Di Zhang1, Menglin Bao2, Sven Beer3, Kunshan Gao1, Juntian Xu2, Dong Yan1, and Cong Zhou1 and John Beardall1,4
1State Key Laboratory of Marine Environmental Science & College of Ocean and Earth Sciences, Xiamen University, Xiamen 361105, China
2 Co-Innovation Center of Jiangsu Marine Bio-industry Technology, Jiangsu Ocean University, Lianyungang 222005, China
3Department of Plant Sciences and Food Security, Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel
4School of Biological Sciences, Monash University, Clayton, VIC 3800, Australia
The commercially important red alga Pyropia yezoensis is subject, in its natural environment, to environmental stressors including high levels of both photosynthetically active radiation (PAR, 400-700 nm) and ultraviolet radiation (UVR, 280-400 nm). UVB radiation (280-315 nm) in particular is known to inhibit photosynthesis and growth in a range of algae. However, in outdoor exposure of P. yezoensis for 9 days and in the absence of UVR, elevated partial pressure of CO2 (pCO2) at levels predicted for the end of the century led to enhanced photosynthetic carbon assimilation rate (CAR) and relative growth rates (RGR). Both UVA (315-400 nm) and UVB caused significant inhibition of CAR and RGR, but the inhibition caused by UVA was considerably reduced under high pCO2. UVB inhibition was more severe and unaffected by pCO2. Rapid chlorophyll fluorescence curves showed that high levels of PAR alone induced photoinhibition of inter-photosystem electron transport. In the presence of UVR, photoinduced inhibition was mainly identified in the O2-evolving complex (OEC) and PSII. Such inhibition appeared to ameliorate the function of downstream electron acceptors, protecting PSI from over-reduction. In turn, the stable PSI activity increased the efficiency of cyclic electron transport (CET) around PSI, dissipating excess energy and supplying ATP for CO2 assimilation. When algae were grown under elevated CO2, CET became further enhanced, which maintained OEC stability, thus markedly alleviating UVR-induced photoinhibition. Thus, coordination between PSII and PSI endows P. yezoensis with a highly efficient photochemical performance in response to UVR, especially under future increased CO2 levels.