Influence of cadmium and salinity in the red alga Pterocladiella capillacea: cell morphology, photosynthetic performance and antioxidant systems

This study aimed to determine the effects of cadmium (Cd) on apical segments of Pterocladiella capillacea (S. G. Gmelin) Santelices & Hommersand. For 7 days, these segments were cultivated under different salinities (25, 35 and 45 psu) and different Cd concentrations (0.17–0.70 ppm Cd). The samples were processed for light and confocal microscopy to assess photosynthetic performance, CHN levels, protein levels, as well as NADH dehydrogenase, and catalase activities. Plants cultivated at 0.70 ppm showed a reduction in the number of floridean starch grains by cytochemical analysis. Chloroplast autofluorescence decreased with cadmium exposure, while the kinetic patterns of ETR-light curves showed no significant differences among the treatments. Treatments showed variations in CNH levels, but no clear trend was observed. The content of total soluble protein and mitochondrial NADH dehydrogenase activity varied significantly, depending on the cadmium concentration and salinity levels. However, catalase activity decreased only in samples exposed to both cadmium and salinity treatments. Based on these lines of evidence, the present results showed a higher sensitivity of P. capillacea to increasing levels of cadmium exposure. When intermediate concentrations of cadmium were associated with low salinity values, its toxic effect on the model alga was intensified. Abiotic factors interfered with the physiological ability of the alga to protect itself from metal toxicity, resulting in metabolic compromise and reduction of primary productivity, i.e., photosynthesis and carbohydrate accumulation in the form of starch granules.