Photosynthetic Characteristics of Macroalgae Ulva fasciata and Sargassum thunbergii in the Daya Bay of the South China Sea, with Special Reference to the Effects of Light Quality

The changes in underwater light in field usually occur not only in intensity but in spectrum, affecting the photophysiology of marine photoautotrophs. In this study, we comparably examined the photosynthesis of two dominating macroalgae in the Daya Bay, Chlorophyta Ulva fasciata and Phaeophyta Sargassum thunbergii , under white light, as well as under red, green and blue light. The results showed that the net photosynthetic O 2 evolution rate (Pn) of U. fasciata under field light increased from 25.2 ± 3.06 to 168 ± 1.2 µmol O 2 g FW −1 h −1 from dawn to noon, then decreased to 42.4 ± 0.20 µmol O 2 g FW −1 h −1 at dusk. The Pn of S. thunbergii exhibited a similar diel change pattern, but was over 50% lower than that of U. fasciata . The maximal photosynthetic rate (Pmax) of U. fasciata derived from the photosynthesis vs. irradiance curve under white light (i.e., 148 ± 15.8 µmol O 2 g FW −1 h −1 ) was ~30% higher than that under blue light, while the Pmax of S. thunbergii under white light (i.e., 39.2 ± 3.44 µmol O 2 g FW −1 h −1 ) was over 50% lower than that under red, green and blue light. Furthermore, the daily primary production (PP) of U. fasciata was ~20% higher under white than blue light, while that of S. thunbergii was 34% lower, indicating the varied light spectral compositions influence algal photosynthetic ability and thus their primary production in field, and such an influence is species-specific.