Nickel tolerance and accumulation by bacteria from rhizosphere of nickel hyperaccumulators in serpentine soil ecosystem of Andaman, India

Rhizosphere microorganisms harboring nickel hyperaccumulators, Rinorea bengalensis (Wall.) O. K. and Dichapetalum gelonioides ssp. andamanicum (King) Leenh. endemic to serpentine outcrops of Andaman Islands, India, were screened for their tolerance and accumulation of Ni. The rhizosphere soils from both the plants were rich in total and available Ni along with Co, Cr, Fe and Mg but poor in microbial density and were dominated by bacteria. Out of total 123 rhizosphere microorganisms (99 bacteria and 24 fungi), bacteria were more tolerant to Ni than fungi. Viable cells of selected Ni-tolerant bacterial isolates (MIC = 13.6–28.9 mM Ni) belonging to Pseudomonas, Bacillus and Cupriavidus were capable of accumulating nickel (209.5–224.0 μM Ni g⁻¹ protein) from aqueous solution. Cupriavidus pauculus KPS 201 (MTCC 6280), showing highest degree of nickel tolerance (MIC 28.9 mM Ni) and uptake (224.0 μM Ni g⁻¹ protein, 60 min) was used for detailed study. Kinetics of nickel uptake in C. pauculus KPS 201 followed a linearized Lineweaver-Burk plot. The K _m and V _max for nickel uptake by minimal medium grown-cells approximated 1.5 mM Ni and 636.9 μM Ni g⁻¹ protein, respectively. The uptake process was inhibited by Co, Cu, Cd, Mg, Mn and Zn, however, complete inhibition was not achieved even in presence of 500 mM Mg. Metabolic inhibitors, sodium azide (1.0 mM) and carbonyl cyanide m-chlorophenylhydrazone (0.4 mM) strongly inhibited nickel uptake suggesting the process as an energy dependent one. The present study clearly shows that bacteria in the rhizosphere of Ni-hyperaccumulators are capable of tolerating high concentration of Ni and also possesses nickel uptake potential. The Ni-hyperaccumulators in combination with these Ni-resistant bacteria could be an ideal tool for nickel bioremediation.