Contribution of the UV component of natural sunlight to photoinhibition ofphotosynthesis in six species of subtidal brown and red seaweeds

Field-collected specimens of three species of Laminaria and three species o f subtidal red algae (Delesseria sanguinea, Plocamium cartilagineum and Phy llophora pseudoceranoides) were exposed to natural summer sunlight on Helgo land (southern North Sea) for up to 4 h at 15 degreesC. Dark-adapted variab le fluorescence (F-v : F-m) was measured immediately after these treatments , and following 6, 24 and 48 h of recovery in moderate irradiances of white light. The response of plants to the full spectrum of natural sunlight was compared with that to PAR alone, UV-A + visible, UV-A + UV-B, or UV-A alon e. The F-v:F-m values of all species were reduced to minimal values after 4 h in all of these treatments, but those of the more resistant species (Lam inaria spp. and P. pseudoceranoides) were higher after shorter exposures to UV radiation alone than to PAR with or without UV. The recovery of F-v:F-m in all species was also more rapid in the two treatments that contained UV radiation alone than in those that included PAR. These results suggest tha t it is the high irradiances of PAR in natural sunlight which are responsib le for the photoinhibition of photosynthesis of subtidal seaweeds and that the current ambient irradiances of UV radiation (either UV-B or UV-A) in no rthern temperate latitudes would not contribute significantly to this photo inhibition.