Mechanisms of biodegradation of metal-citrate complexes by Pseudomonas fluorescens

Abstract

Biodegradation of metal-citrate complexes by Pseudomonas fluorescens depends on the nature of the complex formed between the metal and citric acid. Bidentate Fe(III)-, Ni-, and Zn-citrate complexes were readily biodegraded, but the tridentate Cd-and Cu-citrate, and U-citrate complexes were not. The biodegradation of Ni-and Zn-citrate commenced after an initial lag period; the former showed only partial (70%) degradation, whereas the latter was completely degraded. Uptake studies with 14 C-labeled citric acid and metal-citrate complexes showed that cells grown in medium containing citric acid transported free citric acid at the rate of 28 nmol min ؊1 and Fe(III)-citrate at the rate of 12.6 nmol min ؊1 but not Cd-, Cu-, Ni-, U-, and Zn-citrate complexes. However, cells grown in medium containing Ni-or Zn-citrate transported both Ni-and Zn-citrate, suggesting the involvement of a common, inducible transport factor. Cell extracts degraded Fe(III)-, Ni-, U-, and Zn-citrate complexes in the following order: citric acid ‫؍‬ Fe(III)-citrate > Ni-citrate ‫؍‬ Zn-citrate > U-citrate. The cell extract did not degrade Cd-or Cu-citrate complexes. These results show that the biodegradation of the U-citrate complex was limited by the lack of transport inside the cell and that the tridentate Cd-and Cu-citrate complexes were neither transported inside the cell nor metabolized by the bacterium.Citric acid is a multidentate ligand and forms stable complexes with various metal ions (19). The rate and extent of biodegradation of several metal-citrate complexes by microorganisms vary. For example, Pseudomonas pseudoalcaligenes degraded Mg-citrate at a much lower rate than Ca-, Fe(III)-, and Al(III)-citrate (29). Studies with a Klebsiella sp. showed that citric acid and Mg-citrate were readily degraded, whereas Cd-, Cu-, and Zn-citrate were resistant to biodegradation (9). Both these studies also showed that metal toxicity was not responsible for the lack of or the lower rate of degradation of certain metal-citrate complexes but gave no other explanation. Biodegradation studies with Pseudomonas fluorescens showed that bidentate complexes of Fe(III)-, Ni-, and Zn-citrate were readily biodegraded, whereas complexes that involve the hydroxyl group of citric acid, the tridentate Cd-and Cu-citrate complexes, and U-citrate complex were not biodegraded (17). No relationship between biodegradability and stability of the complexes was observed. The tridentate Fe(II)-citrate complex, although recalcitrant, was readily biodegraded after oxidation and hydrolysis to the bidentate Fe(III)-citrate form, denoting a structure-function relationship in the metabolism of the complex (16).Microorganisms have selective transport systems for the uptake of metals of known biological function (11,23,28). Heavy metals are transported into cells by preexisting transport systems and are subsequently either chemically neutralized, sequestered, or removed from the cells by rapid efflux (6,35). Metals affect the transport of citric acid in bacteria. For example, the...

Presence of Fe3+ and Zn2+ promoted biotransformation of Cd–citrate complex and removal of metals from solutions

“…MBR were similar to that by Pseudomonas fluorescens [7,8,15] [7]. Therefore, the non-biodegradable tridentate Fe(III)-citrate complex was the predominant complex in the 1:1 Fe:citrate medium at pH Table 2 Distribution and uptake of metals in Pseudomonas sp.…”

“…Bidentate complexes, such as Fe(III)-and Zn-citrate can be transported as intact complex inside the bacteria and then was biodegraded [7]. Tridentate and binuclear complexes, such as Cd-and U-citrate are recalcitrant to biodegradation due to the lack of transport systems for metal-citrate complexes or the inability of the citrate degrading enzymes to utilize the complexes as substrate or both [8].…”

Microbial Transformations of Uranium Complexed with Organic and Inorganic Ligands

“…We have shown that the metabolism of the metal-citrate complex is dependent upon the type of complex formed between the metal and citric acid. Fe(III) forms a bidentate complex with citric acid and was metabolized, whereas U forms a binuclear complex with citric acid and was recalcitrant (Francis et al 1992;Francis and Dodge,1993;Joshi-Tope and Francis 1995). The resting cells of the sulfate-reducing Desulfovibrio desulfuricans and the facultative iron-reducing Shewanella halotolerans bacteria reduced U(VI) complexed with oxalate or citrate to U(IV) under anaerobic conditions with little precipitation of uranium.…”

Impact of the Mg 2+ -citrate transporter CitM on heavy metal toxicity in Bacillus subtilis

“…The Mg 2+ -citrate transporter CitM of Bacillus subtilis is the best-studied example and accepts the toxic heavy metal ions Zn 2+ , Ni 2+ and Co 2+ instead of Mg 2+ in the metal-citrate complex . Bacteria capable of taking up and metabolizing heavy metal ions complexed to citrate have been implicated in preventing the mobilization of toxic metals from waste dumps (Joshi-Tope and Francis 1995). Alternatively, these organisms may play a role in bioremediation of heavy-metal-polluted sites by active accumulation of the toxic ions after heterotrophic leaching with citrate (White et al 1997;Peters 1999).…”

“…A limited number of bacteria of the genera Pseudomonas, Citrobacter, Bacillus and Klebsiella have been described that transport metal-citrate complexes across the cytoplasmic membrane into the cell (Willecke et al 1973;Bergsma and Konings 1983;Madsen and Alexander 1985;Brynhildsen and Rosswall 1989;Joshi-Tope and Francis 1995). The Mg 2+ -citrate transporter CitM of Bacillus subtilis is the best-studied example and accepts the toxic heavy metal ions Zn 2+ , Ni 2+ and Co 2+ instead of Mg 2+ in the metal-citrate complex .…”