Cellular Mycosporine-like amino acids protect photosystem II of the Dinoflagellate Scrippsiella sweeneyae from ultraviolet radiation damage

Photo-damage to photosystem II (PSII) from ultraviolet radiation (UVR) was determined using chlorophyll fluorescence in relation to sunscreen factors on the dinoflagellate Scrippsiella sweeneyae based on the cellular mycosporine-like amino acid contents (CMAAs, cell) and cell diameter (= light path, d). Three different CMAAs, cell were prepared by acclimating cells to three levels (30.8, 15.2, and 7.7 W m− 2) of photosynthetically active radiation (PAR). PAR-acclimated cells were exposed to PAR (0.64 W m− 2) + UVR (3.94 W m− 2 = 2.51 W m− 2 UVB + 1.43 W m− 2 UVA) for 12 min. High PAR (HL) and medium PAR (NDF1) treatments acclimated cells to induce shinorine and porphyra-334 (longer λmax at 333 and 334 nm); whereas, the low PAR (NDF2) treatment acclimated cells to induce mycosporine-glycine and palythine (shorter λmax at 310 and 320 nm). Absorption spectra for the individual MAAs were reconstructed using the λmax and CMAAs, cell and were summed to reconstruct the absorption of the total CMAAs, cell (m2 cell− 1) to estimate the sunscreen factor (S[λ]) at λmax. The highest S(λmax) was obtained for cells that acclimated to the highest PAR (highest CMAAs, cell and longest d); whereas, the lowest S(λmax) was obtained for cells acclimated to the lowest PAR (the lowest CMAAs, cell and the shortest d). CMAAs, cell contributed approximately 94%, whereas d contributed < 6%, of the sunscreen factor (Sλmax). UVR-induced damage was indexed with a temporal decrease in the optimum quantum yield (Fv/Fm) in the Photosystem II. The highest damage was observed for cells acclimated to the lowest Sλmax (lowest CMAAs, cell and shortest d); whereas, the lowest damage was observed for cells acclimated to the highest Sλmax (highest CMAAs, cell and longest d). The CMAAs, cell mitigated most of the UVR-induced damage in photosystem II of the dinoflagellate S. sweeneyae.