Commercial Arthrospira platensis Extract Modifies the Photophysiology of Cladocopium goreaui, Coral Endosymbiont Microalgae

Arthrospira platensis extract is incorporated into sunscreen formulations for its beneficial and UV-protective properties on cultured human cells. However, its effects have not yet been assessed on non-target organisms such as endosymbiotic microalgae in coral tissue. To evaluate its effects, we investigated the photophysiology of the cultured dinoflagellate Cladocopium goreaui using PAM fluorometry (RLC, OJIP) after a 5-day exposure to different extract concentrations. Our results show that, through a hormetic effect, A. platensis enhances the performance index (Pi_Abs) at 0.018 mg L−1 by increasing the number of active reaction centers (RC/ABS) and improving electron transfer efficiency (φEo, ψEo) along the electron transport chain. Conversely, beyond 108.8 mg L−1, negative impacts appear on PSII, increasing the apparent antenna size (ABS/RC) and impairing the oxygen-evolving complex (K-peak), ultimately reducing the maximum relative electron transport rate (rETRm). This relative toxicity, obtained only for the highest concentrations, supports its potential incorporation into cosmetic formulations. This study contributes to improving the ecotoxicity assessment of cosmetic products on non-target organisms.