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Chemotrophs are organisms that obtain energy by the oxidation of electron donating molecules in their environments. These molecules can be organic (organotrophs) or inorganic (lithotrophs). The chemotroph designation is in contrast to phototrophs which utilize solar energy. Chemotrophs can be either autotrophic or heterotrophic.
- Chemoautotrophs (or chemotrophic autotroph), (Gk: Chemo = chemical, auto = self, troph = nourishment) in addition to deriving energy from chemical reactions, synthesize all necessary organic compounds from carbon dioxide. Chemoautotrophs generally only use inorganic energy sources. Most are bacteria or archaea that live in hostile environments such as deep sea vents and are the primary producers in such ecosystems. Evolutionary scientists believe that the first organisms to inhabit Earth were chemoautotrophs that produced oxygen as a by-product and later evolved into both aerobic, animal-like organisms and photosynthetic, plant-like organisms. Chemoautotrophs generally fall into several groups: methanogens, halophiles, sulfur reducers, nitrifiers, anammoxbacteria and thermoacidophiles.
- Chemoheterotrophs (or chemotrophic heterotrophs) (Gk: Chemo = chemical, hetero = (an)other, troph = nourishment) must ingest organic building blocks that they are incapable of creating on their own. Most chemoheterotrophs derive energy from organic molecules like glucose.
Iron and manganese oxidizing bacteria
In the deep oceans, iron oxidizing bacteria derive their energy needs by oxidizing Iron II to Iron III. The extra electron obtained from this reaction powers the cells, replacing or augmenting traditional phototrophism.
- In general, iron oxidizing bacteria can only exist in areas with high iron concentrations - such as new lava beds or areas of hydrothermal activity (where there is dissolved Fe). Most of the ocean is devoid of iron, due to both the oxidative effect of dissolved oxygen in the water and the tendency of prokaryotes to uptake the iron.
- Lava beds supply bacteria with iron straight from the Earth's mantle, but only newly formed igneous rocks have high enough levels of unoxidized iron. In addition, since 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 though is how exactly iron bacteria extract the iron out of the rock. It is accepted that some mechanism exists which eats away at the rock, perhaps through specialized enzymes or compounds which bring more FeO to the surface. It has been long debated about how much of the weathering of the rock is due to biotic and how much can be attributed to abiotic processes.
- 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 Mn2+ into Mn4+. Manganese is much rarer than iron in oceanic crust, but is much easier for bacteria to extract from the igneous glass. In addition, each manganese oxidation yields around twice the energy as an iron oxidation due to the gain of twice the number of electrons. Much still remains unknown about manganese oxidizing bacteria because they have not been cultured and documented to any great extent.
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