Regional photoprotection strategies in Indian Ocean phytoplankton: Arabian Sea vs South Tropical Indian Ocean responses to PAR/UVR stress.

Phytoplankton photoprotection is a key yet understudied determinant of marine ecosystem resilience to climate-driven environmental stressors, particularly in the Indian Ocean (IO), where observational constraints limit projections of productivity under warming and stratification. While photophysiological adaptation has been extensively characterised in Atlantic and Pacific systems, IO phytoplankton communities remain poorly resolved. We assessed the photophysiological responses of natural phytoplankton assemblages from two contrasting IO bioregions-the productive, monsoon-influenced Arabian Sea (AS) and the oligotrophic South Tropical IO (STIO)-using onboard microcosm experiments that combined fast repetition rate fluorometry, HPLC pigment analysis, and microscopy. Surface and subsurface chlorophyll maximum waters were exposed to photosynthetically active radiation (PAR) alone or combined with ultraviolet-A/B radiation (PAR + UVR) for 72h under in situ temperature and natural light conditions. Dynamic photoinhibition, non-photochemical quenching (NPQ: xanthophyll cycle-mediated energy dissipation), and photoprotective pigment accumulation emerged as primary photoprotective mechanisms, but with distinct regional expression reflecting environmental stress gradients. In AS, rapid photoinhibition recovery (Fv/Fm returning to morning maxima by evening), antenna-associated NPQ, and inactive xanthophyll cycling characterised responses to high UVR stress. Conversely, STIO communities exhibited gradual recovery of photoinhibition, PSII reaction centre NPQ, and active xanthophyll cycle operation (diatoxanthin-diadinoxanthin) under sustained high-PAR conditions. These responses reflect the combined influence of physiological acclimation and community composition (AS: diatom-dominated; STIO: picophytoplankton-dominated), which indicates regional environmental gradients. Under projected surface-ocean stratification and mixed-layer shoaling, altered light climates and community restructuring will modify photoprotection mechanisms available to IO phytoplankton, with implications for primary productivity, carbon cycling, and ecosystem vulnerability to cumulative environmental change.