Elevated levels of the carcinogen arsenic (As), from both geogenic and anthropogenic sources, are found in coastal wetland soils. In the coastal environment, the saltwater inundation and ebb tide can induce anaerobic conditions in soils. However, the effect of anaerobic microbe on As reduction and mobility in coastal wetland with high sulfate has not been reported, and the underlying mechanisms of this process remain poorly understood.

This study investigated speciation transformation and redistribution of As in coastal wetland soil under anaerobic conditions using incubation experiments and molecular biology techniques. The effect of microbial sulfidogenesis on these processes was examined by addition of sulfate to the incubation systems. The biotic incubation results clearly revealed that As was reduced to As(III) and released in the early stage of the incubation. Dissolved As(III) and total As concentrations all increased and reached the maximum values of 12.0 and 64.8 μg/L, respectively, either sulfate addition or not. In contrast, As(V) reduction was not observed, and total As concentration was lower in abiotic experiments. In the later stage of the incubation, dissloved As(III) and total As concentrations decreased and resequestered into the solid phase. The XRD and soil sequential extraction analysis indicated that the new mineral of Fe₂O₃ was formed, and the dissolved As in the solid was re-adsorbed on new formed Fe₂O₃ mineral primarily in the later stage of the incubation. Moreover, the relative abundances of the bacterial 16S rRNA gene were examined. The results indicated that the Proteobacteria(52-60%), Chloroflexi(14-19%), Actinobacteria(6-10%), and Firmicutes(7%) occupied the majority in biotic experiments. The sulfate reducing bacteria Desulfocapsa from the Proteobacteria was detected in biotic incubation. Our findings suggest that the anaerobic microbial including sulfate reducing bacteria (Desulfocapsa) can stimulate the reduction and release of As in coastal wetland sulfur-rich soil, although part of the released As re-adsorbed on new formed iron mineral.