Aerobic Hydrocarbon-Degrading Alphaproteobacteria: Sphingomonadales

The bacterial order Sphingomonadales includes many Sphingomonadaceae and Erythrobacteraceae isolates that have the ability to degrade a wide range of hydrocarbons. Hydrocarbon-degrading members of the Sphingomonadaceae (mainly belonging to the genera Sphingomonas, Sphingobium, Novosphingobium, and Sphingopyxis) are common Gram-negative aerobic organisms that have been isolated from a wide variety of environments, including temperate and polar soils, marine sediments, and plant surfaces and tissues. They degrade a broad range of mono- and polycyclic aromatic compounds, and the genetics and enzymology of these processes have been elucidated in some detail. Recently, the genome sequences of several hydrocarbon-degrading strains have been determined. In sphingomonads, the biodegradation gene loci are often widely distributed in the genome and are not colocalized as in other hydrocarbon-degrading genera, which has hindered genetic manipulation. The relevant degradative genes are very often located on specific large plasmids ("megaplasmids"). Sphingomonad strains are easy to cultivate and are strong candidates for bioremediation applications, but the recent results suggest that their role in the degradation of recalcitrant aromatics in contaminated soils in situ may be less than previously thought.