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

Chromium (Cr) soil pollution is a pressing global concern that demands thorough assessment. The pollution-induced community tolerance (PICT) methodology serves as a highly sensitive tool capable of directly assessing metal toxicity within microbial communities. In this study, 10 soils exhibiting a wide range of properties were subjected to Cr contamination, with concentrations ranging from 31.25 to 2000 mg Cr kg−1, in addition to the control. Bacterial growth, assessed using the [3H]-leucine incorporation technique, was used to determine whether bacterial communities developed tolerance to Cr, i.e. PICT to Cr in response to Cr additions to different soil types. The obtained results revealed that at concentrations of 1000 or 2000 mg Cr kg−1, certain bacterial communities showed inhibited growth, likely attributable to elevated Cr toxicity, while others continued to thrive. Interestingly, with Cr concentrations below 500 mg Cr kg−1, bacterial communities demonstrated two distinct responses depending on soil type: 7 of the 10 studied soils exhibited an increased bacterial community tolerance to Cr, while the remaining 3 soils did not develop such tolerance. Furthermore, the Cr level at which bacterial communities developed tolerance to Cr varies among soils, indicating varying levels of Cr toxicity between studied soils. The dissolved organic carbon (DOC) and the fraction of Cr extracted with distilled water (H2O-Cr) played an essential role in shaping the impact of Cr on microbial communities (R2=95.6 %). These factors (DOC and H2O-Cr) contribute to increased Cr toxicity in soil, i.e. during the selection phase of the PICT methodology.