METHANOTROPHIC BACTERIA

Methane-utilizing bacteria (methanotrophs) are a diverse group of gram -negative bacteria that are related to other members of the Proteobact eria. These bacteria are classified into three groups based on the pat hways used for assimilation of formaldehyde, the major source of cell carbon, and other physiological and morphological features. The type I and type X methanotrophs are found within the gamma subdivision of th e Proteobacteria and employ tile ribulose monophosphate pathway for fo rmaldehyde assimilation, whereas type II methanotrophs, which employ t he serine pathway for formaldehyde assimilation, form a coherent clust er within the beta subdivision of the Proteobacteria. Methanotrophic b acteria are ubiquitous. The growth of type II bacteria appears to be f avored in environments that contain relatively high levels of methane, low levels of dissolved oxygen, and limiting concentrations of combin ed nitrogen and/or copper. Type I methanotrophs appear to be dominant in environments in which methane is limiting and combined nitrogen and copper levels are relatively high. These bacteria serve as biofilters for the oxidation of methane produced in anaerobic environments, and when oxygen is present in soils, atmospheric methane is oxidized. Thei r activities in nature are greatly influenced by agricultural practice s and other human activities. Recent evidence indicates that naturally occurring uncultured methanotrophs represent new genera. Methanotroph s that are capable of oxidizing methane at atmospheric levels exhibit methane oxidation kinetics different from those of methanotrophs avail able in pure cultures. A limited number of methanotrophs have the gene tic capacity to synthesize a soluble methane monooxygenase which catal yzes the rapid oxidation of environmental pollutants including trichlo roethylene.