Cadmium adsorption by rhizobacteria: implications for New Zealand pastureland

Cadmium (Cd) accumulation by plants is a concern for human health and for d ecreased productivity of plants and animals. Uptake of Cd by plants is also fundamental in the process of phytoremediation, whereby plants are used to remove Cd from polluted soils. Plants have a substantial microflora associ ated with their rhizospheres that influence nutrient mobilisation and avail ability to the plant. This study investigated the effects of Cd on the grow th and Cd uptake of Pseudomonas fluorescens (CR3) a lux-marked rhizoplane b acterial isolate from clover-root tips, as well as four other bacterial iso lates from the clover (Trifolium spp.) rhizosphere. The bacteria studied ar e from a ryegrass (Lolium perenne)/clover pastureland near Palmerston North . In the first study, CR3 was grown in a media containing 0, 0.1, 0.5, 1 an d 5 mg l(-1) Cd. Optical density, a luminescence measurement of metabolic a ctivity and Cd adsorption were measured over a 48-hour period. There was no significant retardation of growth in media containing less than 1 mg l(-1) Cd. The lag phase was significantly longer in the 1 and 5 mg l(-1) treatme nts. The metabolic activity in these treatments, as indicated by luminescen ce, was also significantly lower during the exponential growth phase. Cadmi um accumulated in the bacteria to circa is 100 times that of the Cd concent ration of the media in which they were grown. Accumulation of Cd was also a ssessed under different growth-media pH conditions and with different bacte rial isolates. Cadmium absorption by cells decreased with reduced pH and ab sorption varied between different species. Sequestration of Cd by rhizosphe re microorganisms may have an important influence on plant Cd uptake. Furth er research is still required, however, to establish whether the accumulati on of Cd by rhizobacteria inhibits, or accelerates, Cd uptake by the host p lant. (C) 2001 Elsevier Science B.V. All rights reserved.