In situ bioremediation practices that include subsurface addition of fermentable electron donors to stimulate reductive dechlorination by anaerobic bacteria have become widely employed to combat chlorinated solvent contamination in groundwater. At a contaminated site located near Baton Rouge, Louisiana (USA), toluene was transiently observed in groundwater at concentrations that sometimes far exceeded the US drinking water maximum contaminant level (MCL) of 1 mg/L after a fermentable substrate (agricultural feed grade cane molasses) was injected into the subsurface with the intent of providing electron donors for reductive dechlorination. Here, we present data that demonstrate that indigenous microorganisms can biologically produce toluene by converting phenylacetic acid, phenylalanine, phenyllactate, and phenylpyruvate to toluene. When grown in defined medium with phenylacetic acid at concentrations ≤350 mg/L, the molar ratio between toluene accumulated and phenylacetic acid supplied was highly correlated ( R ≥ 0.96) with a toluene yield exceeding 0.9:1. Experiments conducted using C labeled compounds (phenylacetic acid-2-C and l-phenylalanine-3-C) resulted in production of toluene-α-C, confirming that toluene was synthesized from these precursors by two independently developed enrichment cultures. Results presented here suggest that monitoring of aromatic hydrocarbons is warranted during enhanced bioremediation activities where electron donors are introduced to stimulate anaerobic biotransformation of chlorinated solvents.
Bioremediation approaches that involve subsurface injection of fermentable substrates have become common for achieving anaerobic transformation of halogenated alkanes (e.g., 1,2-dichloroethane) and alkenes (e.g., vinyl chloride) to nontoxic final products (e.g., ethene) in groundwater. It was previously reported that subsurface microbial communities could produce toluene as an unintended consequence of subsurface molasses injection. Here, we report a combination of field monitoring data and laboratory studies that demonstrate that subsurface microbial communities can also produce p-cresol (4-methylphenol) as an unintended consequence of enhanced anaerobic bioremediation strategies targeting chlorinated solvents. Following the subsurface injection of agricultural feed-grade cane molasses, p-cresol was observed to transiently accumulate in groundwater sampled from multiple wells, reaching concentrations as high as 63.2 mg/L. Enrichment cultures established with site groundwater consistently produced p-cresol when provided with 4-hydroxyphenylacetic acid. The results from additional experiments with molasses-grown biomass and two casein-derived products with varying aromatic amino acid composition provide compelling evidence that anaerobic transformation of proteins from biomass produced following the addition of a readily fermentable substrate into groundwater can supply a sufficiently large reservoir of aromatic amino acids to account for p-cresol and toluene concentrations observed at the field scale.
The perceived direction of rotation of a 3-D cloud of dots can be biased by a prior rotation (Jiang, Pantle, and Mark, 1998 Perception & Psychophysics 60 275-286). In a series of experiments, it is shown that the temporal rotation bias is reversed by a 180 degrees change of head orientation between two rotation sequences; i.e. the perceived direction of rotation reverses for the second of two sequences when head orientation is changed. The bias is, therefore, viewer-centered. Perceptual reversals are not obtained when the orientation of the head is changed and returned to its original position between rotation sequences. It was also found that the viewer-centered bias combined additively with viewer-independent near-far luminance information. Finally, the bias was manifest when 3-D depth was re-established, but not maintained, between rotation sequences. A model, in descriptive and flowchart forms, is used to explain the integration of world-centered information and a viewer-centered temporal bias on the presence/absence of perceptual reversals of the rotating virtual sphere. In the model, the temporal bias is the result of the coupling of depth values to persisting 2-D retinal motion signals.
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