Bacterial Metabolism of n-Alkanes and Ammonia under Oxic, Suboxic and Anoxic Conditions

Abstract

n-Alkanes are widespread in the biosphere. Due to the lack of functional groups, these alkanes exhibit low chemical reactivity. However, many microorganisms have evolved pathways to utilise n-alkanes as a growth substrate, and moreover, fortuitous alkane oxidation may play an important role in alkane degradation. This review discusses the ecology of n-alkane-degrading and ammonia-oxidising bacteria with a focus on alkane metabolism in the transition from oxic to anoxic conditions, the pathways of n-alkane and ammonium oxidation, and the enzymes catalysing n-alkane and ammonia activation. n-Alkane degrading bacteria occur in oxic as well as strictly anoxic environments, and they live in very diverse habitats, including marine or fresh water, soils, sediments or aquifers. Aerobic ammonium-oxidising as well as methanotrophic bacteria are often found in stratified habitats such as biofilms and sediments. Aerobic pathways involving oxygenases that catalyse the initial activation of n-alkanes and ammonium are well known. However, anaerobic ammonium oxidation as well as anaerobic utilisation of hydrocarbons have been demonstrated only in the past decade and are the subject of current research efforts. Enzyme systems that catalyse aerobic alkane oxidation involve a number of well-characterised monooxygenases such as cytochrome P450 monooxygenases, multi-component alkane monooxygenases (also known as ω-hydroxylase systems), methane monooxygenases, and ammonia monooxygenase. Alternative enzymes, for example an n-alkyl hydroperoxide-forming dioxygenase, have also been postulated, but contrary to the monooxygenases, an n-alkane oxidising dioxygenasehas not yet been biochemically characterised. The oxygenase components of soluble methane monooxygenase and alkane monooxygenase contain binuclear iron centres that mediate dioxygen activation, whereas particulate methane monooxygenase, ammonia monooxygenase, and presumably distinct butane monooxygenases are copper-containing enzymes. Little is known about the impact of the oxygen concentration on bacterial alkane degradation, and it has not yet been investigated which pathways and enzymes are active in bacteria which utilise alkanes at suboxic or even quasi-anoxic conditions. Methane monooxygenase as well as ammonia monooxygenase have low half-saturation constants for oxygen and, in addition, both have an ample substrate spectrum. Activation of n-alkanes by cooxidation has been demonstrated for both types of enzymes. In suboxic to quasi-anoxic habitats, in which alkane, ammonium and methane oxidising bacteria as well as other organotrophic microorganisms live in close vicinity, a cooperative effect with respect to n-alkane degradation may occur.