BioPhyt. Plant Colonizers of a Mercury Contaminated Site: Trace Metals and Associated Rhizosphere Bacteria

Background and Aims: Mercury (Hg) contamination poses severe human and environmental health risks. We aimed to understand the colonization of Hg-contaminated sites by native plants and to characterize plant-associated bacterial communities in the rhizosphere of Diplotaxis muralis. Methods: We conducted a field study at a Hg-contaminated site in Romania by characterizing its spontaneous flora, plant metal accumulation, and the diversity of the rhizosphere microbiome of D. muralis at the site. Soil and plant samples were analyzed using portable X-ray fluorescence and metagenomic 16S rRNA amplicon sequencing. Results: Site-specific trace metal distribution across the site drove plant species distribution in the highly contaminated soil, with Lotus tenuis and D. muralis associated with higher Hg concentrations. In addition, for the bacterial communities in the rhizosphere soil of D. muralis, there was no observable alpha diversity decrease with increasing Hg. Notably Actinomycetota had an average of 24% relative abundance in the rhizosphere communities that also tested positive for the presence of merA, whereas in the absence of merA, relative abundance was approximately 2%. Conclusions: Our results suggest a dependence of species-wise plant survival on local trace metal levels in soil, as well as an intricate interplay of the latter with rhizosphere bacterial diversity. Understanding these dynamics could have implications for phytoremediation stakeholders, as it may allow for the selection of plant species and appropriate soil microbial inoculates with elevated Hg tolerance or potential for bioremediation for mitigating ecological risks associated with Hg contamination.