Chemotroph

Chemotrophs are organisms that obtain energy by the oxidation of electron donors in their environments.[1] These molecules can be organic (chemoorganotrophs) or inorganic (chemolithotrophs). The chemotroph designation is in contrast to phototrophs, which use solar energy. Chemotrophs can be either autotrophic or heterotrophic. Chemotrophs are 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.

Blacksmoker in Atlantic Ocean
A black smoker in the Atlantic Ocean providing energy and nutrients

Chemoautotroph

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.[2][3]

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.[4][5]

Chemoheterotroph

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.[6][7][8][9] 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.

Flowchart

AutoHeteroTrophs flowchart
Flowchart to determine if a species is autotroph, heterotroph, or a subtype

See also

Notes

  1. ^ Chang, Kenneth (12 September 2016). "Visions of Life on Mars in Earth's Depths". The New York Times. Retrieved 12 September 2016.
  2. ^ Dobrinski, K. P. (2005). "The Carbon-Concentrating Mechanism of the Hydrothermal Vent Chemolithoautotroph Thiomicrospira crunogena". Journal of Bacteriology. 187 (16): 5761–5766. doi:10.1128/JB.187.16.5761-5766.2005. PMC 1196061. PMID 16077123.
  3. ^ Rich Boden, Kathleen M. Scott, J. Williams, S. Russel, K. Antonen, Alexander W. Rae, Lee P. Hutt (June 2017). "An evaluation of Thiomicrospira, Hydrogenovibrio and Thioalkalimicrobium: reclassification of four species of Thiomicrospira to each Thiomicrorhabdus gen. nov. and Hydrogenovibrio, and reclassification of all four species of Thioalkalimicrobium to Thiomicrospira". International Journal of Systematic and Evolutionary Microbiology. 67 (5): 1140–1151. doi:10.1099/ijsem.0.001855. PMID 28581925.CS1 maint: Multiple names: authors list (link)
  4. ^ Kelly, D. P., & Wood, A. P. (2006). The chemolithotrophic prokaryotes. In: The prokaryotes (pp. 441-456). Springer New York, [1].
  5. ^ Schlegel, H.G. (1975). Mechanisms of chemo-autotrophy. In: Marine ecology, Vol. 2, Part I (O. Kinne, ed.), pp. 9-60, [2].
  6. ^ Davis, Mackenzie Leo; et al. (2004). Principles of environmental engineering and science. 清华大学出版社. p. 133. ISBN 978-7-302-09724-2.
  7. ^ Lengeler, Joseph W.; Drews, Gerhart; Schlegel, Hans Günter (1999). Biology of the Prokaryotes. Georg Thieme Verlag. p. 238. ISBN 978-3-13-108411-8.
  8. ^ Dworkin, Martin (2006). The Prokaryotes: Ecophysiology and biochemistry (3rd ed.). Springer. p. 989. ISBN 978-0-387-25492-0.
  9. ^ Bergey, David Hendricks; Holt, John G. (1994). Bergey's manual of determinative bacteriology (9th ed.). Lippincott Williams & Wilkins. p. 427. ISBN 978-0-683-00603-2.

References

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

Cold seep

A cold seep (sometimes called a cold vent) is an area of the ocean floor where hydrogen sulfide, methane and other hydrocarbon-rich fluid seepage occurs, often in the form of a brine pool. Cold does not mean that the temperature of the seepage is lower than that of the surrounding sea water. On the contrary, its temperature is often slightly higher. The "cold" is relative to the very warm (at least 60 °C or 140 °F) conditions of a hydrothermal vent. Cold seeps constitute a biome supporting several endemic species.

Cold seeps develop unique topography over time, where reactions between methane and seawater create carbonate rock formations and reefs. These reactions may also be dependent on bacterial activity. Ikaite, a hydrous calcium carbonate, can be associated with oxidizing methane at cold seeps.

Decomposer

Decomposers are organisms that break down dead or decaying organisms, and in doing so, they carry out the natural process of decomposition. Like herbivores and predators, decomposers are heterotrophic, meaning that they use organic substrates to get their energy, carbon and nutrients for growth and development. While the terms decomposer and detritivore are often interchangeably used, detritivores must ingest and digest dead matter via internal processes while decomposers can directly absorb nutrients through chemical and biological processes hence breaking down matter without ingesting it. Thus, invertebrates such as earthworms, woodlice, and sea cucumbers are technically detritivores, not decomposers, since they must ingest nutrients and are unable to absorb them externally.

Extremotroph

An extremotroph (from Latin extremus meaning "extreme" and Greek troph (τροφ) meaning "food") is an organism that feeds on matter that is not typically considered to be food to most life on Earth. "These anthropocentric definitions that we make of extremophily and extremotrophy focus on a single environmental extreme but many extremophiles may fall into multiple categories, for example, organisms living inside hot rocks deep under the Earth's surface."

Methanogen

Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They are prokaryotic and belong to the domain of archaea. They are common in wetlands, where they are responsible for marsh gas, and in the digestive tracts of animals such as ruminants and humans, where they are responsible for the methane content of belching in ruminants and flatulence in humans. In marine sediments the biological production of methane, also termed methanogenesis, is generally confined to where sulfates are depleted, below the top layers. Moreover, methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments. Others are extremophiles, found in environments such as hot springs and submarine hydrothermal vents as well as in the "solid" rock of Earth's crust, kilometers below the surface.

Precambrian body plans

Until the late 1950s, the Precambrian era was not believed to have hosted multicellular organisms. However, with radiometric dating techniques, it has been found that fossils initially found in the Ediacara Hills in Southern Australia date back to the late Precambrian era. These fossils are body impressions of organisms shaped like disks, fronds and some with ribbon patterns that were most likely tentacles.

These are the earliest multicellular organisms in Earth's history, despite the fact that unicellularity had been around for a long time before that. The requirements for multicellularity were embedded in the genes of some of these cells, specifically choanoflagellates. These are thought to be the precursors for all multicellular organisms. They are highly related to sponges (Porifera), which are the simplest multicellular organisms.

In order to understand the transition to multicellularity during the Precambrian, it is important to look at the requirements for multicellularity—both biological and environmental.

Serpentinite

Serpentinite is a rock composed of one or more serpentine group minerals, the name originating from the similarity of the texture of the rock to that of the skin of a snake. Minerals in this group, which are rich in magnesium and water, light to dark green, greasy looking and slippery feeling, are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle. The mineral alteration is particularly important at the sea floor at tectonic plate boundaries.

Thermus aquaticus

Thermus aquaticus is a species of bacteria that can tolerate high temperatures, one of several thermophilic bacteria that belong to the Deinococcus–Thermus group. It is the source of the heat-resistant enzyme Taq DNA polymerase, one of the most important enzymes in molecular biology because of its use in the polymerase chain reaction (PCR) DNA amplification technique.

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