Nitrate Stimulates Anaerobic Microbial Arsenite Oxidation in Paddy Soils

Abstract

Arsenic (As) bioavailability to rice plants is elevated in flooded paddy soils due to reductive mobilization of arsenite [As(III)]. However, some microorganisms are able to mediate anaerobic As(III) oxidation by coupling to nitrate reduction, thus attenuating As mobility. In this study, we investigated the impact of nitrate additions on As species dynamics in the porewater of four As-contaminated paddy soils. The effects of nitrate on microbial community structure and the abundance and diversity of the As(III) oxidase (aioA) genes were quantified using 16S rRNA sequencing, quantitative PCR, and aioA gene clone libraries. Nitrate additions greatly stimulated anaerobic oxidation of As(III) to As(V) and decreased total soluble As in the porewater in flooded paddy soils. Nitrate additions significantly enhanced the abundance of aioA genes and changed the microbial community structure by increasing the relative abundance of the operational taxonomic units (OTUs) from the genera Acidovorax and Azoarcus. The aioA gene sequences from the Acidovorax related OTU were also stimulated by nitrate. A bacterial strain (ST3) belonging to Acidovorax was isolated from nitrate-amended paddy soil. The strain was able to oxidize As(III) and Fe(II) under anoxic conditions using nitrate as the electron acceptor. Abiotic experiments showed that Fe(II), but not As(III), could be oxidized by nitrite. These results show that nitrate additions can stimulate As(III) oxidation in flooded paddy soils by enhancing the population of anaerobic As(III) oxidizers, offering a potential strategy to decrease As mobility in As-contaminated paddy soils.

Anaerobic Bacterial Immobilization and Removal of Toxic Sb(III) Coupled With Fe(II)/Sb(III) Oxidation and Denitrification

“…The addition of nitrate significantly increased bacterial anaerobic Fe(II) oxidation and produced Fe(III)-containing precipitate, which would provide a sorption phase for metal(loid)s. These findings are consistent with previous studies of Acidovorax sp. ST3 isolated from As-contaminated paddy soils (Zhang et al, 2017). As expected, we confirmed that Sb(III) could be efficiently oxidized and immobilized by the microbially produced Fe(III)-containing precipitate.…”

“…The resuspension was used as an inoculum for anaerobic Sb(III) oxidation. For nitrate addition, we used 1 mM KNO 3 , which is similar to the amount of nitrate used in anaerobic As(III) oxidation (Zhang et al, 2017). To further investigate the effect of nitrate on anaerobic Sb(III) oxidation, different concentrations (0, 1, and 5 mM) of nitrate were added to the culture.…”

Phylogenetic Structure and Metabolic Properties of Microbial Communities in Arsenic-Rich Waters of Geothermal Origin

“…This suggested that microbial communities could activate this process as a result of an increase in nitrate concentration due to anthropic contamination (e.g., agricultural activity). Nitrate-linked microbial transformation of As was reported in As-contaminated aquatic environments, and some microorganisms were found to mediate anaerobic As(III) oxidation by coupling to nitrate reduction ( Zhang et al, 2017 ). Differently from alluvial environments ( Islam et al, 2004 ), Fe- and Mn-related metabolisms were not found in our samples, since they were not likely to play an essential role in this geothermal area ( Piscopo et al, 2006 ).…”

“…Microbial activity is linked to the biogeochemistry of arsenic and microorganisms are able to directly catalyze As transformations in natural environments or mediate these processes by coupling with other redox reactions such as nitrate-, sulfate-, manganese-, and iron-reduction ( Rittle et al, 1995 ; Fendorf et al, 2010 ; Lee, 2013 ; Zhang et al, 2017 ). Aquatic microorganisms have evolved different mechanisms to resist to high As concentrations and metabolize it, including sorption, mobilization, precipitation and redox and methylation transformation ( Huang, 2014 ).…”

Dissolved organic matter differentially influences arsenic methylation and volatilization in paddy soils

“…Soil properties and agriculture practices perturb the biogeochemical cycle of As (Zhang et al, 2015;Zhu et al, 2017), with the responses to nitrate (Zhang et al, 2017), iron (Yang et al, 2018) and organic matter (Williams et al, 2011) being well characterized. DOM mediates the functional state of soil bacterial communities (Li et al, 2019); in the present study, its quality was shown to cause a differential response in arsM diversity.…”