Effects Of Anaerobic Arsenite Oxidizing Bacteria On Arsenic Speciation In Flooded Paddy Soils

Arsenic(As)is a kind of highly toxic metalloid.At present,with the rapid development of industry in China,the As pollution in the environment is becoming more and more serious.High concentration of As in contaminated paddy soil can be harmful to cereal crops,for depressing its yield and quality.Because of the cultivation and its high efficiency on absorption of As(?)through silicon pathway,rice is usually enriching more As than other crops.Besides larges areas of paddy soil are contaminated by As,thus rice have became the largest food source of As in Southeast Asia and south regions of China.Some denitrifying microorganisms can oxidize As(?)under nitrate-reducing condition,since As(?)is generally thought to be less mobile in the environment and less toxic to organisms than As(?).The aim of this study was to learn the process of As(?)-oxidization coupled to denitrification mediated by microorganisms and screen a large number of As(?)oxidizers from different As-contaminated paddy soil,then study the physiological and biochemical characteristics of the microorganisms in the process of anaerobic As(?)oxidation coupled to nitrate reduction.Then explore the effect of nitrate on the anaerobic oxidation of As(?)in paddy soil,build a aioA gene clone library and construct phylogenetic trees for the analysis of which microorganisms are participated in the oxidation of As(?).A novel chemoautotrophic As(?)-oxidizing bacterium preserved in Laboratory,designated strain SY,was able to derive energy from the oxidation of As(?)to As(?)under both aerobic and anaerobic conditions using O2 or NO3-as the respective electron acceptor.Inoculation of the washed SY cells into a flooded soil greatly enhanced As(?)oxidation to As(?)both in the solution and adsorbed phases of the soil.Strain SY is phylogenetically closely related to Paracoccus niistensis with a 16S rRNA gene similarity of 96.79%.The isolate contains both the denitrification and ribulose 1,5-bisphosphate carboxylase/oxygenase gene clusters,underscoring its ability to denitrify and to fix CO2 while coupled to As(?)oxidation.Deletion of the aioA gene encoding the As(?)oxidase subunit A abolished the As(?)oxidation ability of strain S Y and led to increased sensitivity to As(?),suggesting that As(?)oxidation is a detoxification mechanism in this bacterium under aerobic and heterotrophic growth conditions.Analysis of the aioA gene clone library revealed that the majority of the As(?)-oxidizing bacteria in the soil were closely related to the genera Paracoccus of a-Proteobacteria.Our results provide direct evidence for As(?)oxidation by Paracoccus species and suggest that these species may play an important role in As(?)oxidation in paddy soils under both aerobic and denitrifying conditions.Through the screening of As(?)oxidizing bacteria from different As-contaminated paddy soil,6 isolates with the ability to oxidize As(?)was found which mainly including Rhizobium,Bosea and ensifer,4 strains have been amplified As(?)oxidase gene(aioA).Then we studied the physiological and biochemical characteristics of As(?)oxidizing bacteria which can couple anaerobic As(?)oxidation to denitrification,for learning more of the microbial As(?)oxidation mechanism in the anaerobic denitrifying conditions.Flooding of As-contaminated paddy soil was conducted to study the effect of nitrate on the anaerobic As(?)oxidation in denitrifying conditions.From the quantitative results of aioA gene,when nitrate was added to the flooded soil,it stimulated the growth of anaerobic As(?)oxidizing bacteria in the soil,and promote the oxidation of As(?).Through the establishment of aioA gene clone library,we found that the aioA gene sequences from 3mM nitrate-amended microcosms at day 28 of incubation showed Acidovorax-related OTUs were stimulated by nitrate.These findings confirm that microbial-mediated anaerobic As(?)-oxidization linked to nitrate-reduction has the potential to reduce dissolved-As concentrations in the As-contaminated soil.Although As(?)reduction reaction is the predominant reaction under anaerobic conditions in paddy soil,but the oxidation of As(?)by anaerobic microbes should not be overlooked.Under certain conditions,stimulating the growth of these microbes or enhancing the As(?)oxidation process may reduce the toxicity and bioavailability of As in soil,which may lead to the reduction of As accumulation in plants.