Variation of carbon isotope fractionation in hydrogenotrophic methanogenic microbial cultures and environmental samples at different energy status

Abstract

Methane is a major product of anaerobic degradation of organic matter and an important greenhouse gas. Its stable carbon isotope composition can be used to reveal active methanogenic pathways, if associated isotope fractionation factors are known. To clarify the causes that lead to the wide variation of fractionation factors of methanogenesis from H 2 plus CO 2 (a CO2ÀCH4 ), pure cultures and various cocultures were grown under different thermodynamic conditions. In syntrophic and obligate syntrophic cocultures thriving on different carbohydrate substrates, fermentative bacteria were coupled to three different species of hydrogenotrophic methanogens of the families Methanobacteriaceae and Methanomicrobiaceae. We found that C-isotope fractionation was correlated to the Gibbs free energy change (DG) of CH 4 formation from H 2 plus CO 2 and that the relation can be described by a semi-Gauss curve. The derived relationship was used to quantify the average DG that is available to hydrogenotrophic methanogenic archaea in their habitat, thus avoiding the problems encountered with measurement of low H 2 concentrations on a microscale. Boreal peat, rice field soil, and rumen fluid, which represent major sources of atmospheric CH 4 , exhibited increasingly smaller a CO2ÀCH4 , indicating that thermodynamic conditions for hydrogenotrophic methanogens became increasingly more favourable. Vice versa, we hypothesize that environments with similar energetic conditions will also exhibit similar isotope fractionation. Our results, thus, provide a mechanistic constraint for modelling the 13 C flux from microbial sources of atmospheric CH 4 .

Influence of Methanogenic Populations in Holocene Lacustrine Sediments Revealed by Clone Libraries and Fatty Acid Biogeochemistry

“…5). The very negative 13 C CH4 value at 7.8 m depth (-68.3‰) can correspond to an advanced stage of CO 2 reduction (Whiticar et al 1986) derived from the use of refractory OM by syntrophic consortia (Penning et al 2005). Deeper in the sediment (ca.…”

Methanogenic pathway, 13C isotope fractionation, and archaeal community composition in the sediment of two clear‐water lakes of Amazonia

“…Thus, d 13 C values measured in methanogenic environments have frequently been exploited to determine the relative importance of the acetotrophic vs. hydrogenotrophic path of CH 4 production . However, it has also been noticed that the fractionation factors involved in CH 4 production from either acetate or CO 2 can vary with the d 13 C of the substrate (i.e., organic carbon, acetate, and CO 2 ) (Conrad et al 2009), the H 2 partial pressure and energetic conditions for hydrogenotrophic methanogenesis (Penning et al 2005;Sugimoto and Fujita 2006), the acetate concentration (Goevert and Conrad 2009), and the composition of the active methanogenic archaeal community (Penning et al 2006;Goevert and Conrad 2009). The importance of each of these controls can presently not be predicted for a particular environment, so that fractionation factors must be determined for each individual CH 4 -producing environment.…”

Carbon and hydrogen isotope fractionation during methanogenesis: A laboratory study using coal and formation water

“…While these general ranges appear to be correct in most cases, the strict interpretation of CH 4 carbon isotopic data is not warranted (Chanton et al, 2004;Chidthaisong et al, 2002;Waldron et al, 1998;Whiticar, 1999). For example, high α values are typical of hydrogenotrophic methanogenesis, but in an environment with a high supply of free energy available to the methanogens, hydrogenotrophic methanogenesis may also cause a low α value (Conrad, 2005;Penning et al, 2005). As such, various multi-element isotope approaches can be useful to help identify and evaluate the many processes leading to biogenic CH 4 production .…”

“…This is because microbes preferentially utilize the lighter isotope of C ( 12 C) that has lower bond energies, and therefore biogenically derived CH 4 is generally 13 C-depleted. Isotope fractionation is maximized under conditions where the methanogenic substrate is in abundant supply, and depends on several factors including temperature, microbial growth phase, the hydrogen supply, the methanogen species, and the methanogenic pathways involved (Botz et al, 1996;Conrad, 2005;Penning et al, 2005).…”

“…This is because microbes preferentially utilize the lighter isotope of C ( 12 C) that has lower bond energies, and therefore biogenically derived CH 4 is generally 13 C-depleted. Isotope fractionation is maximized under conditions where the methanogenic substrate is in abundant supply, and depends on several factors including temperature, microbial growth phase, the hydrogen supply, the methanogen species, and the methanogenic pathways involved (Botz et al, 1996;Conrad, 2005;Penning et al, 2005).Moreover, isotopic fractionation of biogenic CH 4 may increase throughout the culture's exponential growth phase before stabilizing during the stationary phase (Valentine et al, 2004).The current understanding of the isotopic composition of biogenic CH 4 is in the range of -110 ‰ to -50 ‰ and -400 ‰ to -150 ‰, for δ 13 C-CH 4 and δ 2 H-CH 4 , respectively (Conrad, 2005;Golding et al, 2013;Whiticar, 1999;Whiticar et al, 1986). In addition, CH 4 produced from H 2 +CO 2 is typically 13 C-depleted relative to that produced from acetate (Chanton et al, 2004;Conrad et al, 2011;Whiticar, 1999).…”