Multiple cryptic N-Fe-Mn-C redox couplings/cycles regulate the fate of sedimentary methane of a subtropical lake
A complex and active microbial community was observed to exhibit both aerobic and anaerobic activity in deep methanogenic sediments of Lake Kinneret, well below the sulfidic zone. Using a combination of geochemical and microbial activity profiles together with sediment incubations, we were able to show that these communities manage to survive under energy limited, highly reduced conditions, through triple cryptic cycles of N-Mn/Fe-CH4. One of our key findings was the presence of NO2- (up to ~0.5µM) in the methanogenic zone under elevated Fe(II) and Mn(II) levels where a notable increase in NC10 bacteria abundance coincided with decreasing CH4 concentrations. Controlled stable isotope probing (SIP) anaerobic incubation experiments with the methanogenic sediment labeled with 15NH4+ showed production of enriched 15N-NO2- and 15N-N2, indicating indeed their anaerobic production by NH4+ oxidation, most likely coupled to MnOx reduction. A cryptic sink for NO2- was observed by a series of top-core sediment experiments and was shown to be coupled to Fe(II) oxidation, where Fe(II) partly originated from anaerobic oxidation of methane (AOM). Nevertheless, NC10 activity in this setting is proposed as a third cryptic cycle for dark oxygen production through NO2- disproportionation that enables aerobic CH4 oxidation by NC10 and by other aerobic methanotrophs. The complex microbial couplings between N-Mn/Fe-CH4 demonstrated here are significant in attenuating the emissions of the greenhouse gas CH4 from lake sediments by 10-15% of the produced CH4, additionally to insights provided on survival mechanisms of microbes under low energy conditions.