Biodegradation of nickel-citrate and modulation of nickel toxicity by iron

Biodegradation of 1:1 nickel:citric acid by Pseudomonas fluorescens proceeded after a lag (similar to 17 h) at the rate of 11 +/- 1 mu mol h(-1), with only partial mineralization of the complex. The incomplete degradation of the complex was not attributed to changes in its structure, but was due to the toxicity of the Ni released. Addition of 1:1 Ni:citric acid inhibited glucose metabolism by the bacterium. The toxicity of the released Ni was evident only when it attained a threshold concentration of >0.3 mM in the culture medium. Speciation calculations showed that Ni released after metabolism of the complex was present as Ni2+ ion and nickel carbonate. Addition of iron as a ferric hydroxide or 1:1 Fe:citric acid to 1:1 Ni:citric acid resulted in the complete metabolism of the Ni-citrate complex, with concurrent removal of the released Ni from solution by coprecipitation with iron.