Ocean acidification alters the photosynthetic responses of a coccolithophorid to fluctuating UV and visible radiation

Mixing of seawater subjects phytoplankton to fluctuations in photosynthetically active radiation (PAR, 400-700nm) and ultraviolet radiation (UVR, 280-400nm). These irradiance fluctuations are now superimposed upon ocean acidification and thinning of the upper mixing layer through stratification, that alters mixing regimes. We therefore examined the photosynthetic carbon fixation and photochemical performance of a coccolithophore, Gephyrocapsa oceanica grown under high, future (1000 μatm) and low, current (390 μatm) CO2 levels, under regimes of fluctuating irradiances with or without UVR. Under both CO2 levels, fluctuating irradiances, as compared to constant irradiance, led to lower non-photochemical quenching (NPQ) and less UVR-induced inhibition of carbon fixation and PSII electron transport. The cells grown under high CO2 showed a lower photosynthetic carbon fixation rate, but lower NPQ and less UVB (280-315 nm)-induced inhibition. UVA (315-400 nm) led to less enhancement of the photosynthetic carbon fixation in the high CO2-grown cells under fluctuating irradiance. Our data suggest that ocean acidification and fast mixing or fluctuation of solar radiation will act synergistically to lower carbon fixation by G. oceanica, though ocean acidification may decrease UVB-related photochemical inhibition.

Jin P., Gao K., Villafañe V. E., Campbell D. A. & Helbling W., in press. Ocean acidification alters the photosynthetic responses of a coccolithophorid to fluctuating UV and visible radiation. Plant Physiology. Article.

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