PAR and UVBR effects on photosynthesis, viability, growth and DNA in different life stages of two coexisting Gigartinales: implications for recruitment and zonation pattern

The effects that ultraviolet radiation (UVR, 280 to 400 nm) and photosynthetically active radiation (PAR, 400 to 700 nm) had on early life stages of Mastocarpus stellatus and Chondrus crispus were studied to determine if differences in UVR tolerance could influence their recruitment success on the upper eulittoral shores of Helgoland (North Sea). Photosynthesis, germination capacity, DNA damage and carpospore repair were measured after exposures to different time lengths and intensities of PAR+UV-A+UV-B, PAR+UV-A or PAR alone, and also after recovery in low white light. Germination and photosynthesis of the low light adapted carpospores of both species were inhibited as PAR was increased. Supplemental UV-A and UV-B had a small additional effect on the F-v/F-m of M, stellatus but this effect was more pronounced in C. crispus. However, photosynthesis of both species significantly recovered after 48 h. Carpospore viability in C, crispus was more sensitive than in M. stellatus to UVR, while a higher dose was needed to achieve 50 % germination inhibition in M stellatus. Furthermore, UV-B-induced DNA damage, measured as cyclobutane-pyrimidine dimers (CPDs), was less in M stellatus spores, which also exhibited an efficient DNA repair mechanism compared with C crispus. In contrast, growth and chlorophyll a contents in young gametophytes of both species were not affected by repeated UV exposures. Higher total carotenoid was measured in plants exposed to UVR, indicating a photoprotection role, because photosynthesis completely acclimated to UVR after 3 d. Furthermore, DNA damage was not detected on mature fronds of both species when exposed to the full solar spectrum. Therefore, the susceptibility of carpospores to UVR could influence species recruitment to the upper eulittoral zone.