Bacterial allies in chromium hyperaccumulation: native rhizobacterial dynamics of profusely growing Dactyloctenium aegyptium in highly tainted tannery sludge.

Abstract

Tannery sludge has highly toxic heavy metals like chromium (Cr), posing environmental and health risks. This research investigates the potential of Dactyloctenium aegyptium (L.) Willd. and the associated rhizobacterial communities for bacterial-assisted phytoremediation of tannery sludge, having 6403.16 ± 0.71 mg/kg Cr. The analyses of culturable bacterial communities resulted in the exploration of two highly potent plant growth-promoting strains (CRB2 and CRB5), out of the six culturable strains obtained. SEM imaging depicted robust bacterial colonization on the plant root surface, confirming active plant-microbe interaction. D. aegyptium showed significant Cr accumulation (4936 ± 1.34 mg/kg) within the tissues, followed by substantial translocation to shoots and leaves (TF = 1.01). With a BCF of 1.66 for Cr, D. aegyptium bespeaks evident hyperaccumulation potential. TEM imaging revealed the granular metal deposition in the plant tissues. Post-plant growth, the sludge exhibited an 80.3% reduction in Cr concentration, alongside enhanced physicochemical properties (reduced pH, increased organic matter, reduced metal content). Furthermore, metagenomics analyses showed that the growth of D. aegyptium drastically changed rhizobacterial communities, decreasing species richness and increasing functional pathways associated with stress responses and metal tolerance. Important genes (copA, czcA, nirA), enzymes (dioxygenases, trimethylamine-N-oxide reductase), and proteins (CsgE, DsbG), essential for the nitrogen cycle, chromium detoxification, and plant-microbe associations, were found to be involved in metabolic pathways. The study amalgamates morphophysiological and advanced metagenomic approaches to put forth an understanding of species-specific plant-microbe interactions for the development of scalable and sustainable remediation and engineering of rhizospheric microbiomes for eco-restoration of heavy metal-polluted industrial sites.