Effects of manganese on the physiology and ultrastructure of Sargassum cymosum

We herein report the physiological and cellular changes in the brown seaweed Sargassum cymosum (C. Agardh 1820) after short- (7 days) and long-term (14 days) exposure to manganese (Mn). To accomplish this, S. cymosum segments were cultivated without metal (0 mg L−1 Mn, Control) and with Mn (10, 20, 40 and 80 mg L−1) during 7 and 14 days under laboratory-controlled conditions. Seaweeds had high Mn biosorption capacity. Mn treatments had higher growth rates than Control at both exposure times, but reduction in cell viability after 14 days. Photosynthetic pigments (chlorophylls a and c) increased with Mn treatments, and changes in the composition of carotenoid profile were observed. Phenolic and flavonoid compounds also showed content reductions relative to exposure time and Mn concentration. In confocal analysis, lower autofluorescence of chloroplasts and phenolic compounds was observed. Scanning Electron Microscopy analysis indicated changes on the thallus surface of Mn treated samples, with disruption of cortical cell walls after 14 days. Transmission Electron Microscopy revealed cellular damage, with organelle disruption and disorganization of cell wall fibrils. Medium to high concentrations of Mn proved to be a metabolic stressor for S. cymosum, causing damage to organelle structure and cellular organization, while, at the same time, attenuating acclimation mechanisms for resistance against toxicity. Since this metal is not recognized as a pollutant in Brazilian coastal waters, S. cymosum is at risk for both heavy metal contamination and variations in abiotic parameters, such as salinity or pH, which could, in turn, intensify the metabolic damage to S. cymosum after long-term exposure of Mn.