Abstract
Marine dinoflagellates synthesize ultraviolet absorbing compounds, such as mycosporine-like amino acids (MAAs), at high ultraviolet radiation (UVR) to protect from UVR damage. Short-term responses (360 min) of MAAs and absorption of UVR between 310 nm to 360 nm (a[310–360]) to UVR were examined in the dinoflagellate Prorocentrum micans to confirm reproducibility even at relatively low doses of UVR (UVB: 0.76 W m−2 in 320–340 nm) and (UVA: 0.84 W m−2 in 340–400 nm). By using a low dose of UVR there was no apparent bleaching of cells and it was possible to normalize MAA concentration with chlorophyll a concentration (Chl a). At the beginning of the exposure experiment, MAAs primarily consisted of shinorine (58.5%), palythine (30.2%), mycosporine-glycine (7.0%), palythene (2.6%), and porphyra-334 (1.7%). Net change in the Chl a specific MAA concentration (δMAAs) was estimated as the difference in Chl a specific MAA concentrations between the experiments with photosynthetically active radiation (PAR in 400–700 nm) and PAR+UVR (320–700 nm). The responses to either PAR or PAR+UVR were composed of two phases: the first was shorter than approximately 90 min, and the second included the subsequent phases. The initial temporal change during the first phase in δMAAs (0.031 nmol MAAs [mol Chl a]−1 min−1) consisted of an increase in the four primary MAAs, shinorine, porphyra-334, mycosporine-glycine, and palythene (0.049 nmol MAAs [mol Chl a]−1 min−1), and a decrease in one secondary MAA, palythine (−0.018 nmol MAA [mol Chl a]−1 min−1). Although the temporal increase in δMAAs was limited, a possible main route of MAA transformation had mycosporine-glycine and porphyra-334 as the primary precursors of a sequential series of conversions, by which the cells accumulated shinorine and palythene, possibly at the expense of palythine. The increased MAAs enhanced a(310–360). The cells appear to achieve MAA transformation with an increase in total MAAs to protect from UVR damage.
