Multigenerational adaptation alters methylmercury uptake in cyanobacteria under warming and elevated CO2.

Climate-driven increases in atmospheric CO2 concentrations and temperature strongly regulate the physiology of phytoplankton, the primary entry point for neurotoxic methylmercury (MeHg) in aquatic food webs. How multigenerational adaptation to these stressors reshapes cellular traits governing MeHg uptake remains unclear. Here, we exposed the bloom-forming cyanobacterium Microcystis aeruginosa to long-term adaptation under elevated temperature (+5 °C) or CO2 (+580 ppm), then quantified MeHg uptake and cellular partitioning. Both stressors reduced cellular MeHg accumulation by 36% under warming and 78% under elevated CO2 compared to ambient conditions (25 °C, 420 ppm CO2; p < 0.05). Short-term exposure (15 days) increased intracellular MeHg fractions from 33% (ambient) to 62% (warming) and 51% (elevated CO2), but responses diverged over time (up to 330 days), declining under warming and rising under elevated CO2. Mechanistically, warming stimulated secretion of extracellular soluble proteins that complexed MeHg, while elevated CO2 reduced extracellular bound polysaccharides, limiting MeHg adsorption. These results reveal that climate-driven physiological acclimation and adaptation shape MeHg bioavailability in phytoplankton, highlighting the need to incorporate such processes into models predicting MeHg cycling and risks under climate change.