Metabolic Adaptation of Soil Microbial Communities to Metal Contamination: Assessment of Cadmium Stress

Anthropogenic contamination of soil with heavy metals such as Cadmium (Cd2+) alters the metabolic activities and structure of soil microbial communities and has profound impacts on the soil ecosystem. In this study, we examined the effects of Cd2+ exposure on soil microbial communities by amending soil solutions with different concentrations of Cadmium nitrate tetrahydrate Cd(NO3)2 ● 4H2O (0, 0.5, 1.0, 1.5, and 2.5 mM). 150 µL of the Cd2+ amended and control soil solutions were added to BIOLOG 96-well ecoplates and incubated (26 ±1oC) over a 7-day period for metabolic analysis of 31 selected carbon substrates. Average well-color development (AWCD) of the BIOLOG ecoplates showed a significant difference between the metabolic rates of Cd2+ amended soil communities, indicating alterations in the soil microbial community structure. There was a significant decrease in AWCD in the 2.5 mM Cd2+ sample when compared to lower concentrations (0.5, 1.0, and 1.5 mM) and the control samples. Furthermore, Principal Component Analysis (PCA) of the samples showed 73.2% variation along the PC1 separating communities on Cd2+ exposure basis; however, PC2 (9.4% variation) revealed distinct clusters of communities based on the responses of the bacterial communities to Cd2+ stress, with the community in 2.5 mM exposure significantly different to communities exposed to lower concentrations. Control samples exhibited unique metabolic profiles utilizing carbohydrates, α-Cyclodextrin, Glycyl-L Glutamic Acid, and Phenylethylamine while Cd-exposed communities showed differential responses, for instance, communities exposed to lower Cd2+ concentrations disproportionately utilized polymer and D-mannitol. Bacteria amended with 2.5 mM Cd2+ guild utilization included 36.30% carbohydrates compared to the control utilizing 31.21% carbohydrates. The 2.5 mM Cd2+ bacteria also utilized 22.52% polymers compared to 10.94% polymers for control. For all concentrations amines & amides were utilized the least in a range of 4.01% to 6.95%. In conclusion, our findings showed that soil microbial communities pre-exposed to varying Cd2+ stress manifested different metabolic profiles and utilization rates of carbon substrates. Moreover, AWCD data indicated that Cd2+ addition inhibited the metabolic activity of soil microbial communities. This study underscores the need to identify Cd-resistant species that might play significant roles in metal remediation. Therefore, metagenomics analysis is needed to evaluate bacterial species and functional diversity that are selected for in the Cd2+ amended or polluted soils.