Absorption of Low-Dose Ultraviolet Radiation by Mycosporine-like Amino Acids Induced by the Dinoflagellate Prorocentrum micans

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.