Heliobacteria are a unique subset of

prokaryotic A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' ...

bacteria that process light for energy. Distinguishable from other phototrophic bacteria, they utilize a unique photosynthetic pigment, bacteriochlorophyll ''g'' and are the only known

Gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain is ...

phototroph. They are a key player in

symbiotic Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...

nitrogen fixation Nitrogen fixation is a chemical process by which molecular dinitrogen () is converted into ammonia (). It occurs both biologically and abiological nitrogen fixation, abiologically in chemical industry, chemical industries. Biological nitrogen ...

alongside plants, and use a type I reaction center like green-sulfur bacteria.

RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...

trees place the heliobacteria among the

Bacillota The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have Gram-positive cell wall structure. They have round cells, called cocci (singular coccus), or rod-like forms (bacillus). A few Bacillota, such as '' Megasphaera'', ...

. They have no outer membrane and like certain other Bacillota (

Clostridia The Clostridia are a highly polyphyletic class of Bacillota, including '' Clostridium'' and other similar genera. They are distinguished from the Bacilli by lacking aerobic respiration. They are obligate anaerobes and oxygen is toxic to them ...

), they form heat-resistant

endospore An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria in the phylum Bacillota. The name "endospore" is suggestive of a spore or seed-like form (''endo'' means 'within'), but it is not a true spore (i.e., not ...

s, which contain high levels of

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

and dipicolinic acid. Heliobacteria are the only Bacillota known to be phototrophic.

Metabolism

The heliobacteria are phototrophic: they convert light energy into chemical energy using a type I reaction center. The primary

pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

involved is bacteriochlorophyll ''g'', which is unique to the group and has a unique

absorption spectrum Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, ...

; this gives the heliobacteria their own environmental niche. Phototrophic processes take place at the

cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

, which does not form folds or compartments as it does in

purple bacteria Purple bacteria or purple photosynthetic bacteria are Gram-negative proteobacteria that are phototrophic, capable of producing their own food via photosynthesis. They are pigmented with bacteriochlorophyll ''a'' or ''b'', together with various ...

. Though heliobacteria are phototrophic, they can create energy without light using pyruvate fermentation, which generates significantly less energy than it could with light. Heliobacteria are

photoheterotroph Photoheterotrophs (''Greek language, Gk'': ''photo'' = light, ''hetero'' = (an)other, ''troph'' = nourishment) are heterotrophic phototrophs—that is, they are organisms that use light for energy, but cannot use carbon dioxide as their sole carbon ...

ic, requiring organic carbon sources, and they are exclusively

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Adhesive#Anaerobic, Anaerobic ad ...

. Bacteriochlorophyll ''g'' is inactivated by the presence of

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

, making them

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 (they cannot survive in aerobic conditions). Heliobacteria have been found in

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

hot springs A hot spring, hydrothermal spring, or geothermal spring is a Spring (hydrology), spring produced by the emergence of Geothermal activity, geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow ...

soda lake A soda lake or alkaline lake is a lake on the strongly base (chemistry), basic side of neutrality, typically with a pH, pH value between 9 and 12. They are characterized by high concentrations of carbonate salts, typically sodium carbonate (and ...

s and are common in the waterlogged soils of paddy fields. They are avid nitrogen fixers, so are probably important in the fertility of paddy fields. Heliobacteria are mainly terrestrial phototrophs, contrary to the multitudes of others that are aquatic, and often form mutualistic relationships with the plants near them.

Taxonomy

Heliobacteria should not be confused with ''

Helicobacter ''Helicobacter'' is a genus of gram-negative bacteria possessing a characteristic helical shape. They were initially considered to be members of the genus '' Campylobacter'', but in 1989, Goodwin ''et al.'' published sufficient reasons to justi ...

'', which is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of bacteria with quite different characteristics. The currently accepted taxonomy is based on the

List of Prokaryotic names with Standing in Nomenclature List of Prokaryotic names with Standing in Nomenclature (LPSN) is an online database that maintains information on the naming and taxonomy of prokaryotes, following the taxonomy requirements and rulings of the International Code of Nomenclatu ...

(LPSN) and

National Center for Biotechnology Information The National Center for Biotechnology Information (NCBI) is part of the National Library of Medicine (NLM), a branch of the National Institutes of Health (NIH). It is approved and funded by the government of the United States. The NCBI is lo ...

(NCBI).

Metabolism

Taxonomy

See also

References

Further reading