Contrasting time- and concentration-dependent cellular and photosynthetic responses of the freshwater alga Chlamydomonas reinhardtii to inorganic mercury and methylmercury

Mercury (Hg) is a global pollutant threatening ecosystems, mainly via methylmercury (MeHg) biomagnification in food webs. Microalgae are the primary entry point, yet the toxicity of its different forms in them is still understudied. This study compared, for the first time, the time-dependent effects (3-96 hr) of inorganic Hg (IHg) and MeHg at concentrations of 10-5-10-8 g/L on the green microalga Chlamydomonas reinhardtii. Flow cytometry and chlorophyll fluorescence revealed contrasting physiological and photosynthetic responses. Inorganic Hg was more toxic at 3 hr than MeHg, reducing growth, photosynthetic efficiency, and chlorophyll autofluorescence, and increasing mortality. Effects of IHg decreased over time, with reactive oxygen species (ROS) and mortality below control by 96 hr. In contrast, MeHg at 10-5 g/L caused sustained growth inhibition, elevated ROS, increased chlorophyll autofluorescence, and cell death beyond 24 hr. Differences in energy dissipation and photosystem II (PSII) recovery further indicated distinct toxicity targets. At lower concentrations, both forms triggered time- and concentration-dependent responses, including transient growth inhibition, stimulation, and ROS changes, likely reflecting hormesis and metabolic trade-offs. These responses may affect growth, photosynthesis, and ROS processes. They highlight the need for time-resolved assessments to capture adaptation-toxicity thresholds. This study strengthens the understanding of Hg toxicity in primary producers and its ecotoxicological risks.