Analysis of Changes in the Microbial Biodiversity of Soil Contaminated with Cr(III) and Cr(VI)

Contamination with heavy metals, including chromium that exists in two oxidation states—Cr(III) and Cr(VI)—poses a significant challenge for the soil environment. Both chemical forms of chromium can exert toxic effects on microorganisms that play a key role in maintaining soil fertility and plant health. The aim of the study was to compare the selective toxic effects of Cr(III) and Cr(VI) ions on soil bacterial and fungal taxonomic diversity using NGS technology. The data obtained enabled a comprehensive characterisation of the taxonomic profile of the soil microbiome exposed to both forms of chromium, providing a basis for further research into the adaptation and resistance mechanisms of microorganisms. The calculated diversity indices, in particular the Shannon-Wiener index, suggest that Cr(VI) is more toxic to bacteria than Cr(III). In soil contaminated with chromium, the relative abundance of chromium-resistant bacteria of the phylum Actinobacteriota increased to the detriment of chromium-sensitive Acidobacteriota and Proteobacteriota. The abundance of Ascomycota, the dominant fungal phylum, increased in soil with Cr(III) and decreased in soil with Cr(VI). Cr(III) promoted the growth of bacteria of the genera Phycicoccus and Arthrobacter and Penicillium fungi. In turn, Cr(VI) stimulated the growth of bacteria of the genera Mycoplana and Cellulosimicrobium, and Trichoderma fungi. The study demonstrated that microbial resistance mechanisms are influenced by the chemical form of chromium. In addition, the increased abundance of chromium-resistant taxa highlights their potential for the bioremediation of soils contaminated with this element.