A new assay of bacterial selection with Pb reveals an unexpected effect of Pb on bacterial behavior: implications for remediation

Abstract

Soil pollution by lead (Pb) is a major health concern due to Pb toxicity. Phytoextraction could remove Pb, but this technique is limited by the low Pb mobility in soils. Pb mobility can be increased by bioaugmentation, which consists in adding selected bacteria in soil to increase Pb bioavailability. Nonetheless, many bioaugmentation and phytoextraction experiments have failed because bacterial selections did not take into account the presence of metals. Therefore, we developed a microplate assay with Pb-enriched growth media for the rapid selection of bacterial strains. Selection criteria included the ability of bacteria to grow in soils, to promote plant growth and to increase Pb availability. Results show that 100-250 µM Pb induced a decrease of production of indole acetic acid (IAA), a plant growth promotor, by up to 49% for Cupriavidus metallidurans, compared to the control without Pb. This finding implies that application in real soil conditions with C. metallidurans would have probably failed, thus strengthening the value of our selection method in the presence of Pb. By contrast, 100-250 µM Pb induced a 8.5-11-fold higher degradation of aminocyclopropane carboxylic acid (ACC) by C. metallidurans. Surprisingly , Pseudomonas putida did not degrade ACC without Pb, but degraded ACC at 100-250 µM Pb. This observation means that Pb activates ACC degradation, which should reduce plant stress because ACC is the precursor of the ethylene phytohormone. Overall, our selection method in the presence of Pb allows to reveal new bacterial properties, which would not have been disclosed by current methods that do not take into account the effect of metals. Our method allows also to test simultaneously about 200 bacterial isolates. In addition, our findings show for the first time that Pb changes the production of IAA and the degradation of ACC.