Daily variation in cellular content of UV-absorbing compounds mycosporine-like amino acids in the marine dinoflagellate Scrippsiella sweeneyae

Sudden exposure experiments to high PAR (photosynthetically available radiation) or high PAR + UVR (ultraviolet radiation) were conducted for the marine dinoflagellate Scrippsiella sweeneyae acclimated to either low PAR or high PAR to determine the induction of cellular mycosporine-like amino acid (MAA) in relation to photosynthesis status. When the exposure to high PAR (30.8 W m^-2) was provided at different time in the light period for S. sweeneyae acclimated to low PAR (7.7 W m^-2) which suppressed photosynthesis, S. sweeneyae could enhance the induction of MAA but it only occurred in the first half of the light period. When UVR exposure was provided for the culture acclimated to high PAR which enhanced photosynthesis, cellular MAA content did not increase during the entire light period, but displayed daily variation similar to the control for two and half days. Daily variation of cellular MAA content did not synchronized with that of cell volume and cellular chlorophyll a content. The individual MAAs also revealed similar daily variations with different phase, which increased for a few hours in the beginning of the light period, except for cellular palythine content. Thus the total cellular MAA content revealed daily variation with changing the relative composition within a few hours. As one of the biological protective strategies against harmful UVR in sunlight, the daily vertical migration in the bloom forming dinoflagellates might be accompanied by the daily variation of cellular MAA content for a photosynthesis at daytime.