Introduction

Abstract

The surface of the world is in contact with the oxygenated atmosphere and all readily apparent life forms are oxygen dependent, but there are many situations where oxygen is deficient. Organisms able to utilize the available nutrients in such environments must be capable of an anaerobic way of life. Such anaerobic life is almost entirely bacterial and indeed most prokaryotic species, unlike most eukaryotes, are capable of prolonged growth in the complete absence of oxygen. Many of these organisms, such as Escherichia coli, grow more efficiently in the presence of oxygen and are facultative anaerobes. Even types often regarded as obligate aerobes, such as pseudomonads, behave as facultative anaerobes if they can obtain energy from anaerobic respiration, commonly by the reduction of nitrate.