Soil organic matter amount determines the behavior of iron and arsenic in paddy soil with microbial fuel cells.

Arsenic (As) mobility in paddy soils is mainly controlled by iron (Fe) oxides and iron reducing bacteria (IBR). The Fe reducing bacteria are also considered to be enriched on the anode of soil microbial fuel cells (sMFC). Thus, the sMFC may have an impact on elements' behavior, especially Fe and As, mobilization and immobilization in paddy soils. In this study, we found dissolved organic matter (DOC) abundance was a major determinate for the sMFC impact on Fe and As. In the constructed sMFCs with and without water management, distinctive behaviors of Fe and As in paddy soil were observed, which can be explained by the low or high DOC content under different water management. When the sMFC was deployed without water management, i.e. DOC was abundant, the sMFC promoted Fe and As movement into the soil porewater. The As release into the porewater was associated with the enhanced Fe reduction by the sMFC. This was ascribed to the acidification effect of sMFC anode and the increase of Fe reducing bacteria in the sMFC anode vicinity and associated bulk soil. However, when the sMFC was coupled with alternating dry-wet cycles, i.e. DOC was limited, the Fe and As concentrations in the soil porewater dramatically decreased by up to 2.3 and 1.6 fold, respectively, compared to the controls under the same water management regime. This study implies an environmental risk for the in-situ application of sMFC in organic matter rich wetlands and also points out a new mitigation strategy for As management in paddy soils.

Relic DNA does not obscure the microbial community of paddy soil microbial fuel cells.

Soil Microbial Fuel Cells (MFCs) are devices that can generate electricity by using the flooded soil's anode respiring microbial consortium. When the MFC starts to work, the microbial community in the anode vicinity rapidly changes. This shift in the microbial community results in many dead cells that may release their DNA (relic DNA) and obscure culture independent estimates of microbial community composition. Although relic DNA is expected to increase in MFCs, the effect of relic DNA has not been investigated in the soil MFCs system. In this study the effect of the MFCs on the soil microbial community composition within the soil profile and the influence of relic DNA were investigated. Microbial community analysis revealed that the MFCs deployment significantly influenced the community composition within the soil profile. The phylum Proteobacteria (34.4% vs 23.6%) and the class Deltaproteobacteria (16.8% vs 5.9%) significantly increased in the MFCs compared to the control, while the phylum Firmicutes (24.0% vs 28.7%) and the class Sphingobacteria (5.3% vs 7.0%) were more abundant in the control. Furthermore, the archaeal phyla Euryarchaeota (40.7% vs 52.3%) and Bathyarchaeota (10.1% vs 17.3%) were significantly lower in the MFCs, whereas the phylum Woesearchaeota (DHVEG6) (24.4% vs 19.4%) was slightly enhanced. Moreover, the results showed that relic DNA can affect the relative abundance of Geobacter and Candidatus Methanoperedens, however, it has no significant effects on the microbial community structure. These results indicate that MFCs can influence the soil microbial community profile, nevertheless the relic DNA generated has minimum effect on the culture independent estimates of microbial community composition.