Abstract. The effects of chemical speciation on mineralization of organic compounds was studied using citric acid as a model substance. The degradation of ~4C-labeled AI-, Co-, Cu-, and Zn-citrate was followed in chemically well-defined media inoculated with mixed cultures of microorganisms (soil extracts from two soils). The degradation of citrate was completely inhibited when the acid was bound to Zn, Cu, or Co and partly inhibited when bound to A1. The mineralization of citrate as well as histidine was also followed by incubation of the complexes (Cu, Zn, A1) in the two soils. No effect of metals on the degradation of histidine was seen. The degradation of citrate in soils was also unaffected when complexed to Cu and Zn, whereas AI exerted an inhibited decomposition in both soils.
The uptake of 14C-labeled cadmium-, copper-and zinc-citrate into cells of Klebsiella oxytoca was followed.The study was made in order to examine if the earlier reported disability of the bacterium to degrade these complexes was due to an inhibition in transport through the cell membrane. Citrate complexed to cadmium, copper or zinc was taken up at a similar rate to the free citric acid. However, the metal-citrate complexes were not metabolized as shown by the marked accumulation of 14C in the cells as compared with the 14C content in the cells incubated with free citric acid. This was confirmed by the results from trichioroacetic acidprecipitation showing that no 14C was incorporated into macromolecules when the citrate was complexed to the different metals. It is suggested that the inhibited degradation was due to effects on the interaction between enzyme (aconitase) and substrate in the conversion of citrate to iso-citrate. The role of complex configuration on the mineralization of metal-citrate is discussed and also tested in mineralization studies of other metal-citrate complexes (aluminum-, calcium-, cobalt-, manganese-and nickel-citrate).
The influence of glucose concentration on Cd, Cu, Hg, and Zn toxicity to a Klebsiella sp. was studied by following the degradation of 14C-labeled glucose at pH 6.0. Uptake of '4C into the cells was also determined. The carbon concentrations ranged from 0.01 to 40 mg liter-', which are equivalent to soluble C concentrations in natural environments. The toxicity of Cu, Cd, and Zn to a KlebsieUa sp. was affected considerably by the C concentration. Copper at 10-s M was toxic when the carbon concentration was 10 or 40 mg liter-', while at 0.01 to 1.0 mg liter-' no toxicity was observed. Cadmium and zinc were toxic at 10-2 M in media containing 0.01 to 1.0 mg of C liter-'. At C concentrations greater than 1.0 mg liter-', the inhibition of glucose degradation and carbon assimilation was observed at 10-3 M Cd and Zn. The toxicity of mercury seemed to be independent of the C concentration. Results of this study showed that the nutritional state of an organism may have a profound effect on its sensitivity to metals. Metals taken up by an energy-driven transport system may be less toxic under conditions of C starvation. The C concentration should be taken into account when evaluating results from toxicity studies, especially as most microorganisms in nature live under energy-limited conditions.
The effects of Cd2+, Cu2+, Mg2+, and Zn2+ on the decomposition of citric acid by a Klebsiella sp. were studied by monitoring the degradation of [14C]citrate. The carbon concentration used was 10 ,ug of C liter-', and the media were designed to provide at least 95% of the citrate complexed to the metal studied. After 72 h of incubation, 80% of the uncomplexed citric acid and 76% of the magnesium citrate had been decomposed. A marked inhibition was observed when Cd2+, Cu2+, or Zn2+ was bound to the organic anion; only 23% of the cadmium citrate, 14% of the zinc citrate, and 5% of the cuprous citrate had been decomposed. The effects were not the result of toxicity, since experiments run with [14C]glucose (nonchelating compound) instead of citrate resulted in similar decomposition rates regardless of the presence of the metal. To examine whether the binding of a metal to citrate enhanced its uptake by the Klebsiella sp., we studied the relative uptake of 65Zn in citrate-and in glucose-containing media. No such effect could be observed, with the uptake of Zn2+ being higher in the glucose-containing media. The study shows that metals may render low-molecular-weight organic acids, such as citric acid, resistant to bacterial degradation. This stresses the importance of metals in influencing microbial decomposition of organic compounds, not only as a result of toxicity.
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