Biotechnological applications of serpentine soil bacteria for phytoremediation of trace metals

Serpentine or ultramafic soils are produced by weathering and pedogenesis of ultramafic rocks that are characterized by high levels of Ni, Cr, and sometimes Co, but contain low levels of essential nutrients such as N, P, K, and Ca. A number of plant species endemic to serpentine soils are capable of accumulating exceptionally high concentrations of Ni, Zn, and Co. These plants are known as metal “hyperaccumulators.” The function of hyperaccumulation depends not only on the plant, but also on the interaction of the plant roots with rhizosphere microbes and the concentrations of bioavailable metals in the soil. The rhizosphere provides a complex and dynamic microenvironment where microorganisms, in association with roots, form unique communities that have considerable potential for the detoxification of hazardous materials. The rhizosphere bacteria play a significant role on plant growth in serpentine soils by various mechanisms, namely, fixation of atmospheric nitrogen, utilization of 1-aminocyclopropane-1-carboxylic acid (ACC) as the sole N source, production of siderophores, or production of plant growth regulators (hormones). Further, many microorganisms in serpentine soil are able to solubilize “unavailable” forms of heavy metal–bearing minerals by excreting organic acids. In addition, the metal-resistant serpentine isolates increase the efficiency of phytoextraction directly by enhancing the metal accumulation in plant tissues and indirectly by promoting the shoot and root biomass of hyperaccumulators. Hence, isolation of the indigenous and stress-adapted beneficial bacteria serve as a potential biotechnological tool for inoculation of plants for the successful restoration of metal-contaminated ecosystems. In this study, we highlight the diversity and beneficial features of serpentine bacteria and discuss their potential in phytoremediation of serpentine and anthropogenically metal-contaminated soils.