Exposures to UV Radiation Diminish the Positive Effects of Elevated pCO2 on a Coastal Strain of the Picophytoplankton Synechococcus

The cyanobacteria Synechococcus spp. are key contributors to marine primary productivity. They are found from equatorial to polar waters where these organisms are exposed to changed levels of pCO2 and solar ultraviolet radiation (UVR, 280–400 nm). To investigate how elevated pCO2 modulates the physiological responses of a coastal pico-cyanobacterium to UVR, Synechococcus XM24 was used and acclimated to periodic (90 min per day) near-surface 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 UVR). However, such stimulatory effects were substantially abolished in UVR-exposed cells. UVR significantly inhibited the effective photochemical quantum yield of photosystem II (PSII) and accelerated PSII damage without a compensatory increase in repair, thereby reducing the repair/damage ratio (r/k) 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 addition, UVR exposure led to drastic reductions in cellular POC and PON content, reflecting a potential energy shift from biomass production towards cellular maintenance. Reduced contents of Chl a and carotenoids 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 the coastal Synechococcus XM24 under PAR-alone conditions, exposures to UV radiation can diminish such enhancement.