Effects of pressure, methane concentration, sulfate reduction activity, and temperature on methane production in surface sediments of the Gulf of Mexico

Abstract

Microbial methanogenesis is a known source of methane in marine environment, but the factors affecting the production of methane from different pathways remain largely unconstrained. We investigated the effects of pressure, methane concentration, temperature, and sulfate reduction activity on the conversion of methanogenic substrates to methane using radiotracers in nearshore and offshore surface sediments in the northern Gulf of Mexico. The transformation of bicarbonate to methane increased with methane concentrations under quasi in situ conditions of deep‐sea sediment, potentially reflecting, to some degree, enhancement of the enzymatic back reaction under high‐methane conditions. In coastal sediments, a positive effect of both increased methane concentration and inhibition of sulfate reduction by molybdate on 14CH4 accumulation suggests that anaerobic oxidation of methane and methanogenesis occur concurrently. In contrast, methane production from methanol was suppressed by increased methane concentration likely due to the reduced energy yield of methanogenesis at elevated methane concentrations. Temperature could limit hydrogenotrophic and acetoclastic methanogenesis under in situ conditions, as higher rates were observed at 40°C in nearshore sediments. Inhibitor experiments with 2‐bromoethanesulfonate and molybdate indicated that sulfate‐reducing bacteria preferentially utilized acetate, as well as H2. Methanol was a noncompetitive substrate for methanogens in the deep‐sea sediments but was competitively cycled in coastal sediments. An authentic noncompetitive substrate, methylamine, was channeled predominantly to methane production under all conditions tested. The different response of methanogenic activity to substrate availability, metabolic competition, temperature, and pressure between sites suggest variable environmental controls on the methane production in coastal vs. deep‐sea surface sediments.

Biogeochemistry, microbial activity, and diversity in surface and subsurface deep‐sea sediments of South China Sea

“…Although hydrogenotrophic methanogenesis was also detected at ROV2 and ROV3, the higher turnover of methylated compounds as well as the dominance of methylotrophic methanogens ( see below) suggested the methylotrophic pathway could be important for the concurrent methanogenesis with SR in the sulfate‐reducing sediments. Those results were in aggreement with previous studies, which experiemntally demonstrated that methane production in sulfate‐rich sediment could be driven by the fermentation of methylated compounds (Maltby et al ; Xiao et al ; Zhuang et al ).…”

“…Although the concave‐up shape of methane profile was often interpreted as the diffusion of methane from SMTZ, increasing evidence suggested endogenous methane production could occur in the sulfate‐rich sediments (Maltby et al ; Sela‐Adler et al ; Xiao et al ; Zhuang et al ), as observed in this study. Therefore, the low but detectable concentrations of methane could be a collective result of production, consumption, and diffusion from underlying sediment below SMTZ.…”

“…Shortly after, samples were flushed with N 2 for at least 1 min, and incubated at 4 C for shore-based analysis. Pore fluid samples were extracted from sediments using Rhizon soil moisture samplers (0.1 μm porous polymer, Rhizosphere Research, Wageningen, The Netherlands) (Seeberg-Elverfeldt et al 2005), as previously described (Zhuang et al 2018a).…”

Generation and Utilization of Volatile Fatty Acids and Alcohols in Hydrothermally Altered Sediments in the Guaymas Basin, Gulf of California

“…Slurries were prepared by mixing sediment with anoxic artificial seawater medium (1:1 ratio; slightly modified from Parkes et al ()). A final concentration of 25 mM molybdate and 30 mM 2‐bromoethanesulfonate were employed to inhibit the activity of sulfate reduction and methane production, respectively (Zhuang, Montgomery, Sibert, et al, ). Rates were determined as described above.…”

“…Methanol is often considered a noncompetitive substrate for methanogens in different sedimentary settings (Maltby et al, 2016;Oremland et al, 1982;Zhuang, Montgomery, & Joye, 2019;Zhuang, Montgomery, Sibert, et al, 2018). In these samples, >95% of methanol was channeled into the oxidation pathway, suggesting that methanol was primarily used as an energy source by nonmethanogenic heterotrophs.…”