Zinc and cadmium contamination in an Avicennia schaueriana mangrove environment: Mass balance and tissue distribution

The leachate pond of a Zn processing plant in Rio de Janeiro, Brazil, released Zn and Cd into a mangrove during three decades. Soil, root, stems, leaves and phloem bark samples of Avicenna schaueriana from the mangrove were collected and analyzed. Zn and Cd levels were measured by atomic absorption and were much more concentrated in soil at 76 kg/ha and 424 kg/ha respectively than in the trees (272.3 kg/ha Zn and 0.046 kg/ha Cd). This species shows a protective mechanism against uptake. The high content of Cd in the bark phloem was due to exposure to tidal flooding with high concentrations of Cd, not translocation from the roots. Allometry, and the biomass of roots, stems, leaves and bark showed that only a small percentage was phytoextracted by A. schaueriana, so this species under the described concentrations cannot phytoaccumulate and should not be used in phytoextraction.

Mixed planting with a leguminous plant outperforms bacteria in promoting growth of a metal remediating plant through histidine synthesis

The effectiveness of plant growth promoting bacteria (PGPB) in improving metal phytoremediation is still limited by stunted plant growth under high soil metal concentrations. Meanwhile, mixed planting with leguminous plants is known to improve yield in nutrient deficient soils but the use of a metal tolerant legume to enhance metal tolerance of a phytoremediator has not been explored. We compared the use of Pseudomonas brassicacearum, Rhizobium leguminosarum, and the metal tolerant leguminous plant Vicia sativa to promote the growth of Brassica juncea in soil contaminated with 400 mg Zn kg(-1), and used synchrotron based microfocus X-ray absorption spectroscopy to probe Zn speciation in plant roots. B. juncea grew better when planted with V. sativa than when inoculated with PGPB. By combining PGPB with mixed planting, B. juncea recovered full growth while also achieving soil remediation efficiency of >75%, the maximum ever demonstrated for B. juncea. μXANES analysis of V. sativa suggested possible root exudation of the Zn chelates histidine and cysteine were responsible for reducing Zn toxicity. We propose the exploration of a legume-assisted-phytoremediation system as a more effective alternative to PGPB for Zn bioremediation.