Effects of Microbacterium algeriense C14 on growth and rhizosphere environment of Zinnia elegans under cadmium and nickel stress.

INTRODUCTION: Although plantmicrobe systems have been studied for singlemetal contamination, little is known about their synergistic mechanisms under CdNi costress. Current studies often lack mechanistic explanations of microbial immobilization and plant physiological regulation, which this study aims to address.

METHODS: To explore the remediation effect and intrinsic mechanism of the plant-microbe combined system on cadmium (Cd)-nickel (Ni) co-contaminated soil, Microbacterium algeriense (M. algeriense) C14 and Zinnia elegans (Z. elegans) were selected as the research objects. Through indoor culture and pot experiments, we investigated the CdNi resistance, mineralization mechanism, and effects of strain C14 on seed germination, seedling growth, mature plant growth, physiological metabolism, and heavy metal enrichment of Z. elegans under CdNi costress.

RESULTS: M. algeriense C14 exhibited strong resistance to Ni2+ (within 800 mg·L-1) and Cd2+ (within 120 mg·L-1), and could form stable carboxylate precipitates through the coordination of carboxyl groups in amino acids of its metabolites with heavy metal ions. Inoculation with strain C14 significantly alleviated the inhibitory effect of Cd-Ni co-stress on Z. elegans, improved seed germination indexes, promoted the growth of seedlings and mature plants, increased the soluble protein content, superoxide dismutase (SOD) and peroxidase (POD) activities of plants, and reduced Cd and Ni enrichment in plants. Correlation and PCA analyses confirmed the coordinated regulation of growth, physiology, and heavy metal uptake by the strain.The core mechanism of their synergistic remediation was the comprehensive effect of heavy metal immobilization by the strain, plant growth promotion, and synergistic regulation of plant heavy metal uptake, enrichment and physiological metabolism.

DISCUSSION: This study confirms that the M. algeriense C14 and Z. elegans system is a promising phytomicrobial remediation model. Unlike conventional biosorption or microbially induced carbonate precipitation, strain C14 achieves stable heavy metal immobilization via carboxylate‑mediated mineralization, which differs fundamentally from known mechanisms and offers superior long‑term stability. Meanwhile, M. algeriense C14 shows exceptional dual resistance to Cd and Ni, strong IAA‑producing and growth‑promoting traits, distinguishing it from most previously reported PGPR.