''Paracoccus denitrificans'', is a coccoid

bacterium Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

known for its nitrate reducing properties, its ability to replicate under conditions of

hypergravity Hypergravity is defined as the condition where the force of gravity exceeds that on the surface of the Earth. This is expressed as being greater than 1 '' g''. Hypergravity conditions are created on Earth for research on human physiology in a ...

and for being a relative of the

eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

mitochondrion A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is use ...

(

endosymbiotic theory Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possib ...

Description

''Paracoccus denitrificans'', is a

gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...

, 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 A microbiologist (from Greek ) is a scientist who studies microscopic life forms and processes. This includes study of the growth, interactions and characteristics of microscopic organisms such as bacteria, algae, fungi, and some types of para ...

. The bacterium was reclassified in 1969 to ''Paracoccus denitrificans'' by D.H. Davis. The genome of ''P. denitrificans'' was sequenced in 2004.

Ecology and ecological applications

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 An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme tem ...

. The bacteria are able to obtain energy both from organic compounds, such as

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...

and

methylamine Methylamine is an organic compound with a formula of . This colorless gas is a derivative of ammonia, but with one hydrogen atom being replaced by a methyl group. It is the simplest primary amine. Methylamine is sold as a solution in methanol, ...

, and from inorganic compounds, such as

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

and

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

. The ability to metabolise compounds of hydrogen and sulfur, such as

thiosulfate Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, e ...

has led to the microbe being exploited as a model organism for the study of poorly characterized sulfur compound transformations. The

denitrification Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denit ...

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 Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its ...

and

nitrous oxide Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and ha ...

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 A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...

for the characterisation of the complete denitrification process in order to potentially reduce excessive

release from nitrogen fertilised soils. Metabolically, ''Paracoccus denitrificans'' is a known

chemolithoautotroph A lithoautotroph is an organism which derives energy from reactions of reduced compounds of mineral (inorganic) origin. Two types of lithoautotrophs are distinguished by their energy source; photolithoautotrophs derive their energy from light while ...

- several strains of the microbe have been isolated that grow chemolithoautotrophically using

carbon disulfide Carbon disulfide (also spelled as carbon disulphide) is a neurotoxic, colorless, volatile liquid with the formula and structure . The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical n ...

or carbonyl sulfide as energy sources. It is not a known human

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...

. ''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. Strains of ''Paracoccus denitrificans'' have been isolated from activated sludge that degrade a variety of methylated amines under both aerobic and anaerobic conditions; another strain is chemolithoautotrophically capable of degrading quaternary carbon compounds such as dimethylmalonate under denitrifying conditions. Some strains are capable of ' aerobic denitrification', the complete dissimilation of nitrate to dinitrogen (or nitrous oxide) under aerobic growth conditions. The microbe also can oxidize

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...

to nitrite while growth on organic energy sources, a process known as ' heterotrophic nitrification'. Coupled to denitrification, heterotrophic nitrification allows for the complete transformation of ammonia to dinitrogen by a single organism.

Resemblance to mitochondria

Early research indicated that ''Paracoccus denitrificans'' especially resembled

mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

. The bacteria encloses within itself the biochemistry of the mitochondrial respiratory chain and oxidative phosphorylation. While these features are found randomly distributed in other species of aerobic bacteria, to date all of these are only found in ''Paracoccus denitrificans''. In addition, a feasible mechanism for the evolution of a eukaryotic mitochondrion, from the plasma membrane of an ancestral aerobic bacterium resembling ''P. denitrificans'' to the inner mitochondrial membrane, has been suggested. More recent phylogenetic analysis however puts other bacteria more closely related to mitochondria: see

Proto-mitochondrion The proto-mitochondrion is the hypothetical ancestral bacterial endosymbiont from which all mitochondria in eukaryotes are thought to descend, after an episode of symbiogenesis which created the aerobic eukaryotes. Phylogeny The phylogenetic ana ...

Growth under hypergravity

Recent research carried out on extremophiles in Japan involved a variety of bacteria including ''Paracoccus denitrificans'' being subject to conditions of extreme gravity. The bacteria were cultivated while being rotated in an ultracentrifuge at high speeds corresponding to 403,627 times '' g'' (the normal acceleration resulting from gravity at the Earth's surface). ''Paracoccus denitrificans'' displayed not only survival but also robust cellular growth under these conditions of hyper-acceleration which are usually found only in cosmic environments, such as on very massive stars or in the shock waves of

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...

s. Analysis showed that the small size of prokaryotic cells is essential for successful growth under

. The research has implications on the feasibility of the existence of exobacteria and panspermia.

References

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