From molecular mechanisms to environmental application: the role of carotenoids in bacterial response to metal(loid)s and the development of a biochar-adherent microbial consortium for bioremediation

Bacteria capable of transforming toxic metal(loid)s represent promising tools for the bioremediation of contaminated environments. This doctoral research investigates bacterial–metal(loid) interactions by integrating physiological analyses, multi-omics approaches, and biochar-based microcosm experiments to elucidate fundamental mechanisms of metal tolerance and to assess their applicability in bioremediation. In the first part of the study, the response of Rhodococcus aetherivorans to tellurite stress was examined by comparing the wild-type strain BCP1 with a spontaneous white mutant (BCP1-Wh) isolated under high tellurite concentrations. BCP1-Wh showed increased tellurite resistance, with enhanced planktonic growth and biofilm formation. Although tellurite uptake and ROS levels were comparable between strains, the mutant exhibited higher tolerance to oxidative stress, suggesting compensatory redox defenses. Genetic analyses identified a frameshift mutation in the gene encoding phytoene dehydrogenase. Its association with tellurite resistance was supported by genetic complementation, targeted gene knockout in an independent Rhodococcus strain, and recurrent isolation of spontaneous white mutants from tellurite-exposed BCP1 WT, all showing increased resistance. Multi-omics analyses revealed pronounced lipidomic remodeling in the mutant, while transcriptomic and metabolomic changes were less extensive, consistent with carotenoid loss affecting membrane properties and redox homeostasis. The second part of the study describes the development of a synthetic microbial community (SynCom-As) designed for arsenic detoxification. The consortium, composed of metal-resistant strains, was immobilized on standard and magnetic biochar and tested in arsenic-contaminated soil microcosms. Over two months, microbial dynamics and arsenic speciation were monitored using qPCR, 16S rRNA metabarcoding, SEM, and physicochemical analyses. Biochar treatments increased soil pH and altered community composition, while magnetic biochar promoted a reduction in bioavailable As(V), indicating a combined contribution of microbial activity and sorption processes. Overall, this work provides mechanistic insights into bacterial metal resistance and supports the use of biochar-functionalized microbial consortia as a sustainable strategy for the remediation of metal-contaminated soils.