Abstract

Ultraviolet (UV) radiation is a major constraint to aquatic photosynthetic organisms. Freshwater algae dwelling in natural high altitudinal ponds and lakes must tolerate high UV radiation exposure, especially in oligotrophic waters. Here, we investigated the in situ physiological short-term response of three algal communities to UV removal in lakes at high-elevation in Brazil, by measuring photosynthesis, carotenoid and phenolic contents, nutrient concentration and antioxidant enzymatic activities. Rancho Queimado (RQ), Lagoa do Roncador (LR) and Lagoa Bonita (LB), located in southern and central Brazil, were classified as oligo and ultraoligotrophic and presented species-rich algal communities dominated by species of the class Zygnematophyceae. The algal communities presented different short-term metabolic strategies to circumvent the stressful effects of the UV, influenced by species composition. At RQ, photosynthesis was negatively affected by UV removal, suggesting that photoprotection may be triggered by this radiation; at this site, the algae relied mainly on an efficient superoxide dismutase response. LR algae exhibited a higher contribution of catalase but prioritized the non-enzymatic antioxidant defences (phenolics and carotenoids) as their background and primary barrier against reactive oxygen species, which contributed to the development of higher net photosynthesis than that of the other sites and showed no variation to light treatments. LB algae appeared to be impaired by the increase in the temperature and highly variable irradiance throughout the day, which affected the net photosynthesis and activity of antioxidant enzymes. However, similar to LR, carotenoids and phenolic compounds acted as protagonists against oxidative damage.