Evaluating photosystem II efficiency in Parachlorella kessleri under atrazine exposure using chlorophyll a fluorescence analysis

This study aims to evaluate the toxicity of Atrazine on freshwater algae by employing spectroscopic analysis (specifically the F₆₈₃/F₇₃₀ ratio) and photosynthetic parameters (Fv/Fm, Fv/F₀, ФC, ФPSII, qP, and UQF) as bioindicators. The unicellular alga Parachlorella kessleri was selected for this investigation. Cells of Parachlorella kessleri were cultivated under axenic conditions in Bold's Basal Medium (BBM) and subsequently exposed to various concentrations of Atrazine (260, 520, and 1040 μg/L) for three distinct exposure durations (1, 60, and 360 min). Reflectance, transmittance, basal steady-state emission spectra, and variable chlorophyll-a fluorescence were recorded and analyzed for both control and treated algae groups. The results demonstrated that Atrazine inhibits electron transport in the photosynthetic chain of Parachlorella kessleri, leading to a marked decline in photosynthetic efficiency. The absorbed light energy was predominantly dissipated via the physical de-excitation pathway (ΦC), while regulated heat dissipation remained minimal—even in the absence of the herbicide—suggesting a possible deficiency in the PGR5-dependent photoprotective pathway. Among the tested fluorescence parameters, ΦPSII, derived from Kautsky kinetics, proved to be the most sensitive to Atrazine exposure, highlighting its potential use in algae-based biosensors for herbicide detection in aquatic environments. Although the OJIP method provided a faster response and detected abrupt changes in relative chlorophyll fluorescence (VOP), these changes did not correlate with herbicide concentration. Steady-state fluorescence, especially when corrected for inner filter effects, may offer a complementary, non-invasive tool for monitoring aquatic stressors. However, anomalous responses at high herbicide concentrations indicated the need for further validation under varied conditions.