Lead (II) Ion Inhibition of Respiration and Replication in a Toluene-Enriched Microbial Population

Abstract

ABSTRACT Bioremediation is inhibited in mine leachate water because of high concentrations of heavy metals and low pH. Inhibition of cellular respiration (A N ) and replication (U N ) by lead ions were measured at pH values near 5 for a mixed microbial culture from a wastewater treatment plant, grown aerobically using toluene as the primary carbon source. Gram-negative microorganisms dominated the toluene-selected microbial population. Microorganisms sequestered lead at binding sites having two different strengths. The stronger binding sites were saturated at about 0.5 mg/L Pb(II) concentration; no evidence of saturation of weaker binding sites was observed up to 15 mg/L of Pb(II). Most of the weak-binding capacity for lead was extracellular, which increased with lead ion concentration without saturating; the intracellular portion of the weak-binding capacity saturated at about 1.0 mg of lead per milligram of dry cells. Lead toxicity, measured by inhibition of respiration and replication, increased as lead accumulated at weak binding sites. However, toxicity to respiration and replication exhibited significantly different dose dependences with lead ion exposure. Replication decreased linearly with the amount of lead sequestered by the cells, consistent with nonspecific inhibition of reproductive catalysis. Respiration was correlated with lead ion activity in solution. Dependence of acute respiration toxicity on Pb(II) concentration and pH were consistent with a modified free ion activity model involving bis binding of lead at inhibition sites. Using a few easily determined parameters, inhibition of respiration and inhibition of reproduction for a natural microbial populations by Pb(II)-contaminated water at low pH are described by the modified free-ion activity model coupled with an appropriate dose-response function.