Comparison of the photosynthetic characteristics of two Isochrysis galbana strains under high light

Abstract

Abstract The photoprotection mechanisms in the marine microalga Isochrysis galbana were examined by comparing the photosynthetic characteristics in two I. galbana strains IOAC724S and IOAC683S under high light. The rDNA ITS regions of IOAC724S and IOAC683S were closely clustered in the Neighbor-Joining tree, suggesting that the two strains are very similar to each other genetically. Regulated energy dissipation in photosystem (PS) II (NPQ) and cyclic electron flow with PSI (CEF-I) can protect the photosynthetic apparatus against photodamage. There were no significant differences in NPQ and CEF-I between IOAC724S and IOAC683S under high light. In both strains, NPQ was very low, and CEF-I was maintained at a high level. This suggested that NPQ was not strong enough to dissipate the excess excitation energy, and CEF-I might protect the photosynthetic apparatus against photodamage in I. galbana. Photosynthetic linear electron flow was lower, but the alternative electron flow within PSII (AEF-II) was significantly higher in IOAC724S than in IOAC683S. The higher AEF-II in IOAC724S efficiently removed excess excitation energy, thereby, protecting the photosynthetic apparatus, as indicated by the lower value of quantum yield of the nonregulated energy dissipation of PSII and the lower content of hydrogen peroxide in IOAC724S.