Optimization of culture conditions for improved green biodecolourization of methylene blue by Rhodococcus pyridinivorans strain UCC 0003

Methylene blue is a toxic dye present in the textile industry, and if left untreated, it causes harm to the environment. Therefore, to decolorize methylene blue from industrial effluents, a green approach using Rhodococcus pyridinivorans strain UCC 0003 was attempted. Methylene blue decolorization was measured spectrophotometrically, and the static condition yielded 86% decolorization after 24 h as compared to the shaking mode (20%). Optimization of static conditions using the one-factor-at-a-time approach resulted in 100% decolorization at 30̊C, pH 6, inoculum size of 16% (v/v), and 5% (v/v) banana peel addition as a carbon source. The R. pyridinivorans strain UCC 0003 could successfully and completely decolorize 0.75 g/l methylene blue for 4 consecutive cycles, which is advantageous from an economic point of view. The rate of methylene blue disappearance was investigated using 10% (v/v) R. pyridinivorans strain UCC 0003 at 30̊C over a certain incubation time with 0.4 g/l to 10.0 g/l methylene blue as the substrate. This study revealed Vmax and Km values of 37.04 g/l/h and 55.69 g/l, respectively, as the kinetic behavior of methylene blue-decolorizing enzymes from the bacterial strain. The properties of the treated solution of methylene blue resembled the control system (distilled water) for the phytotoxicity study, thereby indicating the complete removal of dye toxicity as evidenced by the growth of Vigna radiata and Triticum aestivum, respectively, in the treated methylene blue solution. This local bacterial strain has therefore a huge potential to be used as a green biocatalyst for the bioremediation of methylene blue-containing industrial effluents.