Microbe-citric acid assisted phytoremediation of chromium by castor bean (Ricinus communis L.)

Chromium is one of the highly toxic heavy metals to plant growth and development especially hexavalent chromium (Cr⁺⁶) due to its readily available nature and mobility into the environment. The chelating agents and hyperaccumulator plant can contribute to remediating the heavy metals from the contaminated medium. This study was conducted to analyze the role of citric acid and chromium resistant bacteria in castor bean to remediate Cr⁺⁶ from the polluted soil. The soil was spiked with different levels of citric acid (0, 2.5, 5 mM) and chromium (0, 10, 20 mg kg^-1). The ripened plants were harvested and analyzed for growth parameters, chlorophyll contents, gas exchange parameters, oxidative stress markers, antioxidant enzymes activities and chromium accumulation in different parts of plants. The high concentration of chromium 20 mg kg^-1 drastically reduced the plant growth, decreased photosynthetic rate and increased oxidative stress. The application of CA improved the plant growth even at the highest concentration of chromium which was further boosted by the combined application of CA and chromium resistant bacteria. However, the performance of staphylococcus aureus was found significantly better than Bacillus subtilis due to its better ability to tolerate chromium toxicity even at high concentrations. The findings proved that castor bean has excellent potential to tolerate high chromium concentrations and can be effectively used to remediate metals contaminated soil. Further, CA and metal resistant bacteria can significantly enhance the phytoremediation potential of castor bean and other hyperaccumulator plants. The bacteria assisted phytoremediation coupled with the chelating agent can be a practical approach to remediate the metals contaminating soils.