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.