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Abstract
Previous studies on diurnal photosynthesis of macroalgal species have shown that at similar levels of photosynthetically active radiation (PAR, 400–700nm) the photosynthetic rate is lower in the afternoon than in the morning. However, the impacts of solar ultraviolet radiation (UVR, 280–400nm) have been little considered. We investigated the diurnal photosynthetic behaviour of the economically significant red alga Gracilaria lemaneiformis in the absence or presence of UV-A+B or UV-B with a flow-through system. While UV-A and UV-B, respectively, inhibited noontime Pmax by 22% and 14% on the sunny days, UV-A during sunrise (PAR below about 50Wm−2) increased the net photosynthesis by about 8% when compared with PAR alone. UV-A + PAR also resulted in higher apparent photosynthetic efficiency in the morning than in the afternoon period than PAR alone. Nevertheless, integrated daytime photosynthetic production under solar PAR alone was higher than with either PAR + UV-A+B or PAR + UV-A. Relative growth rate in the long term (9 days) matched the integrated photosynthetic production in that UV-A led to 9–15% and UV-B to 19–22% reduction, respectively. UV-absorbing compounds were found to be higher in the thalli exposed to PAR+UV-A+B than under PAR alone, reflecting a protective response to UVR.
Acknowledgements
We thank the anonymous reviewers for their constructive comments. This study was supported by “863” project (2006AA10A413) from Ministry of Science and Technology, National Natural Science Foundation of China (Key Project No. 90411018) and Ministry of Education (No.308015). We thank Professors Donat-P. Häder and E. Walter Helbling for helpful comments during their research visits to Shantou University.