical characterization at the micrometre scale remains a challenge. Here, we report the novel hyperspectral technique of micro Energy Dispersive X-ray Absorption Spectroscopy (µED-XAS) tomography which can resolve in both 2D and 3D the spatial distribution of chemical species through the reconstruction of XANES spectra. To document the capability of the technique in resolving chemical species, we first analyse a sample containing 2-30 µm grains of various ferrous- and ferric-iron containing minerals, including hypersthene, magnetite and hematite, distributed in a light matrix of a resin. We accurately obtain the XANES spectra at the Fe K-edge of these four standards, with spatial resolution of 3 µm. Subsequently, a sample of ~1.9 billion-year-old microfossil from the Gunflint Formation in Canada is investigated, and for the first time ever, we are able to locally identify the oxidation state of iron compounds encrusting the 5 to 10 µm microfossils. Our results highlight the potential for attaining new insights into Precambrian ecosystems and the composition of Earth's earliest life forms.

Recognition of fossil prokaryotes in Cretaceous methane seep carbonates: relevance to astrobiology.

Recovery of prokaryotic body fossils from methane seep carbonates such as those of the Cretaceous Tepee Buttes of Colorado serves as a model for sampling in future astrobiological missions. The fossils, found primarily at the interface between paragenetic fabrics, suggest a sharp physicochemical gradient. Evidence of these microbial fossils occurs at a variety of scales. In the field, microbialite is found as meter-scale thrombolitic zones and centimeterscale stromatolitic crusts lining voids inferred to be the sites of ancient methane seepage. Petrographic fabrics suggestive of microbialite include indistinct peloids (0.1-1 mm in diameter) and crusts of authigenic micrite. Primary evidence obtained from scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy analysis comprises pinnate bacteria (0.3 microm in diameter and 1-1.5 microm long), sheaths (2-4 microm in diameter), coccoids (0.5-1 microm in diameter, up to 40 per cluster), and the presence of framboidal pyrite (6-8 microm in diameter). These results are in agreement with studies of other ancient and modern seeps and suggest a morphological conservatism of microbial form that can be incorporated into studies of extraterrestrial environments where it is presumed that reduced gases drive the metabolic activity of prokaryote-like organisms. Target areas that could serve as conduits for reduced gas seeps include tectonic or impact-driven faulting, zones of cryosphere melting, or other disruptions in crustal coherence. Ancient seeps, preserved as localized anomalous evaporite deposits in the sedimentary cover, could be detected by remote sensing.