Rhizosphere microbiome assembly drives metal sequestration in Leucaena leucocephala during tailing phytoremediation

Ghana's water and soil resources face severe challenges due to heavy metal contamination from gold mining operations. Although Leucaena leucocephala exhibits potential for phytoremediation, little is known about the contribution of its rhizosphere microbiomes to metal uptake and tolerance in multiple-metal contaminated tailings in field conditions.

We investigated the rhizosphere bacterial community dynamics in L. leucocephala across three soil treatments (garden soil, 1:1 soil-tailings mixture, and pure tailings) using 16S rRNA amplicon sequencing and atomic absorption spectrophotometry. Briefly, transplanted seedlings of L. leucocephala were harvested at three-month intervals for three consecutive harvests to assess metal accumulation and changes in the microbiome.

Leucaena leucocephala demonstrated notable tolerance to elevated metal concentrations (>10,000 mg/kg Fe and Mn) under acidic conditions (pH 4.57-5.97). Maximum metal uptake occurred at final harvest, with Fe reaching 14,605 ± 1.40 mg/kg in shoots and Mn reaching 12,279 ± 1.13 mg/kg in roots. The elevated concentrations of metals reduced overall bacterial diversity, except for selected metal-tolerant Actinobacteria, Proteobacteria, and Acidobacteria, which dominated bacterial communities across all treatments. The initial proliferation of Nocardioides and Streptomyces corroborated nutrient and metal-induced stress, while key genera such as Arthrobacter, Gaiella, Skermanella, and Chelatococcus showed strong positive associations with metal accumulation and maintained essential ecological functions.

Rhizosphere bacterial communities undergo stress-specific assembly processes, with specific taxa facilitating L. leucocephala's exceptional phytoremediation capacity. These findings provide insights into microbiome-enhanced strategies for mine site rehabilitation.