Key microbial communities involved in humic acid-Fe mineral transformation in high arsenic aquifers and their effects on arsenic release

Abstract:

[Objective] To investigate the variation of composition and abundance of microbial communities involved in the transformation of arsenic-bearing ferrihydrites under different humic acid concentrations and their effects on arsenic release, and to predict the role of functional microbial communities in participating the transformation processes of organic matter-arsenic-bearing iron minerals on arsenic release from high arsenic aquifers. [Methods] Iron-reducing microbial populations were enriched from a high arsenic groundwater and a high sediment sample from the same depth from Hetao Plain, Inner Mongolia Autonomous Region. Anaerobic microcosms were constructed by amendments of bacterial enrichment and different concentrations of humic acid (0, 1.5, 7, and 14 mg C/L) into arsenic-bearing ferrihydrites. The variation of arsenic and iron species during the 50-day incubation was monitored. The composition succession of microbial communities was analyzed by high-throughput sequencing, and the transformation of iron minerals was visualized by X-ray diffractometer (XRD). [Results] The iron-reducing functional communities enriched in the high arsenic groundwater group (group G) and the sediment group (group S) were significantly different, with Aeromonadaceae as the specific dominant family in group G while Shewanellaceae as the specific dominant family in group S. Results of microcosm experiments showed that the iron reduction rates and amount in group S were relatively faster and higher than those in group G. The liquid phase arsenic speciation in group G and group S were different. As(V) was the dominant arsenic form in group G throughout the incubation period. In contrast, in group S, As(V) was the main arsenic species in the early stage, and As(III) became dominant and was up to 3.4 μmol/L in the incubation of 20 d. It was assumed that the microbial communities capable of arsenic reduction were dominated at this time. In the final stage of incubation, the released arsenic in both group G and group S showed different degrees of fixation under the addition of humic acid. Different concentrations of humic acid led to different amounts of arsenic release. XRD results showed that the transformation of arsenic-bearing ferrihydrites was relatively higher in group S, with goethite the dominant secondary mineral in both groups. Redundancy analysis (RDA) indicated that the addition of arsenic and humic acid influenced the overall succession of microbial community composition. Comamonadaceae, Desulfobacteraceae, and Burkholderiaceae became the predominant populations in group G, while populations of Pseudomonadaceae, Comamonadaceae, and Burkholderiaceae were dominated in group S. [Conclusion] The amendments of different concentrations of humic acid into arsenic-bearing ferrihydrites led to differentiated successions of microbial communities, which might play different effects on iron mineral transformation and arsenic release.