From single to synthetic PGPR: Exploring the strategies of submerged macrophyte remediation in cadmium polluted sediment

Plant growth-promoting rhizobacteria (PGPR) are used to assist phytoremediation. Synthetic PGPR can adapt to diverse sediment environments more effectively than single PGPR. However, their mechanisms of application in submerged macrophytes remediation of cadmium polluted sediment remains challenging. Here, we investigated the effect of synthetic PGPR on enhancing phytoremediation in cadmium polluted sediment and the underlying mechanisms promoting growth of submerged macrophytes. We conducted a comparison regarding the phytoremediation efficiencies of submerged macrophytes within cadmium polluted sediment under the conditions of the absence of PGPR, single PGPR and synthetic PGPR inoculation. The results indicated that the lower concentration of cadmium in the sediment was achieved under synthetic PGPR inoculation of two treatments, specifically decreased by 26.18 % and 25.55 %, because of the highest biota-sediment accumulation factor. Synthetic PGPR induced 44 %–88 % increase in the biomass of submerged macrophytes by regulating enzyme activities and photosynthesis system compared with the treatment with cadmium pollution only. The PGPR promoted significant increase of chlorophyll, the optimal/maximal quantum yield of PSⅡ (Fv/Fm) and carotenoids content with the highest increases in macrophytes inoculated with synthetic PGPR. Compared with the treatments inoculated with single PGPR, synthetic PGPR enhanced the activities of antioxidant enzymes to assist submerged macrophytes in resisting cadmium stress. Consequently, synthetic PGPR promoted the growth and cadmium accumulation of submerged macrophytes. Our research further observed that the translocation factor of the submerged macrophytes was below 1, showing that cadmium was not easily transferred within the macrophytes. Our study provides new perspectives into strategy development in microbe-assisted submerged macrophytes remediation of cadmium polluted sediment.