Radiotrophic fungus
Encyclopedia
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
to convert gamma radiation into chemical energy for growth. This proposed mechanism may be similar to anabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
pathways for the synthesis
Biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
of reduced organic carbon (e.g., carbohydrates) in phototroph
Phototroph
Phototrophs are the organisms that carry out photosynthesis to acquire energy. They use the energy from sunlight to convert carbon dioxide and water into organic material to be utilized in cellular functions such as biosynthesis and respiration.Most phototrophs are autotrophs, also known as...
ic organisms, which capture photons from visible light with pigments such as chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
whose energy is then used in photolysis of water to generate usable chemical energy (as ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
) in photophosphorylation
Photophosphorylation
The production of ATP using the energy of sunlight is called photophosphorylation. Only two sources of energy are available to living organisms: sunlight and reduction-oxidation reactions...
of photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. However, whether melanin-containing fungi employ a similar multi-step pathway as photosynthesis, or some chemosynthesis
Chemosynthesis
In biochemistry, chemosynthesis is the biological conversion of one or more carbon molecules and nutrients into organic matter using the oxidation of inorganic molecules or methane as a source of energy, rather than sunlight, as in photosynthesis...
pathways, is unknown.
These were first discovered in 2007 as black molds growing inside and around the Chernobyl Nuclear Power Plant
Chernobyl Nuclear Power Plant
The Chernobyl Nuclear Power Plant or Chornobyl Nuclear Power Plant is a decommissioned nuclear power station near the city of Pripyat, Ukraine, northwest of the city of Chernobyl, from the Ukraine–Belarus border, and about north of Kiev. Reactor 4 was the site of the Chernobyl disaster in...
. Research at the Albert Einstein College of Medicine
Albert Einstein College of Medicine
Albert Einstein College of Medicine is a graduate school of Yeshiva University. It is a not-for-profit, private, nonsectarian medical school located on the Jack and Pearl Resnick Campus in the Morris Park neighborhood of the borough of the Bronx of New York City...
showed that three melanin-containing fungi, Cladosporium
Cladosporium
Cladosporium is a genus of fungi including some of the most common indoor and outdoor molds. Species produce olive-green to brown or black colonies, and have dark-pigmented conidia that are formed in simple or branching chains....
sphaerospermum, Wangiella dermatitidis, and Cryptococcus neoformans
Cryptococcus neoformans
Cryptococcus neoformans is an encapsulated yeast that can live in both plants and animals. Its teleomorph is Filobasidiella neoformans, a filamentous fungus belonging to the class Tremellomycetes. It is often found in pigeon excrement....
, increased in biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
and accumulated acetate
Acetate
An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
faster in an environment in which the radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
level was 500 times higher than in the normal environment. Exposure of C. neoformans cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
to these radiation levels rapidly (within 20–40 minutes of exposure) altered the chemical properties of its melanin and increased melanin-mediated rates of electron transfer (measured as reduction of ferricyanide
Ferricyanide
Ferricyanide is the anion [Fe6]3−. It is also called hexacyanoferrate and in rare, but systematic nomenclature, hexacyanidoferrate...
by NADH) 3 to 4-fold compared with unexposed cells. Similar effects on melanin electron-transport capability were observed by the authors after exposure to non-ionizing radiation, suggesting that melanotic fungi might also be able to use light or heat radiation for growth.
However, melanization may come at some metabolic cost to the fungal cells: in the absence of radiation, some non-melanized fungi (that had been mutated in the melanin pathway) grew faster than their melanized counterparts. Limited uptake of nutrients due to the melanin molecules in the fungal cell wall or toxic intermediates formed in melanin biosynthesis have been suggested to contribute to this phenomenon. It is consistent with the observation that despite being capable of producing melanin, many fungi do not synthesize melanin constitutively (i.e., all the time), but often only in response to external stimuli or at different stages of their development. The exact biochemical processes in the suggested melanin-based synthesis of organic compounds or other metabolite
Metabolite
Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction. Alcohol is an example of a primary metabolite produced in large-scale by industrial...
s for fungal growth, including the chemical intermediates (such as native electron donor and acceptor molecules) in the fungal cell and the location and chemical products of this process, are unknown.
See also
- Chernobyl disasterChernobyl disasterThe Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...
(section: Flora and fauna)