Mitigation of arsenic toxicity in rice by the co-inoculation of arsenate reducer yeast with multifunctional arsenite oxidizing bacteria.

The study aimed to explicate the role of microbial co-inoculants for the mitigation of arsenic (As) toxicity in rice. Arsenate (AsV) reducer yeast Debaryomyces hansenii NBRI-Sh2.11 (Sh2.11) with bacterial strains of different biotransformation potential was attempted to develop microbial co-inoculants. An experiment to test their efficacy (yeast and bacterial strains) on plant growth and As uptake was conducted under a stressed condition of 20 mg kg-1 of arsenite (AsIII). A combination of Sh2.11 with an As(III)-oxidizer, Citrobacter sp. NBRI-B5.12 (B5.12), resulted in ∼90% decrease in grain As content as compared to Sh2.11 alone (∼40%). Reduced As accumulation in rice roots under co-treated condition was validated with SEM-EDS analysis. Enhanced As expulsion in the selected combination under in vitro conditions was found to be correlated with higher As content in the soil during their interaction with plants. Selected co-inoculant mediated enhanced nutrient uptake in association with better production of indole acetic acid (IAA) and gibberellic acid (GA) in shoot, support microbial co-inoculant mediated better biomass under stressful condition. Boosted defense response in association with enhanced glutathione-S-transferase (GST) and glutathione reductase (GR), activities under in vitro and in vivo conditions were observed. These results indicated that the As(III) oxidizer-B5.12 accelerated the As detoxification property of the As(V) reducer-Sh2.11. Henceforth, the results confer that the coupled reduction-oxidation process of the co-inoculant reduces the accumulation of As in rice grain. These co-inoculants can be further developed for field trials to achieve higher biomass with alleviated As toxicity in rice.

A case study for demonstrating the application of U.S. EPA's monitored natural attenuation screening protocol at a hazardous waste site.

Natural attenuation assessment data, collected at a Superfund site located in Louisiana, USA, are presented. The study site is contaminated with large quantities of DNAPL waste products. Source characterization data indicated that chlorinated ethene and ethane compounds are the major contaminants of concern. This case study illustrates the steps involved in implementing the U.S. EPA's [U.S. EPA, 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water, by Wiedmeier, T.H., Swnason, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Hass, P.E., Miller, R.N., Hansen, J. E., Chapelle, F.H., Office of Research and Development, EPA/600/R-98/128] monitored natural attenuation (MNA) screening protocol at this chlorinated solvent site. In the first stage of the MNA assessment process, the field data collected from four monitoring wells located in different parts of the plume were used to complete a biodegradation scoring analysis recommended by the protocol. The analysis indicates that the site has the potential for natural attenuation. In the second stage, a detailed conceptual model was developed to identify various contaminant transport pathways and exposure points. The U.S. EPA model and BIOCHLOR was used to assess whether the contaminants are attenuating at a reasonable rate along these transport paths so that MNA can be considered as a feasible remedial option for the site. The site data along with the modeling results indicate that the chlorinated ethene and chlorinated ethane plumes are degrading and will attenuate within 1000 ft down gradient from the source, well before reaching the identified exposure point Therefore, MNA can be considered as one of the feasible remediation options for the site.