Characterization of physicochemical parameters and bioavailable heavy metals and their interactions with microbial community in arsenic-contaminated soils and sediments

Mobility and toxicity of heavy metal contamination in the environment are highly dependent on its bioavailability. Most of previous studies focused on total heavy metal contents and their influence on microbial community in soils and sediments. Little were concerned about bioavailable fractions. In the current study, soil and sediment samples were collected near an abandoned realgar mine in Shimen County, China. Bioavailable heavy metals including Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Cd, Sb, and Pb in the samples were extracted using three-step sequential extraction method. Interactions among physicochemical parameters, total and bioavailable heavy metals, and microbial community in the collected samples were investigated. The study area has been severely contaminated by As with a concentration up to 2158 mg·kg-1 detected. The result of principal component analysis showed that the abundance of operational taxonomic units (OTUs) in the soils were obviously different from those in the sediments. In the soil samples, pH made a dominant contribution on the OTU abundance of microbial community. Correlation analyses revealed that the alpha diversity indices and microbial taxon were most correlated with bioavailable fractions of heavy metals in all the samples. That means bioavailable heavy metals rather than total heavy metals or physicochemical parameters played a more important role on richness and diversity of microbial community. Little connections were observed between microbial community and As no matter total concentration or bioavailable fraction. However, bioavailable Fe and Mn were recognized as the major driving force shaping the taxonomic structure of microbial community due to their relatively high concentrations and high affinity to other heavy metal contamination in soils and sediments.

Long-term performance of rapid oxidation of arsenite in simulated groundwater using a population of arsenite-oxidizing microorganisms in a bioreactor

A population of arsenite-oxidizing microorganisms enriched from the tailing of the Shimen realgar mine was used to generate biofilms on the surfaces of perlites. This bioreactor is able to completely oxidize 1100 μg/L As(III) dissolved in simulated groundwater into As(V) within 10 min; after 140 days of operation, approximately 20 min were required to completely oxidize the same concentration of As(III). Analysis for the 16S rRNA genes of the microbial community showed that Bacteroidetes and Proteobacteria are dominant in the reactor. Six different bacterial strains were randomly isolated from the reactor. Function and gene analysis indicated that all the isolates possess arsenite-oxidizing activity, and five of them are chemoautotrophic. Further analysis showed that a large diversity of AioAs and two types of RuBisCOs are present in the microbial community. This suggests that many chemoautotrophic arsenite-oxidizing microorganisms were responsible for quick oxidation of arsenite in the reactor. We also found that the reactor is easily regenerated and its number is readily expanded. To the best of our knowledge, the arsenite-oxidizing efficiency, which was expressed as the minimum time for complete oxidization of a certain concentration of As(III) under a single operation, of this bioreactor is the highest among the described bioreactors; it is also the most stable, economic and environment-friendly.