USING THE METABOLIC CAPACITY OF RHODOBACTER-SPHAEROIDES TO ASSESS HEAVY-METAL TOXICITY

Heavy metals can produce differential toxic effects an bacterial growt h depending on the kind of metabolism the cells are using in relation to oxygen. Rhodobacter sphaeroides is a purple, nonsulfur, photosynthe tic, gram-negative bacterium that grows preferentially in anaerobic co nditions acting as a photoheterotroph, but it can also behave as a pho toautotroph. In aerobic conditions, this microorganism has a chemohete rotrophic metabolism. Aerobic and anaerobic metabolism may differ in t heir susceptibility to heavy metal toxicity. The metabolic duality of R. sphaeroides allowed us to assay this issue. After incubation of R. sphaeroides, several indicators of bacterial growth such as optical de nsity, dissolved oxygen concentration, protein content, and pigment co ntent were determined. The parameter values obtained showed that the i nhibitory effect exerted by zinc and cadmium on R. sphaeroides was str onger in aerobic conditions than in anaerobic ones. Microbial growth w as considerably reduced when heavy metal concentration was higher than 1 mg L(-1). Besides, R. sphaeroides was quite tolerant to copper. Cul tures to which copper was added from 0.01 to 10 mg L(-1) showed a rela tively constant growth, both in aerobic and anaerobic conditions. Thus , we have shown that assessment of heavy metal toxicity using bacteria l tests must take into account that bacteria may react in a different way when grown in anaerobic environments. (c) 1993 John Wiley and Sons , Inc.