Tracking photoinhibition and failure dynamics in algae under combined exposure to UVB and water-accommodated oil...

Increasing UV radiation and persistent petroleum-derived contaminants in aquatic environments have raised concerns regarding their combined effects on primary producers. Microalgae, such as Chlorella vulgaris, are particularly vulnerable owing to their surface-dwelling nature and their high sensitivity to impairment of photosystem II (PSII). The hazards associated with photosynthetic failure under ultraviolet (UV) stress in the presence of oil-derived mixtures remain inadequately quantified, particularly under environmentally realistic light spectra. This study addressed this knowledge gap by characterizing the joint effects of UV radiation and petroleum hydrocarbons on algal PSII function and viability. This study aimed to quantify failure thresholds using PAM fluorometry and survival analysis, and to link these with passive sampling of freely dissolved hydrocarbons. A programmable solar simulator was employed to deliver PAR + UVA and PAR + UVA + UVB exposures for 72 h, whereas water-accommodated fractions (WAFs) of crude oil, with and without an anionic surfactant, were administered at environmentally relevant concentrations. The PDMS passive samplers were calibrated for the target polycyclic aromatic hydrocarbons (PAHs) and their uptake kinetics were validated. Time-to-failure was defined as the crossing of an Fv/Fm ≤ 0.4 threshold, and hazard curves were generated across treatment groups. Notably, failure hazards increased significantly under UVB irradiation, particularly in WAF treatments combined with surfactants at critical micelle concentrations, indicating synergistic phototoxicity. These findings highlight the elevated risk posed by the combined UV and chemical stressors on phytoplankton productivity. The framework developed in this study provides a robust method for quantifying failure risks under multi-stressor conditions. Future research should explore the dynamics of chronic adaptation and the community-level effects of natural systems.