A microbial biosensor to predict bioavailable nickel in soil and its transfer to plants

Ralstonia eutropha strain AE2515 was constructed and optimised to serve as a whole-cell biosensor for the detection of bioavailable concentrations of Ni2+ and Co2+ in soil samples. Strain AE2515 is a Ralstonia eutropha CH34 d erivative containing pMOL1550, in which the cnrYXH regulatory genes are tra nscriptionally fused to the bioluminescent luxCDABE reporter system. Strain AE2515 was standardised for its specific responses to Co2+ and Ni2+. The d etection limits for AE2515 were 0.1 muM Ni2+ and 9 muM Co2+, respectively. The signal to noise (S/N) bioluminescence response and the metal cation con centration could be linearly correlated: for Ni2+ this was applicable withi n the range 0.1-60 muM and between 9 and 400 muM for Co2+. The AE2515 biose nsor strain was found to be highly selective for nickel and cobalt: no indu ction was observed with Zn(II), Cd(II), Mn(II), Cu(III) and Cr(VI), In mixe d metal solutions, the bioluminescent response always corresponded to the n ickel concentrations. Only in the presence of high concentrations of Co2+ ( 2 mM), the sensitivity to nickel was reduced due to metal toxicity. AE2515 was used to quantify the metal bioavailability in various nickel-enriched s oils, which had been treated with additives for in situ metal immobilisatio n. The data obtained with strain AE2515 confirmed that the bioavailability of nickel was greatly reduced following the treatment of the soils with the additives beringite and steel shots. Furthermore, the data were found to c orrelate linearly with those on the biological accumulation of Ni2+ in spec ific parts of important agricultural crops, such as maize and potato. There fore, the test can be used to assess the potential transfer of nickel to or ganisms of higher trophic levels, in this case maize and potato plants grow n on nickel-enriched soils, and the potential risk of transfer of these ele ments to the food chain. (C) 2001 Elsevier Science Ltd. All rights reserved .