Effects of UV-B radiation on growth rates, pigment content and ultrastructure of red (wild type), greenish-brown and green strains of Gracilaria birdiae (Gracilariales, Rhodophyta)

Based on physiological characteristics, we hypothesized that different strains of Gracilaria birdiae from two distinct geographical areas of the Brazilian coast (2500 km apart) would have different responses to long-term exposure to UV-B radiation (UV-B). The locations differ in their environmental conditions: one is a warmer area, Ceara State (CE), closer to the equator; the other is a colder area, Espirito Santo State (ES), closer to the Tropic of Capricorn. To test the hypothesis that the CE population is more resistant to UV-B than the ES population, apical segments of the red (RDCE, RDES), green (GR(CE)) and greenish-brown (GB(CE)) strains were cultivated in the laboratory under two treatments: control (PAR) and artificial UV-B (PAR + UV-B). Algal performance was evaluated by considering growth rates, pigment content and ultrastructural analysis. Compared with the control, all strains showed a decrease in growth rates after exposure to UV-B. Of all strains, RDES showed the greatest sensitivity to UV-B. However, a decrease in growth rate and morphological changes were observed to a lesser extent in the RDCE strain. Moreover, exposure to UV-B resulted in a decrease in the concentrations of phycobiliproteins in the RDCE strain. The GB(CE) strain showed an increase in phycoerythrin (PE)/allophycocyanin (APC) and phycocyanin (PC)/allophycocyanin (APC) ratios after exposure to UV-B, suggesting this strain had a higher tolerance to the radiation. No differences in the chlorophyll a and carotenoid content were found between the control and UV-B treated samples for all strains. Ultrastructural changes, such as damage to chloroplasts and mitochondria, were present in all strains after exposure to UV-B. In summary, our findings support the hypothesis that the population from Ceara State has adapted to the higher irradiation and is thus more resistant to increased UV-B. Additionally, of the strains tested, the GB(CE) and RDCE strains appear to be more resistant to this radiation. (AU)