Increase of bacterial community induced-tolerance to Cr in response to soil properties and Cr level in soil

Abstract. Chromium (Cr) pollution in soils is a global concern that should be assessed. Pollution Induced Community Tolerance (PICT) methodology is a highly sensitive tool that can directly indicate metal toxicity in the microbial community. Ten soils with a wide range of properties were spiked with 31.25, 62.5, 125, 250, 500, 1000 and 2000 mg Cr·kg^-1, in addition to the control. Bacterial growth (using [³H]-leucine incorporation technique) was used to determine PICT, that is, whether bacterial communities developed tolerance in response to Cr additions to different soil types. Some bacterial communities did not grow normally at 1000 or 2000 mg Cr·kg^-1, probably due to high Cr toxicity, while others did. Regarding below 500 mg Cr·kg^-1, bacterial communities showed two responses depending on soil type: 7 of the 10 studied soils showed increased tolerance to Cr, while for the remaining 3 soils did not develop tolerance to Cr. Furthermore, the Cr level from which bacterial communities developed tolerance was dependent on the soil, i.e. Cr was more toxic in some of studied soils. The Cr effect on microbial communities was mainly determined by Dissolved Organic Carbon (DOC) and the fraction of Cr extracted with distilled water (H₂O-Cr) (R² = 95.6 %). Their effect on Cr in the soil might lead to an increase in toxicity (selection phase of PICT).