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

Abstract

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.