Responses of mixed methanotrophic consortia to variable Cu/Fe ratios.

Methane mitigation in landfill top cover soils is mediated by methanotrophs whose optimal methane (CH) oxidation capacity is governed by environmental and complex microbial community interactions. Optimization of CH remediating bio-filters need to take microbial responses into account. Divalent copper (Cu) and iron (Fe) are present in landfills at variable ratios and play a vital role in methane oxidation capacity and growth of methanotrophs. This study, as a first of its kind, therefore quantified effects of variable Cu and Fe (5:5, 5:25 and 5:50 μM) ratios on mixed methanotrophic communities enriched from landfill top cover (LB) and compost soils (CB). CH oxidation capacity, CH removal efficiencies, fatty acids content/profiles and polyhydroxybutyrate (PHB; a biopolymer) contents were also analysed to quantify performance and potential co-product development. Mixed methanotroph cultures were raised in 10 L continuous stirred tank reactors (CSTRs, Bioflo & Celligen 310 Fermentor/Bioreactor; John Morris Scientific, Chatswood, NSW, Australia). Community structure was determined by amplifying the V3-V4 region of 16s rRNA gene. Community structure and, consequently, fatty acid-profiles changed significantly with increasing Cu/Fe ratios, and responses were different for LB and CB. Effects on methane oxidation capacities and PHB content were similar in the LB- and CB-CSTR, decreasing with increasing Cu/Fe ratios, while biomass growth was unaffected. In general, high Fe concentration favored growth of the type -II methanotroph Methylosinus in the CB-CSTR, but methanotroph abundances decreased in the LB-CSTR. Increase in Cu/Fe ratio increased the growth of Sphingopyxis in both systems, while Azospirllum was co-dominant in the LB- but absent in the CB-CSTR. After 13 days, methane oxidation capacities and PHB content decreased by ∼50% and more in response to increasing Fe concentrations. Although methanotroph abundance was ∼2% in the LB- (compared to >50% in CB-CSTR), methane oxidation capacities were comparable in the two systems, suggesting that methane oxidation capacity was maintained by the dominant Azospirllum and Sphingopyxis in the LB-CSTR. Despite similar methanotroph inoculum community composition and controlled environmental variables, increasing Cu/Fe ratios resulted in significantly different microbial community structures in the LB- and CB-CSTR, indicative of complex microbial interactions. In summary, our results suggest that a detailed understanding of allelopathic interactions in mixed methanotrophic consortia is vital for constructing robust bio-filters for CH emission abatement.