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The future of the northern peatland carbon (C) sink is uncertain as the effects of warming on microbial metabolisms are unclear. While increased microbial CO2 emissions are expected under warming, the response of microbial photosynthesis remains unknown, complicating predictions of net microbial effects on peatland carbon emissions. Here, using a continental-scale experimental study, we show that warming amplifies microbial photosynthesis by 3.4 mgC m−2 h−1 per 1 °C increase. By 2100, this increase translates to a gain of 51.1 Tg of carbon per year from the northern peatland area under the pessimistic SSP 5-8.5 climatic change scenario, offsetting ~14% of projected heterotrophic CO2 emissions in northern peatlands. By linking field and microcosm experiments, we further show that enhanced microbial photosynthesis accelerates peatland CO2 uptake as photosynthetic microbial-C subsidies stimulate nutrient mineralization. These results underscore the importance of photosynthetic microbes for mitigating carbon emissions and supporting long-term carbon storage in peatlands.