The Carbon-Isotope Record of the Sub-Seafloor Biosphere

Abstract

Sub-seafloor microbial environments exhibit large carbon-isotope fractionation effects as a result of microbial enzymatic reactions. Isotopically light, dissolved inorganic carbon (DIC) derived from organic carbon is commonly released into the interstitial water due to microbial dissimilatory processes prevailing in the sub-surface biosphere. Much stronger carbon-isotope fractionation occurs, however, during methanogenesis, whereby methane is depleted in 13C and, by mass balance, DIC is enriched in 13C, such that isotopic distributions are predominantly influenced by microbial metabolisms involving methane. Methane metabolisms are essentially mediated through a single enzymatic pathway in both Archaea and Bacteria, the Wood–Ljungdahl (WL) pathway, but it remains unclear where in the pathway carbon-isotope fractionation occurs. While it is generally assumed that fractionation arises from kinetic effects of enzymatic reactions, it has recently been suggested that partial carbon-isotope equilibration occurs within the pathway of anaerobic methane oxidation. Equilibrium fractionation might also occur during methanogenesis, as the isotopic difference between DIC and methane is commonly on the order of 75‰, which is near the thermodynamic equilibrium. The isotopic signature in DIC and methane highly varies in marine porewaters, reflecting the distribution of different microbial metabolisms contributing to DIC. If carbon isotopes are preserved in diagenetic carbonates, they may provide a powerful biosignature for the conditions in the deep biosphere, specifically in proximity to the sulphate–methane transition zone. Large variations in isotopic signatures in diagenetic archives have been found that document dramatic changes in sub-seafloor biosphere activity over geological time scales. We present a brief overview on carbon isotopes, including microbial fractionation mechanisms, transport effects, preservation in diagenetic carbonate archives, and their implications for the past sub-seafloor biosphere and its role in the global carbon cycle. We discuss open questions and future potentials of carbon isotopes as archives to trace the deep biosphere through time.

Experimental and theoretical determinations of hydrogen isotopic equilibrium in the system CH4H2H2O from 3 to 200 °C

“…Additionally, as discussed in the introduction, carbon isotope compositions between methane and CO 2 in marine sediments are commonly similar to values calculated based on theoretical estimates of equilibrium and, on this basis, it has been proposed that methane may form in or reach isotopic equilibrium with CO 2 in marine sediments (Bottinga, 1969;Yoshinaga et al, 2014;Meister and Reyes, 2019;Gropp et al, 2021).…”

“…Based on this, they suggested that methanogens in deep-sea sediments could promote CH 4 -CO 2 carbon isotopic equilibration during methane generation. Meister and Reyes (2019) further argued that methanogens can produce methane in carbon isotopic equilibrium with CO 2 because the carbon isotopic composition of methane formed by the reduction of CO 2 from the experiments of are similar to the values expected for equilibrium at the corresponding growth temperatures (35-85°C). Thus the problem has, over the past 50 years, come full circle, with the initial proposal that methanogens catalyze equilibration of carbon between methane vs. CO 2 in marine sediments, though initially dismissed, receiving renewed support.…”

The fossil record of igneous rock

“…For the large variability of the C isotopes, explanations also include slow metabolisms that result in large fractionations, but also the pathway of methanogenesis. Methanogenesis occurring through the carbonate reduction pathway processes produces a 13 C-depleted methane and a strongly 13 C-rich residual CO 2 (Meister and Reyes, 2019). Calcite formed from the latter becomes isotopically heavy, particularly if there are very low concentrations of other C sources that can dilute the methane-related signature, which can explain the heavy δ 13 C values of > > +20‰ in these crystalline rock fractures (Drake et al, 2019).…”

Sensors for Cryogenic Isotope-Separation Column

“…Various chemical compounds based on enriched ( 13 C) have a broad field of application [ 1 ]: Applications in medical fields: in clinical studies, differential diagnosis of multiple diseases, state of illness judgment, treatment evaluation, research on organ function and development of new drugs [ 2 ], such as PET diagnosis [ 3 ], 13 C-urea breath test [ 4 ], tumor therapy (boron neutron capture therapy) research on drugs [ 5 ], etc. ; Applications in life sciences fields: in the spectrum research of the structure and function of protein species by nuclear magnetic resonance (NMR) and mass spectrometer (MS), including isotope-coded affinity tag testing method (ICATTM), stable isotope labeled amino acid cell cultivation (SILAC), absolute quantification protein analysis method (AQUATM) [ 6 ]; Applications in energy metabolism: in sports medicine, child nutrition, food nutrition, weight loss and diet for astronaut using he-isotope tracing method [ 7 ]; Applications in agricultural scientific research fields: in the research of plant physiology and biochemistry, nutrition of soil and plant, plant protection, the improvement of rice, flower, agricultural products and other crops, nitrogen cycle of grassplot [ 8 ]; Applications in the research of environmental science: monitoring of ecosystem pollution, determining of the length of food chains, ascertaining the pollutant sources in air and water and defining the distribution area of plant [ 9 , 10 ]; Applications in analysis and testing: in the testing of food [ 11 ], pesticide residue analysis [ 12 ], drug and excitant, heroin testing [ 13 ], also in the research of geology, geochemistry, paleontology and ecology [ 14 , 15 ]. …”

“…Applications in analysis and testing: in the testing of food [ 11 ], pesticide residue analysis [ 12 ], drug and excitant, heroin testing [ 13 ], also in the research of geology, geochemistry, paleontology and ecology [ 14 , 15 ].…”