Reduction of Cr(VI) and Bioaccumulation of Chromium by Gram Positive and Gram Negative Microorganisms not Previously Exposed to CR-Stress

Abstract

Resistance to Cr(VI) is usually associated with its cellular exclusion, precluding enrichment techniques for the isolation of organisms accumulating Cr(VI) via bioreduction to insoluble Cr(III). A technique was developed to screen for potential Cr(VI) reduction in approx. 2000 isolates from a coastal environment, based on the non-specific reduction of selenite and tellurite to Se0 and Te0, and reduction of tetrazolium blue to insoluble blue formazan. The most promising strains were further screened in liquid culture, giving three, which were identified by 16S rRNA sequence analysis as Bacillus pumilus, Exiguobacterium aurantiacum and Pseudomonas synxantha, all of which reduced 100 microM Cr(VI) anaerobically, without growth. The respective removal of Cr(VI) was 90% and 80% by B. pumilus and E. aurantiacum after 48 h and 80% and by P. synxantha after 192 h. With the gram positive strains Cr(VI) promoted loss of flagella and, in the case of B. pumilus, lysis of some cells, but Cr was deposited as an exocellular precipitate which was identified as containing Cr and P using energy dispersive X-ray microanalysis (EDAX). This prompted the testing of Citrobacter sp. N14 (subsequently re-assigned by 16S rRNA sequence analysis and biochemical studies as a strain of Serratia) which bioprecipitates metal cation phosphates via enzymatically-liberated phosphate. This strain reduced Cr(VI) at a rate comparable to that of P. synxantha but Cr(III) was not bioprecipitated where La(III) was removed as LaPO4, even though a similar amount of phosphate was produced in the presence of Cr(III). Since B. pumilus removed most of the Cr(VI), with the formation of cell-bound CrPO4 implicated, this suggests that this strain could have future bioprocess potential.

Phosphate-Mediated Remediation of Metals and Radionuclides

“…To further highlight such adaptation, recent investigations of terrestrial and marine bacterial isolates belonging to the genera Aeromonas, Bacillus, Myxococcus, Pantoea, Pseudomonas, Rahnella, and Vibrio demonstrated that Cr, Pb, and U were removed from solution as phosphate minerals under both oxic and anoxic growth conditions [57,60,74,152,[154][155][156][157][158]. Our recent work further examined lead and 6 Advances in Ecology uranium precipitates produced by Rahnella sp.…”

“…Furthermore, contaminated sites that are characterized by acidic to circumneutral porewater pH represent environments that can support stable mineral formation (Figures 2(a) and 2(b)), provided that carbonates are not present in significant concentrations (i.e.,P CO 2 < 10 −3.5 atm) [176,177]. Interestingly, investigations of microbial reduction of Cr, Np, Pu, and U have been shown to support subsequent phosphate precipitation reactions via thermodynamic modeling, chromatographic separation of actinides based on valence state, and X-ray analytical methods [154,155,172,178,179]. Unlike U, that is capable of forming phosphate minerals in both hexavalent and tetravalent states [50,179], the reduction of Cr, Np, and Pu is initially required for these contaminants to participate in phosphate precipitation reactions [154,155,172,178].…”

“…Interestingly, investigations of microbial reduction of Cr, Np, Pu, and U have been shown to support subsequent phosphate precipitation reactions via thermodynamic modeling, chromatographic separation of actinides based on valence state, and X-ray analytical methods [154,155,172,178,179]. Unlike U, that is capable of forming phosphate minerals in both hexavalent and tetravalent states [50,179], the reduction of Cr, Np, and Pu is initially required for these contaminants to participate in phosphate precipitation reactions [154,155,172,178]. To date, only pure culture or coculture studies have identified such coupled microbial interactions that offer an additional approach to control contaminant toxicity and mobility that are perpetuated by valence state cycling.…”

Developments in Bioremediation of Soils and Sediments Polluted with Metals and Radionuclides – 1. Microbial Processes and Mechanisms Affecting Bioremediation of Metal Contamination and Influencing Metal Toxicity and Transport

“…The best-documented organism for metal phosphate biomineralization, a Citrobacter sp., (now reclassified as a Serratia sp. on the basis of molecular methods, the presence of the phoN phosphatase gene and the production of pink pigment under some conditions (Pattanapipitpaisal et al 2002) grows well on cheaply available substrates and viable cells are not required for metal uptake since this relies on hydrolytic cleavage of a supplied organic phosphate donor (Macaskie et al 1992). The expense of adding organic phosphate was calculated to be the single factor which limited the economic viability of this approach (Roig et al 1995); moreover organophosphorus compounds are often highly toxic.…”

Production of an extracellular carboxymethyl cellulase from Exiguobacterium aurantiacum AN2 by submerged fermentation

“…Many aerobic and anaerobic bacteria have been isolated from different habitats exhibited their potential capacity to secrete cellulases (Nakamura and Kitamura, 1982;Schwarz, 2001;Milala et al, 2005;Immanuel et al, 2006). Many strains belong to genus Exiguobacterium have wide applications in biotechnological, industrial and agricultural fields (Bogdanova et al, 2001;Pattanapipitpaisal et al, 2002;Anderson and Cook, 2004;Romsaiyud et al, 2005;Kumar et al, 2006;Okeke, 2008). Moreover, various enzymes such as protease, esterase, guanosine kinase, dehydrogenase and ATPases were purified and characterized from several Exiguobacterium strains (Usuda et al, 1998;Suga and Koyama, 2000;W ada et al, 2004;Hwang et al, 2005;Kasana and Yadav, 2007;Hara et al, 2007).…”