Impact of heavy metals in the microalga Chlorella sorokiniana and assessment of its potential use in cadmium bioremediation.

The chlorophyte microalga Chlorella sorokiniana was tested for the bioremediation of heavy metals pollution. It was cultured with different concentrations of Cu, Cd, As (III) and As (V), showing a significant inhibition on its growth at concentrations of 500 µM Cu, 250 µM Cd, 750 µM AsO and 5 mM AsO or higher. Moreover, the consumption of ammonium was also studied, showing significant differences for concentrations higher than 1 mM of Cu and As (III), and 5 mM of As (V). The determination of intracellular heavy metals concentration revealed that Chlorella sorokiniana is an outstanding Cd accumulator organism, able to accumulate 11,232 mg kg of Cd, and removing 65% of initial concentration of this heavy metal. Finally, antioxidant enzymes, such as catalase (CAT) and ascorbate peroxidase (APX), and enzymes involved in the production of glutamate and cysteine, such as glutamine syntethase (GS), glutamate dehydrogenase (GDH), O-acetylserine (thiol) lyase (OASTL) and NAD-isocitrate dehydrogenase (NAD-IDH) were studied both at gene expression and enzymatic activity levels. These enzymes exhibited different grades of upregulation, especially in response to Cd and As stress. However, GS expression was downregulated when Chlorella sorokiniana was cultured in the presence of these heavy metals.