Chemotrophs are organisms that obtain energy by the oxidation of electron donors in their environments. These molecules can be organic (chemoorganotrophs) or inorganic (chemolithotrophs). The chemotroph designation is in contrast to phototrophs, which utilize solar energy. Chemotrophs can be either autotrophic or heterotrophic. Chemotrophs are commonly found in ocean floors where sunlight cannot reach them because they are not dependent on solar energy. Ocean floors often contain underwater volcanos that can provide heat as a substitute for sunlight's warmth.
Chemoautotrophs (or chemotrophic autotroph) (Greek: Chemo (χημία) = chemical, auto (αὐτός) = self, troph (τροφιά) = nourishment), in addition to deriving energy from chemical reactions, synthesize all necessary organic compounds from carbon dioxide. Chemoautotrophs use inorganic energy sources such as hydrogen sulfide, elemental sulfur, ferrous iron, molecular hydrogen, and ammonia. Most chemoautotrophs are extremophiles, bacteria or archaea that live in hostile environments (such as deep sea vents) and are the primary producers in such ecosystems. Chemoautotrophs generally fall into several groups: methanogens, halophiles, sulfur oxidizers and reducers, nitrifiers, anammox bacteria, and thermoacidophiles. An example of one of these prokaryotes would be Sulfolobus. Chemolithotrophic growth can be dramatically fast, such as Hydrogenovibrio crunogenus with a doubling time around one hour.
The term "chemosynthesis", coined in 1897 by Wilhelm Pfeffer, originally was defined as the energy production by oxidation of inorganic substances in association with autotrophy - what would be named today as chemolithoautotrophy. Later, the term would include also the chemoorganoautotrophy, that is, it can be seen as a synonym of chemoautotrophy.
Chemoheterotrophs (or chemotrophic heterotrophs) (Gr: Chemo (χημία) = chemical, hetero (ἕτερος) = (an)other, troph (τροφιά) = nourishment) are unable to fix carbon to form their own organic compounds. Chemoheterotrophs can be chemolithoheterotrophs, utilizing inorganic energy sources such as sulfur or chemoorganoheterotrophs, utilizing organic energy sources such as carbohydrates, lipids, and proteins. Most animals and fungi are examples of chemoheterotrophs.
Iron- and manganese-oxidizing bacteria
In the deep oceans, iron-oxidizing bacteria derive their energy needs by oxidizing ferrous iron (Fe2+) to ferric iron (Fe3+). The electron conserved from this reaction reduces the respiratory chain and can be thus used in the synthesis of ATP by forward electron transport or NADH by reverse electron transport, replacing or augmenting traditional phototrophism.
- In general, iron-oxidizing bacteria can exist only in areas with high ferrous iron concentrations, such as new lava beds or areas of hydrothermal activity. Most of the ocean is devoid of ferrous iron, due to both the oxidative effect of dissolved oxygen in the water and the tendency of bacteria to take up the iron.
- Lava beds supply bacteria with ferrous iron straight from the Earth's mantle, but only newly formed igneous rocks have high enough levels of ferrous iron. In addition, because oxygen is necessary for the reaction, these bacteria are much more common in the upper ocean, where oxygen is more abundant.
- What is still unknown is how exactly iron bacteria extract iron from rock. It is accepted that some mechanism exists that eats away at the rock, perhaps through specialized enzymes or compounds that bring more FeO to the surface. It has been long debated about how much of the weathering of the rock is due to biotic components and how much can be attributed to abiotic components.
- Hydrothermal vents also release large quantities of dissolved iron into the deep ocean, allowing bacteria to survive. In addition, the high thermal gradient around vent systems means a wide variety of bacteria can coexist, each with its own specialized temperature niche.
- Regardless of the catalytic method used, chemoautotrophic bacteria provide a significant but frequently overlooked food source for deep sea ecosystems - which otherwise receive limited sunlight and organic nutrients.
Manganese-oxidizing bacteria also make use of igneous lava rocks in much the same way; by oxidizing manganous manganese (Mn2+) into manganic (Mn4+) manganese. Manganese is more scarce than iron oceanic crust, but is much easier for bacteria to extract from igneous glass. In addition, each manganese oxidation donates two electrons to the cell versus one for each iron oxidation, though the amount of ATP or NADH that can be synthesised in couple to these reactions varies with pH and specific reaction thermodynamics in terms of how much of a Gibbs free energy change there is during the oxidation reactions versus the energy change required for the formation of ATP or NADH, all of which vary with concentration, pH etc. Much still remains unknown about manganese-oxidizing bacteria because they have not been cultured and documented to any great extent.
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1. Katrina Edwards. Microbiology of a Sediment Pond and the Underlying Young, Cold, Hydrologically Active Ridge Flank. Woods Hole Oceanographic Institution.
2. Coupled Photochemical and Enzymatic Mn(II) Oxidation Pathways of a Planktonic Roseobacter-Like Bacterium Colleen M. Hansel and Chris A. Francis* Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115 Received 28 September 2005/ Accepted 17 February 2006