The stable carbon isotope¹³C is used as a universal tracer in plant eco-physiology and studies of carbon exchange between vegetation and atmosphere. Photosynthesis fractionates against¹³CO₂so that source sugars (photosynthates) are on average¹³C depleted by 20‰ compared with atmospheric CO₂. The carbon isotope distribution within sugars has been shown to be heterogeneous, with relatively¹³C-enriched and¹³C-depleted C-atom positions. The¹³C pattern within sugars is the cornerstone of¹³C distribution in plants, because all metabolites inherit the¹³C abundance in their specific precursor C-atom positions. However, the intramolecular isotope pattern in source leaf glucose and the isotope fractionation associated with key enzymes involved in sugar interconversions are currently unknown. To gain insight into these, we have analyzed the intramolecular isotope composition in source leaf transient starch, grain storage starch, and root storage sucrose and measured the site-specific isotope fractionation associated with the invertase (EC 3.2.1.26) and glucose isomerase (EC 5.3.1.5) reactions. When these data are integrated into a simple steady-state model of plant isotopic fluxes, the enzyme-dependent fractionations satisfactorily predict the observed intramolecular patterns. These results demonstrate that glucose and sucrose metabolism is the primary determinant of the¹³C abundance in source and sink tissue and is, therefore, of fundamental importance to the interpretation of plant isotopic signals.

Microbial anaerobic oxidation of hydrocarbons is a key process potentially involved in a myriad of geological and biochemical environments yet has remained notoriously difficult to identify and quantify in natural environments. We performed position-specific carbon isotope analysis of propane from cracking and incubation experiments. Anaerobic bacterial oxidation of propane leads to a pronounced and previously unidentified ¹³C enrichment in the central position of propane, which contrasts with the isotope signature associated with the thermogenic process. This distinctive signature allows the detection and quantification of anaerobic oxidation of hydrocarbons in diverse natural gas reservoirs and suggests that this process may be more widespread than previously thought. Position-specific isotope analysis can elucidate the fate of natural gas hydrocarbons and provide insight into a major but previously cryptic process controlling the biogeochemical cycling of globally significant greenhouse gases. Significance Microorganisms can oxidize hydrocarbons anaerobically, but the detection and quantification of this process in natural settings remains difficult, impeding reliable estimation of these processes at the global scale. We have used the technique of position-specific isotope analysis of propane and show that anaerobic biological degradation of propane leads to a specific signature that differs from that of propane produced from thermal decomposition of higher hydrocarbons. When applied to natural gas reservoirs, we show that anaerobic bacterial oxidation of propane can be detected and quantified, which is not the case with the use of conventional methods. Our findings are thus of importance for the detection of subsurface biology, for the understanding of the carbon cycle, and more broadly for environmental sciences.

Wächtershäuser’s proposal of the autotrophic origin of life theory and subsequent laboratory demonstrations of relevant organic reactions have opened a new gate for the exploration of the origin of life. However, this scenario remains controversial because, at present, it requires a high pressure of CO as a source of carbon and reducing energy, although CO must have been a trace C species on the Hadean Earth. We show that, simulating a geoelectrochemical environment in deep-sea hydrothermal fields, CO production with up to ~40% Faraday efficiency was attainable on CdS in CO₂-saturated NaCl solution at ≤–1 V (versus the standard hydrogen electrode). The threshold potential is readily generated in the H₂-rich, high-temperature, and alkaline hydrothermal vents that were probably widespread on the early komatiitic and basaltic ocean crust. Thus, Wächtershäuser’s scenario starting from CO₂ was likely to be realized in the Hadean ocean hydrothermal systems.

Understanding hydrocarbon cycling in the subsurface is important in various disciplines including climate science, energy resources and astrobiology. Mud volcanoes provide insights into biogeochemical processes occurring in the subsurface. They are usually associated with natural gas reservoirs consisting mainly of methane and other hydrocarbons as well as CO₂. Stable isotopes have been used to decipher the sources and sinks of hydrocarbons in the subsurface, although the interpretation can be ambiguous due to the numerous processes involved. Here we report new data for hydrocarbon isotope analysis, including position-specific isotope composition of propane, for samples from the Tokamachi mud volcano area, Japan. The data suggest that C₂₊ hydrocarbons are being biodegraded, with indirect production of methane (“secondary methanogenesis”). Data from chemical and isotopic composition are discussed with regard to 16S rRNA analysis, which exhibits the presence of hydrogenotrophic and acetoclastic methoanogens. Overall, the combination of isotopologue analysis with 16S rRNA gene data allows refining of our understanding of hydrocarbon cycling in subsurface environments.