Thursday, September 08, 2011, 4:30AM - 5:30AM
Full title of talk: "Labile and redox-active organic matter sources influence bacterial community structure and groundwater arsenic mobilization." Research throughout the last decade has generated a generally accepted model for groundwater arsenic (As) mobilization in South and Southeast Asia: labile organic carbon fuels the microbially-mediated reductive dissolution of iron (Fe) oxide minerals, which releases As sorbed to Fe oxides into the groundwater. Humic substances, redox reactive fraction of dissolved organic matter, are present in high- As groundwaters in the GBD (Mladenov, et al. 2010), and could play an important role in groundwater As mobilization (Jiang, et al. 2009; Jiang & Kappler 2008). Recent research at our field site in Araihazar, Bangladesh suggests that natural organic matter in the high- As aquifer contains both labile components and humic substances capable of enhancing Fe-reduction through abiotic or microbially-mediated electron-shuttling (Mladenov et al., 2010). Thus we set out to investigate how different carbon sources influence bacterial community structure and groundwater As mobilization by conducting a sediment incubation experiment. We incubated the sediment and groundwater for 80 days under different treatment conditions: 2 mM acetate (labile carbon); 5 mM AQDS (a redox-active carbon compound); 2 mM acetate and 5 mM AQDS; native humic substances, redox-active components of the DOM; and no addition/control. We found significant (p0.001) shifts in bacterial community structure, as measured by the UniFrac metric (Lozupone & Knight 2005), from the pre-incubation conditions to the post-incubation conditions. In addition, the different treatments induced significant differences in the structure of the bacterial community (p0.001). Also, the relative abundance of Deltaprotebacteria was significantly higher in the incubations amended with 2 mM acetate and 5 mM AQDS than those amended with only 2 mM acetate, only 5 mM AQDS, only native humic substances or groundwater (control). Interestingly, groundwater Fe and As mobilization was also significantly higher in the incubations amended with 2 mM acetate and 5 mM AQDS (p ≤ 0.002). Thus, these results suggest that native Deltaproteobacteria within the aquifer sediment are able to pair the oxidation of labile carbon to the reduction of redox- active, HS-like carbon sources to accelerate Fe- and As- mobilization.