Responses of the Anaerobic Bacterial Community to Addition of Organic C in Chromium(VI)- and Iron(III)-Amended Microcosms

Abstract

Chromium (VI) is toxic to microorganisms and can inhibit the biodegradation of organic pollutants in contaminated soils. We used microcosms amended with either glucose or protein (to drive bacterial community change) and Fe(III) (to stimulate iron-reducing bacteria) to study the effect of various concentrations of Cr(VI) on anaerobic bacterial communities. Microcosms were destructively sampled based on microbial activity (measured as evolution of CO 2 ) and analyzed for the following: (i) dominant bacterial community by PCRdenaturing gradient gel electrophoresis (DGGE) of the 16S rRNA gene; (ii) culturable Cr-resistant bacteria; and (iii) enrichment of iron-reducing bacteria of the Geobacteraceae family by real-time PCR. The addition of organic C stimulated the activities of anaerobic communities. Cr(VI) amendment resulted in lower rates of CO 2 production in glucose microcosms and a slow mineralization phase in protein-amended microcosms. Glucose and protein amendments selected for different bacterial communities. This selection was modified by the addition of Cr(VI), since some DGGE bands were intensified and new bands appeared in Cr(VI)-amended microcosms. A second dose of Cr(VI), added after the onset of activity, had a strong inhibitory effect when higher levels of Cr were added, indicating that the developing Cr-resistant communities had a relatively low tolerance threshold. Most of the isolated Cr-resistant bacteria were closely related to previously studied Cr-resistant anaerobes, such as Pantoea, Pseudomonas, and Enterobacter species. Geobacteraceae were not enriched during the incubation. The studied Cr(VI)-contaminated soil contained a viable anaerobic bacterial community; however, Cr(VI) altered its composition, which could affect the soil biodegradation potential.

Inhibition of Nitrate Reduction by Chromium(VI) in Anaerobic Soil Microcosms

“…Microcosm headspaces were sampled with a gas-lock syringe at 2-to 7-day intervals, and CO 2 was determined by a gas chromatograph (HP 5890 series II) as previously described (26).…”

“…All media contained 0.25 mM potassium chromate. Soil extract was prepared as previously described (26). NRB basal medium contained the following (all per liter): 2.5 g sodium bicarbonate, 0.25 g ammonium chloride, 0.6 g sodium phosphate monobasic, monohydrate, 0.1 g potassium chloride, 10 ml mineral solution (27), and 0.025 g yeast extract.…”

“…The purpose of this study was to extend our work on the effects of Cr(VI) upon microbes in soil that mediate discrete chemoheterotrophic processes such as the use of O 2 (30) or Fe ϩ3 (26) as terminal electron acceptors. We examined denitrification to determine whether the putative direct impact of Cr(VI) on the biochemistry of nitrate reduction would alter community dynamics from what had been observed with other terminal electron acceptors.…”

Complete Genome Sequence of Rahnella aquatilis CIP 78.65

“…Strains belonging to the genus Rahnella have been isolated from soil, freshwater, food, and clinical samples. As a result, members of this genus have been studied in plant biology as they relate to plant growth and nutrient acquisition (13,16,18,26,27), food sciences (4,17,23,24), medical microbiology (1,10,12), and more recently, in studies that demonstrate the potential use of Rahnella isolates for bioremediation (2,8,19,21,25). Therefore, wholegenome sequencing was conducted on Rahnella aquatilis CIP 78.65 to support current research that utilizes this strain as well as to support comparative genomics studies.…”

Complete Genome Sequence of Rahnella sp. Strain Y9602, a Gammaproteobacterium Isolate from Metal- and Radionuclide-Contaminated Soil

“…Subsequent studies indicated that the strain precipitated 95% of soluble uranium [U(VI)] as an insoluble autunite mineral during oxic and anoxic growth conditions when supplied with glycerol-3-phosphate as the sole carbon and phosphorus source (1, 2, 12). Additionally, recent microbial ecology studies of metal-, metalloid-, organic-, and radionuclide-contaminated soils have identified enrichments of Rahnella species that show promise for remediation strategies (6,10,13,15). Whole-genome sequencing was thus conducted on Rahnella sp.…”