How macroalgae can deal with radiation variability and photoacclimation capacity: The example of Gracilaria tenuistipitata (Rhodophyta) in laboratory

Radiation continuously varies in the environment, exposing phototrophic organisms to diverse compositions and ratios among photosynthetically active radiation (PAR), UV-A (Ultraviolet A radiation) and UV-B (Ultraviolet B radiation). In this sense, UV radiation can control, either positively or negatively, the physiology and metabolism of marine organisms in intertidal areas. Our study focused on the exposure of Gracilaria tenuistipitata to variable combinations of radiation, evaluating its physiology, photoprotective strategies and DNA damage under each condition. To accomplish this, algal apical fragments were acclimated to three radiation treatments (PAR, PAR + UV-A and PAR + UV-A + UV-B) for one week. Thereafter, PAR-acclimated samples were moved to PAR (control), PAR + UV-A + UV-B and PAR + UV-B. PAR + UV-A-acclimated samples were moved to PAR + UV-A (control), PAR + UV-A + UV-B and PAR + UV-B, and PAR + UVR-acclimated thalli were placed in PAR + UV-A + UV-B (control), PAR + UV-B and PAR. Samples remained in these conditions for a period of 48 h continuously. Photosynthetic parameters from chlorophyll a fluorescence were evaluated, as well as growth rates, mycosporine-like amino acids, chlorophyll a, and carotenoid contents. In addition, DNA damage was assessed by detecting cyclobutane pyrimidin dimer. Expression of photolyases, ascorbate peroxidase and gluthatione peroxidase was also analyzed at the end of experimental period. Mycosporine-like amino acids were stimulated by UV radiation. DNA damage occurred only when UV-B was provided separately. In some cases, photoacclimation procedures were activated, while in others, it was not enough. In other words, although some tolerance was observed, the species could suffer the consequences of excessive energy present in some specific treatments, e.g., PAR + UV-B. After exposure to variable radiation, it was concluded that our model organism presented acclimation depending of spectral availability, including regulation of photoprotective strategies.