Effects of short-term exposure to ultraviolet-B radiation on photosynthesis and pigment content of red (wild types), greenish-brown, and green strains of Gracilaria birdiae (Gracilariales, Rhodophyta)

Abstract

Based on physiological characteristics, our hypothesis was that different strains of Gracilaria birdiae from two distinct geographical areas of the Brazilian coast, 2,500 km apart, would respond differently to short-term exposure to UV-B radiation. These sites present diverse environment conditions, i.e., one is a warmer area in Ceara State (CE), northeast Brazil, whereas the other is a colder area in Espirito Santo State (ES), southeast Brazil. To test the hypothesis that the northeastern population is more resistant to the stress caused by UV-B than the southeastern, red (RD CE , RD ES ), green (GR CE ), and greenish-brown (GB CE ) strains were cultivated in the laboratory under two different treatments, viz., control (PAR) and artificial UV-B (PAR + UV-B), for 5 h each. Except for the RD CE strain, the effective quantum yield decreased after exposure to UV-B. The photosynthesis recovery observed only for the GR CE strain suggests dynamic photoinhibition. The phycoerythrin/phycocyanin ratio was also higher for the RD CE strain, thereby favoring phycobilisomes acclimatization to changes in irradiance. The absence of alterations in chlorophyll a and carotenoid contents indicated there to be no damage to the photosynthetic apparatus. Results support the notion that the northeastern population, through adapting to higher irradiation, had thus become more resistant to increase UV-B. Furthermore, among the different strains, both the GR CE and RD CE strains appear to be more resistant to this radiation.