Extracellular quinones affecting methane production and methanogenic community in paddy soil.

This study investigated the change of CH4 production and methanogenic community in response to the presence of humic substances (humics) analogue, anthraquinone-2,6-disulfonate (AQDS). Anaerobic experiments used a Chinese paddy soil, and three concentration levels of 0.5, 5, and 20 mM AQDS were conducted. Results suggested that the effect of AQDS on methanogenesis was time-dependent and concentration-dependent. Twenty millimolars of AQDS was toxic for methanogenic activity almost for the entire experimental period. Slight inhibition of methanogenesis by AQDS respiration in the 0.5- and 5-mM AQDS-supplemented treatments occurred within the early period, while CH4 accumulated throughout the later period was approximately five and ten times greater than that of the controls without AQDS, respectively. AQDS reduction coupling to acetate oxidization enriched Geobacter species, and the mcrA-targeted T-RFLP profiles revealed significant increase of Methanosarcina at the expense of Methanobacterium in the 0.5- and 5-mM AQDS treatments. The enriched syntrophic association between Geobacter and Methanosarcina was deduced to be an effective methanogenic pathway for converting acetate to CH4 via direct interspecies electron transfer. This study implied the ecological importance of syntrophic interaction between methanogens and microorganisms enriched by anaerobic respiration of non-methanogenic terminal electron acceptors in paddy soils.