Elevated atmospheric CO2 decreases methylmercury production in freshwater lakes

Abstract Elevated atmospheric carbon dioxide (CO 2 ) level reshapes microbial communities in nature, yet its consequences for neurotoxic methylmercury (MeHg) production in waters remain unclear. Here, we show that elevated CO 2 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 CO 2 -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 CH 4 production) and suppressing Hg methylation. Model simulations predict a 33%–74% global decline in lake MeHg production under future CO 2 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.