Physiological response of the symbiotic dinoflagellate Pelagodinium béii to ultraviolet radiation: synthesis and accumulation of mycosporine-like amino acids

We investigated the physiological response of the symbiotic dinoflagellate Pelagodinium béii to exposure to ultraviolet (UV) radiation, focusing on synthesis and accumulation of UV-absorbing substances (mycosporine-like amino acids, MAAs). For the culture experiments, we used P. béii isolated from the foraminifer Globigerinoides sacculifer , which ubiquitously inhabits the surface waters of oligotrophic oceans. The P. béii produced mainly shinorine, palythenic acid, and mycosporine-glycine as MAAs, and they accumulated MAAs within cells in proportion to UVB exposure within 12 h after UV irradiation. There was no marked accumulation of MAAs with irradiation by only UVA. We observed decreases in growth rate, cellular chlorophyll a content, and photosynthetic activity within 2 days, even under low UVB exposure. Our laboratory experiments showed that P. béii could synthesize MAAs in response to UVB exposure, but that cell growth and photosynthetic function could be inhibited by UVB levels in the surface waters of the natural environment. Our results suggested that free-living P. béii could not survive in the surface waters inhabited by the holobiont of P. béii and its host Globigerinoides sacculifer , even if the stress from nutrient limitation was reduced.