Reduction of methane emissions from manganese-rich constructed wetlands: Role of manganese-dependent anaerobic methane oxidation

The considerable amount of methane (CH4) emitted from constructed wetlands (CWs) during wastewater treatment has the potential to intensify global climate change. Reducing CH4 emissions from CWs is, therefore, an important step in mitigating global warming. In this study, we compared CH4 emission fluxes from long-term operated pilot-scale vertical-flow CWs with natural manganese ore (Mn-CW) and gravel (Gr-CW, control) as a substrate without any seeding sludge or sediment added. The CH4 emission from the Mn-CW (3.01 ± 0.38 g C/m2/day) was significantly lower than the emission from the Gr-CW (5.50 ± 0.24 g C/m2/day). Mn(IV)-dependent anaerobic oxidation of CH4 (Mn-AOM) occurred in the Mn-CW, and was estimated to account for 66% of the total CH4 emissions reduction. The anaerobic methanotrophic archaea lineage ANME-2d present in the Mn-CW was considered the likely methanotroph that mediated Mn-AOM. Anaerobic fermentation, dissimilatory Mn reduction, and hydrogenotrophic methanogenesis were the main processes responsible for the degradation of organics in the Mn-CW. The acetoclastic methanogen archaea were suppressed in the Mn-CW. For the first time, Mn-AOM was demonstrated to be spontaneously present in the freshwater CWs. Our findings also demonstrated a novel approach to mitigate CH4 emissions from CWs, and indicate potential strategies against global climate change.