Response of the agarophyte Gelidium floridanum after in vitro exposure to ultraviolet radiation B: Changes in ultrastructure, pigments, and antioxidant systems

As a source of agar, the red macroalga Gelidium floridanum is a seaweed of great economic importance. However, it grows in a region exposed to high ultraviolet B radiation (UVBR). Therefore, to study the in vitro effect of UVBR on this plant, apical segments of G. floridanum were cultivated and exposed to photosynthetically active radiation (PAR) at 80 μmol photons m ^-2 s ^-1 and PAR + UVBR at 1. 6 W m ^-2 at 3 h per day for 7 days. The samples were processed for electron microscopy, and agar yield, growth rates, mitochondrial activity, protein levels, chlorophyll a, phycobiliproteins, carotenoids and phenolic compounds, and photosynthetic performance were examined. After 7 days of exposure to PAR + UVBR, G. floridanum experienced ultrastructural damage that was primarily observed in the internal organization of chloroplasts, increased cell wall thickness, as well as increased volume of plastoglobuli and free ribosomes. Moreover, this exposure might have caused photodamage and photoinhibition of photosynthetic pigments (chlorophyll a and phycobiliproteins), leading to a decrease in photosynthetic efficiency, relative electron transport rate, and maximum photosynthetic rate. These phenomena were matched with a corresponding decrease in growth rates and depigmentation, combined with partial necrosis of the apical segments exposed to PAR + UVBR. Additionally, the UVBR-induced damage elicited a marked cellular antioxidant response, possibly as a consequence of free radical generation.