Physiological stress responses of Hydrilla verticillata to spent engine oil contamination leading to impairment of photosystem II photochemistry and oxidative destabilization

The indiscriminate release of spent engine oil (SEO) into aquatic ecosystems poses a significant environmental threat due to its complex composition of petroleum hydrocarbons, heavy metals, and oxidative degradation products. The present study evaluated the physiological, biochemical, and photosynthetic responses of Hydrilla verticillata under graded SEO exposure. SEO contamination significantly altered water physicochemical properties, characterized by reduced pH and dissolved oxygen alongside increased electrical conductivity, thereby creating hypoxic and stressful microenvironments. These changes were accompanied by marked inhibition of adventitious root formation, reduced biomass accumulation, and significant depletion of total chlorophyll content. Elevated activities of antioxidant enzymes (SOD, CAT, and POD) indicated activation of defence mechanisms; however, increased malondialdehyde levels confirmed severe oxidative stress and membrane destabilization. Chlorophyll a fluorescence (OJIP kinetics) revealed pronounced impairment of Photosystem II (PSII), characterized by partial inactivation of reaction centers (↓RC/CSm) and significant reductions in phenomenological energy fluxes (↓ABS/CSm, ↓TR₀/CSm, ↓ET₀/CSm). These alterations led to decreased quantum yields of primary photochemistry and electron transport beyond QA (↓ΦP₀, ↓ΦE₀). A compensatory response at the reaction center level was evident from increased specific absorption (↑ABS/RC) accompanied by enhanced energy dissipation (↑DI₀/RC, ↑ΦD₀) and elevated non-photochemical de-excitation constant (Kₙ), indicating intensified thermal energy loss. Multivariate analysis further confirmed a concentration-dependent shift from efficient photochemistry toward oxidative damage and energy dissipation, along with marked declines in performance indices (↓PIabs, ↓PIcsm). Collectively, these findings suggest that SEO severely disrupts PSII structural–functional integrity and metabolic homeostasis in Hydrilla verticillata, highlighting the ecological risk of petroleum-derived contaminants in aquatic systems.