Geomicrobiology of Manganese and Arsenic Cycling in Cambodia SoilsEPA Grant Number: F6F11215
Title: Geomicrobiology of Manganese and Arsenic Cycling in Cambodia Soils
Investigators: Ying, Samantha C.
Institution: Stanford University
EPA Project Officer: Hahn, Intaek
Project Period: September 1, 2006 through September 1, 2008
Project Amount: $111,344
RFA: STAR Graduate Fellowships (2006) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Ecological Indicators/Assessment/Restoration , Fellowship - Geomicrobiology
Arsenic (As) poisoning has become an extensive problem within southeast Asian countries that are gradually replacing the usage of pathogen contaminated surface waters with Asladen well-water. Within the past decade, Cambodians have shifted towards groundwater usage from surface water use, where groundwater samples have been reported to contain up to 100 ug L-1 or more of arsenic. Recently, manganese (Mn) nodules as large as 1 cm in diameter have been discovered in multiple excavation pits near Phnom Penh, Cambodia. Mn oxides are highly charged, reactive precipitates capable of rapid metal and metalloid oxidation, including reduced arsenic (As(III)). Oxidation of As(III) leads to subsequent adsorption of As(V) onto the Mn oxide surface, thus limiting its solubility and mobility in soil and sedimentary environments. In circumneutral soil and water systems, the majority of Mn(II) oxidation is mediated by microbes; however, the molecular mechanisms involved in these microbially mediated catalysis reactions are poorly understood. In this study, we propose to isolate novel terrestrial Mn(II)-oxidizing microbes from areas with high and low arsenic concentrations in Cambodia in order to i) measure adsorption/desorption rates of arsenic on the biogenic Mn oxide surface produced by the isolates, ii) apply molecular genetic approaches (e.g., transposon mutagenesis) to obtain the gene sequence of the Mn-oxidizing protein, iii) purify the Mn oxidizing proteins to further characterize molecular mechanisms and biochemistry involved in terrestrial microbial Mn-oxidation and its influence on As cycling.
A three part experimental approach will be applied to investigate the influence of biogenic Mn oxides from Cambodia soils on arsenic sorption and the molecular mechanisms involved in their formation. First, Mn(II) oxidizers will be isolated from soil and sediment samples from Kean Svay, Kandal province in southeast Cambodia. Second, after isolation and successful growth in liquid culture, the novel Mn-oxidizer will be prepared for genetic studies. Isolates will be screened for development of a genetic system in order to perform transposon mutagenesis to identify genes involved in Mn oxidation. Lastly, arsenic adsorption and desorption studies will be simultaneously performed on the new Mn oxidizing isolate. Adsorption capacity of the biogenic oxides formed by the new isolate will be compared with that of synthetic Mn oxides. The information yielded by this study will be useful towards understanding the impact of Mn oxides on arsenic cycling within Cambodia soils.
We hypothesize that i) under low arsenic concentrations, biogenic oxides will adsorb larger quantities of arsenic than amorphous iron oxides found in the same localities, and ii) novel Mn-oxidizing proteins will have sequences similar to existing putative manganese oxidase sequences; variants found in these Mn-oxidizing proteins may reveal mechanistic benefits imparted upon terrestrial Mn oxidizing bacteria.