Well-hidden methanogenesis in deep, organic-rich sediments of Guaymas Basin.

Deep marine sediments (>1mbsf) harbor ~26% of microbial biomass and are the largest reservoir of methane on Earth. Yet, the deep subsurface biosphere and controls on its contribution to methane production remain underexplored. Here, we use a multidisciplinary approach to examine methanogenesis in sediments (down to 295 mbsf) from sites with varying degrees of thermal alteration (none, past, current) at Guaymas Basin (Gulf of California) for the first time. Traditional (13C/12C and D/H) and multiply substituted (13CH3D and 12CH2D2) methane isotope measurements reveal significant proportions of microbial methane at all sites, with the largest signal at the site with past alteration. With depth, relative microbial methane decreases at differing rates between sites. Gibbs energy calculations confirm methanogenesis is exergonic in Guaymas sediments, with methylotrophic pathways consistently yielding more energy than the canonical hydrogenotrophic and acetoclastic pathways. Yet, metagenomic sequencing and cultivation attempts indicate that methanogens are present in low abundance. We find only one methyl-coenzyme M (mcrA) sequence within the entire sequencing dataset. Also, we identify a wide diversity of methyltransferases (mtaB, mttB), but only a few sequences phylogenetically cluster with methylotrophic methanogens. Our results suggest that the microbial methane in the Guaymas subsurface was produced over geologic time by relatively small methanogen populations, which have been variably influenced by thermal sediment alteration. Higher resolution metagenomic sampling may clarify the modern methanogen community. This study highlights the importance of using a multidisciplinary approach to capture microbial influences in dynamic, deep subsurface settings like Guaymas Basin.

Scale distortion from pressure baselines as a source of inaccuracy in triple-isotope measurements.

RATIONALE: Isotope ratio measurements have become extremely precise in recent years, with many approaching parts-per-million (ppm) levels of precision. However, seemingly innocuous errors in signal baselines, which exist only when gas enters the instrument, might lead to significant errors. These "pressure-baseline" (PBL) offsets may have a variety of origins, such as incoherent scattering of the analyte, isobaric interferences, or electron ablation from the walls of the flight tube. They are probably present in all but ultra-high-resolution instruments, but their importance for high-precision measurements has not been investigated. METHODS: We derive the governing equations for the PBL effect. We compare the oxygen triple-isotope composition of gases on three different mass spectrometers before and after applying a correction for PBLs to determine their effects. We also compare the composition of atmospheric O2 with that of several standard minerals (San-Carlos Olivine and UWG-2) on two high-precision mass spectrometers and compare those results with the differences reported in the literature. RESULTS: We find that PBLs lead to stretching or compression of isotopic variations. The scale distortion is non-mass-dependent, affecting the accuracy of triple-isotope covariations. The governing equations suggest that linear stretching corrections using traditional isotopic delta values (e.g., δ18 O) are rigorous for PBL-induced errors in pure gases. When the reference and sample gases are not comparable in composition or purity, however, a different correction scheme may be required. These non-mass-dependent errors are systematic and may have influenced previous measurements of triple-isotope covariations in natural materials. CONCLUSIONS: Accurate measurements of isotopic variations are essential to biogeochemistry and for testing theoretical models of isotope effects. PBLs are probably ubiquitous, contributing to the interlaboratory disagreements in triple-isotope compositions of materials differing greatly in δ18 O values. Moreover, they may lead to inaccurate determination of triple-isotope compositions and fractionation factors, which has implications for isotopic studies in hydrology and biogeochemistry.

Isotope geochemistry. Biological signatures in clumped isotopes of O2.

The abundances of molecules containing more than one rare isotope have been applied broadly to determine formation temperatures of natural materials. These applications of "clumped" isotopes rely on the assumption that isotope-exchange equilibrium is reached, or at least approached, during the formation of those materials. In a closed-system terrarium experiment, we demonstrate that biological oxygen (O2) cycling drives the clumped-isotope composition of O2 away from isotopic equilibrium. Our model of the system suggests that unique biological signatures are present in clumped isotopes of O2­and not formation temperatures. Photosynthetic O2 is depleted in (18)O(18)O and (17)O(18)O relative to a stochastic distribution of isotopes, unlike at equilibrium, where heavy-isotope pairs are enriched. Similar signatures may be widespread in nature, offering new tracers of biological and geochemical cycling.