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.