Capnophiles are microorganisms that thrive in the presence of high concentrations of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(). Some capnophiles may have a metabolic requirement for carbon dioxide, while others merely compete more successfully for resources under these conditions. The term is a generally descriptive one and has less relevance as a means of establishing a taxonomic or evolutionary relationship among organisms with this characteristic. For example, the ability of capnophiles to tolerate (or utilize) the amount of oxygen that is also in their environment may vary widely and may be far more critical to their survival. Species of '' Campylobacter'' are bacterial capnophiles that are more easily identified because they are also microaerophiles, organisms that can grow in high carbon dioxide as long as a small amount of free oxygen is present, but at a dramatically reduced concentration. (In the

Earth's atmosphere The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weathe ...

carbon dioxide levels are approximately five hundred times lower than that of oxygen, 0.04% and 21% of the total, respectively.)

Obligate anaerobe Obligate anaerobes are microorganisms killed by normal Atmosphere of Earth, atmospheric concentrations of oxygen (20.95% O2). Oxygen tolerance varies between species, with some species capable of surviving in up to 8% oxygen, while others lose vi ...

s are microbes that will die in the presence of oxygen without respect to the concentration of carbon dioxide in their environment, and typically acquire energy through anaerobic respiration or

fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

. In 2004, a capnophilic bacterium was characterized that appears to require carbon dioxide. This organism, ''Mannheimia succiniciproducens'', has a unique metabolism involving

carbon fixation Biological carbon fixation, or сarbon assimilation, is the Biological process, process by which living organisms convert Total inorganic carbon, inorganic carbon (particularly carbon dioxide, ) to Organic compound, organic compounds. These o ...

. While carbon fixation is common to most plant life on earth since it is the key initial step in the biosynthesis of complex carbon compounds during photosynthesis (the Calvin cycle), it is found in relatively few microorganisms and not found in animals. ''M. succiniciproducens'' can attach carbon dioxide to the three-carbon backbone of

phosphoenolpyruvate Phosphoenolpyruvate (2-phosphoenolpyruvate, PEP) is the carboxylic acid derived from the enol of pyruvate and a phosphate anion. It exists as an anion. PEP is an important intermediate in biochemistry. It has the high-energy phosphate, highest-e ...

, an endproduct in

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...

, to generate the four-carbon compound, oxaloacetic acid, an intermediate in the

Krebs cycle The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or TCA cycle (tricarboxylic acid cycle)—is a series of biochemical reactions that release the energy stored in nutrients through acetyl-CoA oxidation. The e ...

. Although ''M. succiniciproducens'' has most of the intermediates in the Krebs cycle, it appears incapable of

aerobic respiration Cellular respiration is the process of oxidizing biological fuels using an inorganic electron acceptor, such as oxygen, to drive production of adenosine triphosphate (ATP), which stores chemical energy in a biologically accessible form. Cellu ...

, instead using fumarate as a final electron acceptor.

Pathogenicity

There are currently at least two relatively well characterized capnophilic groups of microorganisms that include human pathogens. '' Campylobacter'' species can cause intestinal disorders. Other capnophilic pathogens occur in the Gram-negative ''Aggregatibacter'' spp. found in the mouth (e.g. ''

Aggregatibacter actinomycetemcomitans ''Aggregatibacter actinomycetemcomitans'' is a Gram-negative, facultative anaerobe, nonmotile bacterium that is often found in association with localized aggressive periodontitis, a severe infection of the periodontium. It is also suspected to b ...

''). These are a cause of aggressive juvenile

periodontitis Periodontal disease, also known as gum disease, is a set of inflammatory conditions affecting the tissues surrounding the teeth. In its early stage, called gingivitis, the gums become swollen and red and may bleed. It is considered the main c ...

. However, capnophiles are also normal microbiota in some ruminants. ''M. succiniciproducens'', in particular, was isolated from a bovine rumen. Its unusual biochemistry and benign characteristics have attracted commercial interest.

References

{{Extremophile