Long-term effects of soluble and insoluble ferric irons on anaerobic oxidation of methane in paddy soil

Iron-dependent anaerobic oxidation of methane (Fe-AOM) is an important process to reduce methane emissions into the atmosphere. It is well known that iron bioavailability largely influences microbial iron reduction, but the long-term effects of different ferric irons on soil Fe-AOM remain unknown. In this work, paddy soil in the ferruginous zone was collected and inoculated with insoluble ferrihydrite and soluble EDTA-Fe(III) for 420 days. Stable isotope experiments, activity inhibition tests, and molecular biological techniques were performed to reveal the activity, microbial community, and possible mechanism of paddy soil Fe-AOM. The results showed that ferrihydrite was a better electron acceptor for long-term Fe-AOM cultivation. Although EDTA-Fe(III) is highly bioavailable and could stimulate Fe-AOM activity for a short time, it restricted the activity increase in the long term. The abundances of archaea, iron-reducing bacteria (IRB), and gene mcrA largely increased after cultivation, indicating the important roles of mcrA-carrying archaea and IRB. Remarkably, archaeal communities were similar, but bacteria were totally different with different ferric irons. The results of the microbial community and activity inhibition suggested that Fe-AOM was performed likely by the cooperation between archaea (Methanomassiliicoccaceae or pGrfC26) and IRB in the cultures.