Biodegradation of metal-complexing aminopolycarboxylic acids

Aminopolycarboxylic acids (APCAs) are used in a broad range of household products and industrial applications to control the solubility and precipitation of metal ions. The two most widely used APCAs are ethylenediaminetetraacetate (EDTA) and nitrilotriacetate (NTA). Because their application is mostly water-based APCAs are disposed off in wastewater from where they reach the environment if not sufficiently well eliminated. A number of NTA and EDTA-degrading bacterial strains have been isolated. Many of them are strictly aerobic members of the alpha -2 subgroup of Proteobacteria. The two newly established NTA-degrading genera Chelatobacter and Chelatococcus appear to be well represented in most aerobic environments. The biochemistry of NTA and EDTA catabolism has been studied in considerable detail and recently, also information on the degradation of ethylenediaminedisuccinate [S,S]-EDDS, a structural isomer of EDTA of natural origin, was reported. For both, NTA and EDTA, a specific monooxygenase/NADH(2): FMN oxidoreductase enzyme system has been purified and characterised and their regulation under environmentally relevant growth conditions has been studied. The genes for NTA MO and EDTA MO have been cloned and sequenced and they share many similarities. In addition, an oxygen-independent NTA dehydrogenase has been isolated from a denitrifying NTA-degrading bacterium and the breakdown of [S,S]-EDDS was found to be initiated by a lyase. Information on the transport of APCAs is presently limited to EDTA, where there is strong evidence for an energy-dependent carrier that can transport free and Ca2+-complexed EDTA. Metal speciation of APCAs strongly influences both transport and subsequent metabolism. However, from the data available for the intracellular metabolism of APCAs no generalising pattern with respect to influence of metal speciation on degradation can be deduced yet.