Co-inoculation effects of B. licheniformis and P. aeruginosa on soil Cd and As availability and rice accumulation.

There have been studies reporting the effects of multiple bacterial strains on the Cd/As immobilization and transformation in culture media. However, there is limited research to validate the effects of microbial strain combination on plant Cd/As accumulation and antioxidant system in the soil-plant system. By planting the rice (Zhefu 7) with the co-inoculation of bacterial strains (i.e. Bacillus licheniformis and Pseudomonas aeruginosa) after two months with the contaminations of Cd (2 mg/kg), As (80 mg/kg) and Cd + As (2 + 80 mg/kg), we found that the bacterial co-inoculation decreased Cd concentrations in the rhizosphere soil porewater, but had limited effects on mitigating plant Cd accumulation. By contrast, the co-inoculation did not affect the As(III) and As(V) concentrations in the rhizosphere soil porewater, but decreased As(III) and As(V) concentrations by 17% and 17% in the root respectively and by 17% and 37% in rice shoot respectively. Using DNA sequencing, we found the increased abundance in both exogenous Bacillus licheniformis and native microorganisms, indicating that the added strains had synergetic interactions with soil native microorganisms. Regarding on plant antioxidant enzyme system, the bacterial co-inoculation decreased the concentrations of superoxide dismutase (SOD), hydrogen peroxide (H2O2) and malondialdehyde (MDA) by 75%, 74% and 22%, mitigating the As damage to rice root and promote plant growth. However, under Cd and As co-stress, the effects of co-inoculation on mitigating plant As accumulation and enhancing plant stress resistance appear to be diminished. Our findings underscore the importance of microbial co-inoculation in reducing plant As accumulation and preserving plant health under heavy metal stress.

Bacterial coculture enhanced Cd sorption and As bioreduction in co-contaminated systems.

The bacterial interactions that regulate Cd sorption and As bioreduction in co-contaminated systems are poorly understood. We isolated two bacterial strains, i.e., Pseudomonas aeruginosa and Bacillus licheniformis from a Cd and As co-contaminated soil and compared the effects of monoculture and coculture on microbial Cd sorption and As bioreduction efficiency in the media with different Cd (0, 0.5, 5, 10, 50, 100 mg/L) and As(Ⅴ) (0, 90 mg/L) concentrations. Compared with monoculture, the bacterial coculture increased the Cd sorption efficiency by up to 32% and the As bioreduction (As(Ⅴ) to As(Ⅲ)) efficiency by up to 28%, associated with the increased abundance of As reduction gene arsB. Based on SEM-TEM and metabolomics, the enhanced efficiency was attributed to bacterial interactions, supported by the differential secretion of extracellular polymeric substances. Notably, the differential lipids and lipid-like molecules, and organoheterocyclic compounds resulted from bacterial interactions compared to monoculture exhibited the highest Cd sorption and As bioreduction. The increased efficiencies by bacterial coculture were verified by soil incubation experiments. These results provide insight on applying specific bacterial coculture and their metabolites to enhance Cd sorption and As bioreduction in effective and sustainable remediation of co-contaminated environments.

Metagenomic insights into soil microbial communities involved in carbon cycling along an elevation climosequences.

Diversity and community composition of soil microorganisms along the elevation climosequences have been widely studied, while the microbial metabolic potential, particularly in regard to carbon (C) cycling, remains unclear. Here, a metagenomic analysis of C related genes along five elevations ranging from 767 to 4190 m at Mount Kilimanjaro was analysed to evaluate the microbial organic C transformation capacities in various ecosystems. The highest gene abundances for decomposition of moderate mineralizable compounds, i.e. carbohydrate esters, chitin and pectin were found at the mid-elevations with hump-shaped pattern, where the genes for decompositions of recalcitrant C (i.e. lignin) and easily mineralizable C (i.e. starch) showed the opposite trend (i.e. U-shaped pattern), due to high soil pH and seasonality in both low and high elevations. Notably, the gene abundances for the decompositions of starch, carbohydrate esters, chitin and lignin had positive relationships with corresponding C compounds, indicating the consistent responses of microbial functional profiles and metabolites to elevation climosequences. Understanding of adaptation of microbial communities, potential function and metabolites to elevation climosequences and their influencing factors provided a new insight for the regulation of terrestrial C storage.