Assessing the toxicity of heavy metals and potential tolerance of common Bean (Phaseolus vulgaris) while monitoring the population dynamics of the associated Rhizobia

Abstract

Microbially-assisted phytoremediation (MAP) is increasingly recognized as the feasible alternative for removing hazardous heavy metals (HMs) from contaminated environments. However, the dynamics of rhizobial-plant interactions during phytoremediation remain unclear. This study investigated the toxicity of some selected heavy metals (Cobalt, Nickel, and Manganese), the potential tolerance of Phaseolus vulgaris grown in the HMs-rich effluents, and the population dynamics of the associated Rhizobia within the Katsina metropolis. After 80 samples of P. vulgaris collected from Lambun Sarki garden were exposed to 10 mL of 0.5- 2g/L of Ni and Co and 5-20 g/L Mn, respectively, in mesocosms, and the plants treated with 10 mL HMs solutions daily. After three weeks treatment and then Indices of HMs toxicity on seeds and plants (4 and 3, respectively) were monitored in all the treatments. Weekly rhizobial counts on Congo Red Yeast Extract Mannitol Agar (CRYEMA) were taken to monitor rhizobial population dynamics. Pure isolates obtained after three iterations were identified biochemically. One-way ANOVA was employed for statistical analyses using AnalyStat (version 1.6.50). Generally, Ni exerts the highest toxicity, with Mn having less toxicity. Average rhizobial counts increased weekly, with high counts obtained in Ni and Mn treatments. However, they did not differ significantly between weeks (p = 0.061). Thus, longer time intervals (>2 weeks) are required to observe significant shifts in population dynamics. Moreover, HMs concentration did not affect the colony counts (p = 1.00). Metabolism profile of the preliminarily identified Rhizobium sp. and Sinorrhizobium melliloti evidenced HMs removal and plant growth promotion ability. The research demonstrated the phytoremediation ability of P. vulgaris and how rhizospheric population dynamics change during phytoremediation and contributed towards understanding HMs impact as environmental stressors on rhizospheric plant-microbe interactions. Future research targeting the hyperaccumulation capacity of the plants and heavy metals tolerance of the identified rhizobia are recommended, as this may help in knowing the BCF, TF, and BAC of the plants as well as the tolerable amount of the heavy metals to the bacteria.