Seasonal cycles in phytoplankton mycosporine-like amino acids and the attenuation of ultraviolet radiation in coastal surface waters in Sagami Bay

Abstract

The concentration of mycosporine-like amino acids (MAAs) of phytoplankton and the attenuation coefficients of ultraviolet radiation (UVR) were determined in coastal surface waters in Sagami Bay during the period from 2004 to 2012 to examine the seasonal cycle. Four major MAAs, including mycosporine-glycine, palythine, shinorine, and porphyra-334, were detected throughout the year. Mycosporine-glycine was the most dominant type, reaching more than 100 nM, whereas porphyra-334 was the least dominant type, barely reaching 20 nM. The total concentration of the four MAAs was high in the high water temperature (WT) months (February-July) and low in the low WT months (August-January). The seasonal cycle was repeatedly sustained for six years. The attenuation of ultraviolet radiation at two wavelengths, 320 and 340 nm, also followed a similar seasonal cycle: high in the warm period and low in the cool period. The attenuation of UVR at two wavelengths (kd[320] and kd[340]) was linearly correlated with the concentration of MAAs (p<0.01). Solar energy was the environmental driving force responsible for the seasonal cycles of MAAs, which were further enhanced by increased biomass and nutrient availability. Individual groups in the phytoplankton community might share roles to protect the entire community from UVR in the marine ecosystem by producing various MAAs.