Mechanisms of bacterially catalyzed reductive dehalogenation.

Abstract

Nine bacteria were tested for the ability to dehalogenate tetrachloromethane (CT), tetrachloroethene (PCE), and 1,1,1-trichloroethane (TCA) under anaerobic conditions. Three bacteria were able to reductively dehalogenate CT. Dehalogenation ability was not readily linked to a common metabolism or changes in culture redox potential. None of the bacteria tested were able to dehalogenate PCE or TCA. One of the bacteria capable of dehalogenating CT, Shewanella putrefaciens, was chosen as a model organism to study mechanisms of bacterially catalyzed reductive dehalogenation. The effect of a variety of alternate electron acceptors on CT dehalogenation ability by S. putrefaciens was determined. Oxygen and nitrogen oxides were inhibitory but Fe(III), trimethylamine oxide, and fumarate were not. A model of the electron transport chain of S. putrefaciens was developed to explain inhibition patterns. A period of microaerobic growth prior to CT exposure increased the ability of S. putrefaciens to dehalogenate CT. A microaerobic growth period also increased cytochrome concentrations. A relationship between cytochrome content and dehalogenation ability was developed from studies in which cytochrome concentrations in S. putrefaciens were manipulated by changing growth conditions. Inhibitors of electron transport by cytochromes, i.e., carbon monoxide and cyanide, inhibited dehalogenation. Spectrophotometric analyses were done to characterize the cytochromes of S. putrefaciens. Cytochromes c were identified in cytoplasmic, periplasmic, and membrane fractions. Cytochrome b was found only in membrane fractions. Observed results were consistent with a mechanism involving fortuitous CT dehalogenation by bacterial cytochromes in the presence of microbially-produced reductant. Stoichiometry studies using ¹⁴C-CT suggested that CT was first reduced to form a trichloromethyl radical. Reduction of the radical to produce chloroform and reaction of the radical with cellular biochemicals explained observed product distributions. Carbon dioxide or other fully dehalogenated products were not found.