BIODEGRADATION OF CHLORINATED COMPOUNDS GOSSE SCHRAA and ALEXANDER J. B. ZEHNDER Department of Microbiology, Agricultural University, H.v.Suchtelenweg 4, 6703 CT Wageningen, The Netherlands Summary In this paper the important aspects of biodegradation of chlori- nated compounds are discussed; namely, the mechanisms of biode- gradation, the change in the chemical characteristics of mole- cules with chlorine substituents and, some factors affecting biodegradation. The existing knowledge about biodegradation of a number of chlorinated aliphatic hydrocarbons and chlorinated monoaroma tic hydrocarbons is summarized and discussed in more detail for some selected compounds. Special attention is given to the role of molecular oxygen in the breakdown of these chemicals and to the way in which the ring is cleaved in aromatic com- pounds. In addi tion, some general remarks are made about biode- gradation of xenobiotic compounds. 1. INTRODUCTION Halogenated compounds, and among them specifically the chlori- nated hydrocarbons, have been produced synthetically on a large scale during the last few decades (Table I). They are formed as products or by-products in a number of industrial processes and are used as sol- vents, refrigerants, fire retardants, insulators, lubricants and pesticides. But, significant amounts of chlorinated hydrocarbons are also produced during chlorination of drinking water and wastewater (25). Although over 200 halogenated hydrocarbons have been found to be produced naturally (59), most of the chlorinated compounds being released :nto or produced in the environment are of anthropogenic origin. In recent years these man-made chemicals have led to the contamination of air, water and soil (53). Some of these Lases have attracted a lot of attention since a number of the xenobiotics found as pollutants were originally produced as deleterious agents against certain forms of life. Other xenobiotics have, in the meantime, proven harmful to man and animals. To assess the hazard of axe no biotic urn to the biota, introduced accidentally or purposely into the environment, information should be available concerning the following aspects: (i) the acute toxicity of the compound; (ii) its long term effects, especially when present in low concentrations; and (iii) its fate in the environment. Since the scope of this paper is limited to the discussion of the biodegradation of chlorinated aliphatic and aromatic hydrocarbons, we will not go into the toxicological aspects but focus specifically on the biologi- cal mechanisms which lead to the transformation or elimination of these chemicals in the environment. - 278- A. Bjørseth et al. (eds.), Organic Micropollutants in the Aquatic Environment © ECSC, EEC, EAEC, Brussels and Luxembourg 1986