Paracoccus denitrificans

Paracoccus denitrificans, is a coccoid bacterium known for its nitrate reducing properties, its ability to replicate under conditions of hypergravity and for being a relative of the eukaryotic mitochondrion (endosymbiotic theory).

Paracoccus denitrificans, is a gram-negative, coccus, non-motile, denitrifying (nitrate-reducing) bacterium. It is typically a rod-shaped bacterium but assumes spherical shapes during the stationary phase. Like all gram-negative bacteria, it has a double membrane with a cell wall. Formerly known as Micrococcus denitrificans, it was first isolated in 1910 by Martinus Beijerinck, a Dutch microbiologist. The bacterium was reclassified in 1969 to Paracoccus denitrificans by D.H. Davis. The genome of P. denitrificans was sequenced in 2004.

Metabolically Paracoccus denitrificans is very flexible and has been recorded in soil in both aerobic or anaerobic environments. The microbe also has the ability to live in many different kinds of media and environments and is known to be an extremophile. The bacteria are able to obtain energy both from organic compounds, such as methanol and methylamine, and from inorganic compounds, such as hydrogen and sulfur. The ability to metabolise compounds of hydrogen and sulfur, such as thiosulfate has led to the microbe being exploited as a model organism for the study of poorly characterized sulfur compound transformations.

The denitrification properties of Paracoccus denitrificans are an important cause for the loss of nitrogen fertilisers in agricultural soil. This is possibly due to the chemical process called "denitrification" in which nitrogen is converted to dinitrogen to produce nitric oxide and nitrous oxide which cause damage to the atmosphere. Although the enzymatic mechanisms of this denitrification process are well characterised, the exact molecular controles are yet to be fully described. As such, Paracoccus denitrificans has emerged as an important model organism for the characterisation of the complete denitrification process in order to potentially reduce excessive nitrous oxide release from nitrogen fertilised soils.

Metabolically, Paracoccus denitrificans is a known chemolithoautotroph - several strains of the microbe have been isolated that grow chemolithoautotrophically using carbon disulfide or carbonyl sulfide as energy sources. It is not a known human pathogen.

Paracoccus is a biochemically versatile genus, possessing a variety of metabolisms through which a wide range of diverse compounds can be degraded. Accordingly, it has the potential for a wide variety of capabilities and applications in bioremediation.

The denitrifying property of Paracoccus denitrificans has been used in creating a bioreactor, in this case, a tubular gel containing two bacteria, for the removal of nitrogen from wastewater. Paracoccus denitrificans reduces nitrite to nitrogen gas while Nitrosomonas europaea oxidizes ammonia to nitrite, thus fueling the former metabolism. This system simplifies the process of removing nitrogen from wastewater.

Certain strains of the microbe can utilize thiocyanate as an energy source, a capability which could help clean thiocyanate-contaminated wastewater from coke-oven factories. Other strains have been discovered that can degrade halobenzoates under anaerobic denitrifying conditions, and that can degrade sulfonates under anaerobic growth conditions.