Chromium(VI) resistance and removal by actinomycete strains isolated from sediments
Introduction
Chromium is an essential trace metal for living organisms (Anderson, 1997, Cefalu and Hu, 2004). However, it shows a high toxicity, which renders it hazardous even at very low concentration (20 μg l−1) (EPA, 1998, Cheung and Gu, 2003). In nature, (e.g. water and subsurface soils), chromium occurs in two major oxidation states; i.e. Cr(III) and Cr(VI). Cr(VI) induced acute and chronic toxicity, neurotoxicity, dermatotoxicity, genotoxicity, carcinogenicity, immunotoxicity, and general environmental toxicity (Bagchi et al., 2002). Cr(VI) and its compounds are placed on the priority list of toxic chemicals of many countries including USA, UK and Canada (Hedgecott, 1994). As the application of Cr is extensive in several industries like chrome-plating, wood preservation and alloy formation, chromium-associated pollution is an increasing problem (Ryan et al., 2002).
In Tucumán, Argentina, the main hydrographic river basin, the Salí River, crosses the entire state collecting effluents from local industries. The sediment analyses from the former river basin indicate the presence of chromium and other pollutants (Amoroso et al., 1998). On the other hand, a copper processing filter plant, close to the Salí River discharges the water waste in a drainage channel that provides water to a sugar cane culture.
Hexavalent chromium compounds are approximately 1000-fold more cytotoxic and mutagenic than trivalent chromium (Biedermann and Landolph, 1990). Cr(VI) is highly water soluble and mobile, while Cr(III) shows poor solubility and is easily adsorbed on mineral surfaces. Differences in membrane transport may explain the abilities of these two speciations of chromium to induce the formation of reactive oxygen species and produce oxidative tissue damage. Cr(VI) to Cr(III) reduction, therefore, represents a significant immobilization mechanism (Bagchi et al., 2001).
Previous studies have shown that microorganisms can reduce Cr(VI) efficiently and could be used to treat Cr(VI) contaminated water under neutral pH conditions (Lloyd, 2003). This has stimulated the interest in microorganisms, and biological methods are now being explored for metal decontamination as alternatives to the conventionals due their low cost and ecological compatibility. Biological transformation of Cr(VI) to Cr(III) by enzymatic reduction has been recognized as a means of chromium decontamination from effluents (Laxman and More, 2002).
Bacterial Cr(VI) reduction occurs under both aerobic and anaerobic conditions. Most studies were carried out on Gram negative bacteria as Escherichia coli, Vibrio harveyi, Alcaligenes, Enterobacter, Shewanella and Rhodobacter under anaerobic conditions, and Pseudomonas under aerobic conditions (Myers et al., 2000, Nepple et al., 2000, Park et al., 2000, Kwak et al., 2003, Bae et al., 2005). Gram-positive bacteria also have been shown to possess Cr(VI) reduction ability. Several Bacillus species have shown Cr(VI) biosorption and reduction activity (Pal et al., 2005). Among actinomycete species, Microbacterium, Arthrobacter and Streptomyces showed Cr(VI) reduction ability (Pattanapipitpaisal et al., 2001, Laxman and More, 2002, Horton et al., 2006).
Actinomycetes constitute a significant component of the microbial population in most soils. Their metabolic diversity and particular growth characteristics, mycelial form and relatively rapid colonization of selective substrates, indicate them as well suited agents for bioremediation of metal and organic compounds. However, there are very few studies on Cr(VI) resistance and bioreduction by actinomycetes. In previous studies, Amoroso et al. (1998) have reported that metal resistance and biosorption capability may be widespread among actinomycetes growing in contaminated environments. Richards et al. (2002) studied the heavy-metal resistance patterns of Frankia strains. The first report on Cr(VI) reduction by Streptomyces was from Das and Chandra (1990). Later, Amoroso et al. (2001) reported Cr(VI) bioaccumulation by Streptomyces strains. Finally, Laxman and More (2002) determinate Cr(VI) reduction by Streptomyces griseus.
The aim of this study was to determine the Cr(VI) resistance and removal by previously isolated actinomycete strains from contaminated and non-contaminated areas of Tucumán (Argentina).
Section snippets
Bacterial strains and culture conditions
Forty-one actinomycete isolations were used in this study. Twenty-nine were originally isolated from the water reservoir El Cadillal (EC, non-contaminated area), nine from waste water of a copper filter plant (CFP, contaminated area) and three from sugar cane plant (SCP). Culture collection strains (CCS), Rhodococcus erythropoli (ATCC 11048), Pilimelia terevasa (ATCC 25603) and S. fradiae (ATCC 15438) were used as negative Cr(VI) resistance control strains. Actinomycete isolations were grown in
Qualitative and semi-quantitative assays of Cr(VI) resistance
All isolated strains from SCP displayed resistance towards to 5 mM of Cr(VI), 89% from the CFP and only 62% from EC were resistant. Two of the CCS used as negative controls did not grow in the presence of 5 mM Cr(VI).
Twenty-nine strains were selected for testing their resistance towards 3–20 mM Cr(VI). Different levels of growth inhibition for the selected strains were found by semi-quantitative Cr(VI) resistance assays carried out in MM agar medium supplemented with Cr(VI) concentrations ranging
Discussion
Cr(VI) qualitative and semi-quantitative resistance assays reported here were performed in a defined MM, because the supplemented metal does not form complexes with medium components and all the metal added is bioavailable (Amoroso et al., 2001).
Laxman and More (2002) reported higher Cr(VI) tolerance in organic medium than in semi-synthetic medium, probably due to the binding of Cr to the organic constituents of the medium, which reduces toxicity.
The studies on chromium tolerance, as published
Acknowledgements
The authors gratefully acknowledge the financial support of CIUNT, CONICET and ANPCYT, Argentina; and technical assistance of Mr. G. Borchia.
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