MECHANISMS OF BIODEGRADATION OF METAL-CITRATE COMPLEXES BY PSEUDOMONAS-FLUORESCENS

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 C-14-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,vn 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 = in-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.