As a consequence of the activities of modern industry and agriculture, many made-made organic compounds have found their way into our environment, and by persisting there for varying periods of time have caused concern to society. Why do some chemicals persist while others disappear? Detailed answers to this question require an understanding of the degradative segment of the earth's carbon cycle, most of the reactions of which are catalysed by enzymes used by microbes. These organisms owe much of their degradative expertise to their ability to render oxygen gas chemically reactive. This is a process that would be extremely dangerous for any living organism if it were carried out in a haphazard or accidental fashion; but when catalysed and cantrolled by enzymes (oxygenases) of micro-organisms, reaction sequences are started that result in biodegradation of compounds that resist the enzymes of all other living forms.