Predominant methanogenic pathway in the rhizosphere soil of reed in saline-alkaline Zhalong wetland

Abstract:

[Background] Reed wetland is one of the main methane emission sources, and methanogenic archaea are the only known organisms producing ample methane. Whereas, the dominant methane-production pathways in the rhizosphere soil of reed in saline-alkali wetland are unknown. [Objective] To reveal the dominant methanogenic pathway in the saline-alkaline Zhalong wetland. [Methods] High-throughput sequencing of 16S rRNA gene was employed to study the diversity of methanogenic archaea and bacteria in the rhizosphere soil (0–20 cm depth) of reed in Zhalong wetland. The known methanogenic substrates including trimethylamine (TMA), methanol, betaine, acetate, and H₂/CO₂ were used to enrich the methanogenic microorganisms in the wetland soil. The methanogenic rate of each microorganism was measured to determine the predominant methanogenic pathway in the rhizosphere soil, and qPCR was employed to quantify the bacterial and archaeal groups and further predict the bacteria and archaea that jointly convert betaine to CH₄. [Results] The dominant methanoarchaea were determined to be the CO₂-reducing Methanobacterium (36.42%) and Rice Cluster II (11.55%), the methane anaerobic oxidizer Candidatus Methanoperedens (35.06%), the aceticlastic methanogen Methanosaeta (11.29%), methylotrophic Methanosarcina (6.53%), and the H₂-dependent methylotrophic methanogen Methanomassiliicoccus(4.05%). The predominant bacteria were Chloroflexi (21.55%), Proteobacteria (16.88%), Actinobacteria (13.37%), and Acidobacteria (10.00%). The highest methane-producing rate was observed in the media with the addition of TMA and betaine. The dominant bacteria reducing betaine to TMA included Sporomusaceae, Sedimentibacteraceae, Hungateiclostridiaceae, and Clostridiaceae and the dominant archaea producing CH₄ from TMA included Methanosarcina and Methanomassiliicoccus. [Conclusion] Methylotrophic methanogenesis based on the TMA from betaine reduction is the dominant methanogenic pathway in the rhizosphere soil of reed in the low-temperature Zhalong wetland.