Dynamic Response of Photosystem II in Chlorella vulgaris to Heavy Metal Stress: Integrated Mathematical Analysis

This study demonstrates the application of a comprehensive set of mathematical methods—including multivariate statistical analysis, a mathematical model, the JIP-test, and the difference curve method—to analyze the dynamic response of photosystem II (PSII) in Chlorella vulgaris Beijerinck cells exposed to toxicants, specifically cadmium and chromium salts (CdSO4 and K2Cr2O7) at concentrations of 20 and 50 µM each. During the growth of the Chlorella vulgaris culture in a bioreactor, fluorescence transients were recorded automatically every hour and subsequently analyzed using the integrated mathematical approach. The JIP-test was employed to analyze changes in fluorescence transient parameters over prolonged exposure (~20 h) to the toxicants. Principal Component Analysis was used to reduce dimensionality, and then the curves were classified into three groups based on similarities in curve shape characteristics: (1) control and CdSO4 (20 µM), (2) K2Cr2O7 (20 and 50 µM), and (3) CdSO4 (50 µM). The difference curve method was used to examine alterations in the initial segment of the curves, reflecting changes in the oxygen-evolving complex (OEC). A mathematical model was applied to assess the heterogeneity of reaction centers based on OEC activity, calculating the proportions of α- and β-centers, which differ by the presence or absence of a peripheral antenna, respectively. The integrated mathematical approach revealed a decrease in PSII efficiency and OEC activity, while no significant changes were detected in the antenna complex. The use of this complementary set of analytical methods provides a deeper understanding of the mechanisms underlying the adaptation of the photosynthetic apparatus to various stress factors, particularly toxicants.