From under-ice accumulation to open-water oxidation: microbial regulation of methane emissions in high-altitude proglacial lakes

Proglacial lakes, formed by global warming and accelerating deglaciation, represent widespread yet poorly constrained sources of methane (CH 4 ). Although their emissions are closely associated with ice-cover and ice-free conditions, the microbial mechanisms underlying these dynamics are not understood. Here, we integrated multi-season field observations, stable isotope analyses, metagenomic sequencing, and incubation experiments to investigate microbial controls on seasonal CH 4 cycling in newly formed high-altitude proglacial lakes on the Tibetan Plateau. We observed substantial accumulation of dissolved CH 4 in the water column even during the ice-covered period (ICP), despite the enrichment of anaerobic methanotrophs (Candidatus Methylomirabilota) in sediments. Isotope analysis showed that CH 4 oxidation was active during the early ICP but became limited during the late ICP, leading to continued CH 4 accumulation. Following the transition to the ablation period (AP), acetoclastic methanogenesis ( Methanosarcina ) rapidly recovered in bottom sediments. Concurrently, significant aerobic methanotrophy ( Methylobacter ) was established in surface sediments. Temperature was strongly associated with the shift of methanogenic and methanotrophic communities from the ICP to the AP. This microbial reorganization was accompanied by an increased relative abundance of CH 4 metabolism genes and enhanced functional coupling among CH 4 , nitrogen, and sulfur cycling pathways. Overall, our findings demonstrate that microbial succession contributes to the seasonal transition from net CH 4 accumulation beneath the ice to net CH 4 consumption during open-water conditions. Given the potentially greater temperature sensitivity of methanogenesis relative to methanotrophy, continued climate warming may disrupt the balance, potentially transforming rapidly expanding proglacial lakes into stronger sources of atmospheric CH 4 . Video Abstract