Elevated atmospheric CO2 decreases methylmercury production in freshwater lakes

Elevated atmospheric carbon dioxide (CO2) level reshapes microbial communities in nature, yet its consequences for neurotoxic methylmercury (MeHg) production in waters remain unclear. Here, we show that elevated CO2 levels (650 and 1000 ppm) consistently reduced net MeHg production across 45 freshwater lakes spanning 1200 longitudinal kilometers, particularly in eutrophic conditions (54–96%). Elevated CO2-driven shifts in carbon substrates favored hydrogenotrophic methanogens (e.g., Methanobacterium) lacking the hgcA methylation gene over hgcA-harboring acetoclastic strains (e.g., Methanosarcina), decreasing methanogen abundance (18–98% in hgcA copies) and activity (13–53% in net CH4 production) and suppressing Hg methylation. Model simulations predict a 33%–74% global decline in lake MeHg production under future CO2 scenarios, partially counteract MeHg increases associated with intensified algal blooms under warming. This overlooked pathway highlights the need to integrate interacting climate drivers to improve predictions of MeHg risks in a climate-changing future.