Core Species Derived from Multispecies Interactions Facilitate the Immobilization of Cadmium.

Microbial consortia have opened new avenues for heavy-metal remediation. However, the limited understanding of the overall effect of interspecific interactions on remediation efficacy hinders its application. Here, the effects of multispecies growth and biofilm formation on Cd immobilization were explored from direct and multiple interactions through random combinations of two or three rhizosphere bacteria. In monocultures, Cd stress resulted in an average decrease in planktonic biomass of 26%, but through cooperation, the decrease was attenuated in dual (21%) and triple cultures (13%), possibly involving an increase in surface polysaccharides. More than 65% of the co-cultures exhibited induction of biofilm formation under Cd stress, which further enhanced the role of biofilms in Cd immobilization. Notably, excellent biofilm-forming ability or extensive social induction makes Pseudomonas putida and Brevundimonas diminuta stand out in multispecies biofilm formation and Cd immobilization. These two core species significantly increase the colonization of soil microorganisms on rice roots compared to the control, resulting in a 40% decrease in Cd uptake by rice. Our study enhances the understanding of bacterial interactions under Cd stress and provides a novel strategy for adjusting beneficial soil consortia for heavy-metal remediation.

Synergistic effect of biofilm growth and cadmium adsorption via compositional changes of extracellular matrix in montmorillonite system.

The interaction of bacterial biofilm and clay minerals provides great potential for heavy metal remediation in contaminated soil, yet, little is known about how heavy metal, clay minerals and their combinations affect the bacterial biofilm performance and heavy metal adsorption. In this study, the response of biofilm development as well as Cd2+ adsorption in the presence of Cd2+ and montmorillonite has been deciphered. Low concentrations of Cd2+ and montmorillonite or their combinations enhanced biofilm formation by increasing polysaccharides proportion in the biofilm matrix, and the maximum adsorption capacity of Cd2+ by biofilm was increased by 1.5 times. Furthermore, the immobilization of Cd2+ by soil was significantly improved when S14-biofilm was introduced. Such results could gain deeper insight into bacterial survival tactics in the complex systems which makes major contribution to microbial remediation of heavy metal polluted environments.

Complete genome sequence of Raoultella sp. strain X13, a promising cell factory for the synthesis of CdS quantum dots.

A novel cadmium-resistant bacterium, Raoultella sp. strain X13, recently isolated from heavy metal-contaminated soil, and this strain can synthesize CdS quantum dots using cadmium nitrate [Cd(NO4)2] and l-cysteine. Biomineralization of CdS by strain X13 can efficiently remove cadmium from aqueous solution. To illuminate the molecular mechanisms for the biosynthesis of CdS nanoparticle, the complete genome of Raoultella sp. strain X13 was sequenced. The whole genome sequence comprises a circular chromosome and a circular plasmid. Cysteine desulfhydrase smCSE has been previously found to be associated with the synthesis of CdS quantum dots. Bioinformatics analysis indicated that the genome of Raoultella sp. strain X13 encodes five putative cysteine desulfhydrases and all of them are located in the chromosome. The genome information may help us to determine the molecular mechanisms of the synthesis of CdS quantum dots and potentially enable us to engineer this microorganism for applications in biotechnology.

Role of novel bacterial Raoultella sp. strain X13 in plant growth promotion and cadmium bioremediation in soil.

Heavy metal pollution in agricultural soils has become a widespread serious problem with the rapid industrialization and urbanization in the past two decades. Cadmium (Cd2+) is of the most concern in soils due to its high toxicity. It is necessary to develop remediation strategies to remove or neutralize its toxic effects in Cd-contaminated soil. Microbial bioremediation is a promising technology to treat heavy metal-contaminated soils. In this study, Cd-resistant bacterium, isolated from heavy metal-polluted soil in Southern China, was characterized as Raoultella sp. strain X13 on the basis of its biochemical profile and 16S rRNA. We investigated the characterization of Cd2+ distribution in different cellular compartments after Cd2+ uptake. Cd2+ uptake by strain X13 was mainly by ion exchange and chelation binding tightly to the cell wall. In addition, X13 plant growth-promoting characteristics suggested that X13 could solubilize phosphate and produce indole acetic acid. Pot experiments for the remediation of Cd-contaminated soil in situ by X13 inoculation demonstrated that X13 application to Cd-contaminated soils significantly promoted pak choi growth and improved production. We also found that X13 substantially reduced the Cd2+ bioavailability for pak choi. Therefore, strain X13 is an effective treatment for potential application in Cd2+ remediation as well as for sustainable agronomic production programs in Cd-contaminated soils.