Ocean acidification exacerbates the inhibition of fluctuating light on the productivity of Ulva prolifera.

Ulva prolifera, a common species of green macroalgae, is often harmful-algal-bloom causative and significantly impacts local marine ecosystems. Previous studies on the physiological characteristics of U. prolifera have been conducted under constant light (CL). However, light in the natural environment continually changes, and little is known about fluctuating light (FL). Ocean acidification (OA) has been proposed to interact with dynamic surrounding environments to affect the physiological performance of macroalgae. Therefore, we investigated the combined effects of FL (80/300, alternating between 80 μmol photons m-2 s-1 for 2.5 h and 300 μmol photons m-2 s-1 for 1.5 h, with an average light intensity of 160 μmol photons m-2 s-1 and OA (1000 ppm CO2) on U. prolifera. The results clearly showed that FL had no significant effect on the relative growth rate (RGR), whereas OA obviously improved RGR. However, under FL-OA combination conditions, RGR was inhibited significantly, accompanied by a concomitant downgraded photosynthetic performance, while the photoprotective abilities were enhanced. The results would help us accurately predict the primary productivity of macroalgae in coastal waters under future OA conditions with irradiance fluctuations.