Rhizosphere characteristics of the hyperaccumulator plant Sedum alfredii in phytoremediation of multi-metal contaminated soil

Understanding the strategies that roots of hyperaccumulators utilize to extract heavy metals from soils is important for optimizing phytoremediation of contaminated sites. Rhizosphere characteristics of a hyperaccumulator plant species, Sedum alfredii, were investigated when planted in soils contaminated with Cd, Pb, Zn, and Cu. In contrast with its non-hyperaccumulating ecotype (NHE), the hyperaccumulating ecotype (HE) of S. alfredii showed an extrodinary capacity to tolerate and extract the heavy metals in the contaminated soils, as well as very specific rhizobacterial properties. The HE rhizosphere was characterized by extensive root systems and drastically lower soil pH. Enhanced microbial activities, indicated by high microbial biomass carbon and metabolic enzymes, were also noted in the rhizosphere soil with plantation of HE, but not the NHE. The 16S rRNA sequencing of the rhizosphere and bulk soils suggested that HE was characterized by distinct rhizobacterial communities, and harbored abundant Actinomycetales, Cytophagia, Solibacterales, and Roseiflexales, which may contribute to metal activation and uptake. This study revealed the rhizosphere ecology of HE S. alfredii during phytoremediation of polluted soils, suggesting that efficacy of contaminants extraction by the plants may be associated with its extensive root growth, soil acidification, and enhanced rhizomicrobial activities by specific bacterial communities.