The cyanobacterium Synechococcus spp., are key contributors to marine primary productivity. They are found from equator to polar waters where these organisms are exposed to changed levels of pCO₂ and solar UV radiation. To investigate how elevated pCO₂ modulates the physiological responses of picophytoplankton to UVR, Synechococcus XM24 was acclimated to periodic (90 min per day) UVR exposure under ambient (418 μatm) and elevated (1000 μatm) pCO₂ conditions for 17 days. Our results indicate that elevated pCO₂ markedly enhanced its specific growth, cellular pigment accumulation (Chlorophyll a, carotenoids), and particulate organic carbon/nitrogen (POC/PON) quotas in the cells grown under light (without exposure to UV). However, such stimulatory effects were substantially abolished in UVR-exposed cells. UVR significantly inhibited the effective photochemical quantum yield of photosystem II (PSII), accelerated PSII damage and impaired its repairment, raising the damage/repair ratio (k/r) by up to 4-fold. The cells acclimated to the UVR exposures reduced their carbon fixation rate by 56–60% regardless of the pCO₂ levels. In additional, UVR exposure led to drastic reductions in cellular POC and PON content, reflecting a substantial energy shift from biomass production towards cellular maintenance. Reduced contents of photoprotective pigments and Chl a under the combination of UVR and elevated pCO₂ should be responsible for the reductions in photosynthetic carbon fixation and POC/PON contents. Our results imply that while progressive ocean acidification with increasing pCO₂ can stimulate growth of Synechococcus, the magnitude of this enhancement may be diminished by UV radiation, which varies with latitudes, depths, season, and weather conditions.

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