Impact of chromium and lead on Nostoc muscorum: regulation of toxicity by ascorbic acid, glutathione, and sulfur-containing amino acids.

Nostoc muscorum can be protected from Cr and Pb toxicity by supplementing 10 micrograms/ml of ascorbic acid, reduced glutathione, and 5 micrograms/ml of sulfur-containing amino acids (L-methionine and L-cysteine). Appreciable stimulation of different parameters of Nostoc by ascorbic acid and glutathione was noticed in the untreated culture. These substances when used in the presence of metals were found to restore ionic loss, growth, 14CO2 uptake, heterocyst production, and nitrogenase activity thus acting as reducing agents. Sulfur-containing amino acids (at 5 micrograms/ml) exert little stimulatory effect on untreated cultures. These results suggest that sulfur-containing compounds and reducing agents can protect against metal toxicity possibly by providing a reducing power which protects the enzymes from undergoing oxidation. Inhibition of heterocyst frequency and nitrogenase activity by L-methionine and L-cysteine suggests that these amino acids possibly act as a nitrogen source. This study clearly suggests that metal toxicity in N2-fixing cyanobacterium may be appreciably counteracted by reducing substances and amino acids which are likely to occur in natural habitats.