Elevated CO concentration alleviates UVR-induced inhibition of photosynthetic light reactions and growth in an intertidal red macroalga.

The commercially important red macroalga Pyropia (formerly Porphyra) yezoensis is, in its natural intertidal environment, subjected to high levels of both photosynthetically active and ultraviolet radiation (PAR and UVR, respectively). In the present work, we investigated the effects of a plausibly increased global CO concentration on quantum yields of photosystems II (PSII) and I (PSI), as well as photosynthetic and growth rates of P. yezoensis grown under natural solar irradiance regimes with or without the presence of UV-A and/or UV-B. Our results showed that the high-CO treatment (~1000 μbar, which also caused a drop of 0.3 pH units in the seawater) significantly increased both CO assimilation rates (by 35%) and growth (by 18%), as compared with ambient air of ~400 μbar CO. The inhibition of growth by UV-A (by 26%) was reduced to 15% by high-CO concentration, while the inhibition by UV-B remained at ~6% under both CO concentrations. Homologous results were also found for the maximal relative photosynthetic electron transport rates (rETR), the maximum quantum yield of PSII (F/F), as well as the midday decrease in effective quantum yield of PSII (YII) and concomitant increased non-photochemical quenching (NPQ). A two-way ANOVA analysis showed an interaction between CO concentration and irradiance quality, reflecting that UVR-induced inhibition of both growth and YII were alleviated under the high-CO treatment. Contrary to PSII, the effective quantum yield of PSI (YI) showed higher values under high-CO condition, and was not significantly affected by the presence of UVR, indicating that it was well protected from this radiation. Both the elevated CO concentration and presence of UVR significantly induced UV-absorbing compounds. These results suggest that future increasing CO conditions will be beneficial for photosynthesis and growth of P. yezoensis even if UVR should remain at high levels.