Primary nutritional groups

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Primary nutritional groups are groups of organisms, divided in relation to the nutrition mode according to the sources of energy and carbon, needed for living, growth and reproduction. The sources of energy can be light and organic or inorganic compounds; the sources of carbon can be of organic or inorganic origin. 1

The terms aerobic respiration, anaerobic respiration and fermentation do not refer to primary nutritional groups, but simply reflect the different use of possible electron acceptors in particular organisms, such as O2 in aerobic respiration, or NO3-, SO42- or fumarate in anaerobic respiration, or various metabolic intermediates in fermentation. Because all ATP-generating steps in fermentation involve modifications of metabolic intermediates instead of the use of an electron transport chain fermentation is often referred to as substrate-level phosphorylation.

Primary sources of energy

Simplified flowchart for determining if an organism is an autotroph, heterotroph or a subtype.

Phototrophs: Light is absorbed in photo receptors and transformed into chemical energy.
Chemotrophs: Bond energy is released from a chemical compound.

The freed energy is stored as potential energy in ATP, carbohydrates, lipids or proteins. Eventually, the energy is used for life processes as moving, growth and reproduction.

Some bacteria can alternate phototrophy and chemotrophy, depending on availability of light.

Primary sources of reducing equivalents

Organotrophs: Organic compounds are used as electron donor.
Lithotrophs: Inorganic compounds are used as electron donor.

The electrons from reducing equivalents are needed by both, phototrophs and chemotrophs, to keep running reduction-oxidation reactions that transfer energy. The electron donors are taken up from the environment.

Organotrophic organisms are often also heterotrophic, using organic compounds as sources of electrons and carbon at the same time. Similarly, lithotrophic organisms are often also autotrophic, using inorganic sources of electrons and CO2 as inorganic carbon source.

Some lithotrophic bacteria can utilize diverse sources of electrons, depending on availability of possible donors.

Primary sources of carbon

Heterotrophs: Organic compounds are metabolized to get carbon for growth and development.
Autotrophs: Carbon dioxide (CO2) is used as source of carbon.

Energy and carbon

Yellow fungus
Classification of organisms based on their metabolism
Energy source sunlight photo-   -troph
Preformed molecules chemo-
Electron donor organic compound   organo-  
inorganic compound litho-
Carbon source organic compound   hetero-
inorganic compound auto-

A chemoorganoheterotrophic organism is one that requires organic substrates to get its carbon for growth and development, and that produces its energy from oxido-reduction of an organic compound. This group of organisms may be further subdivided according to what kind of organic substrate and compound they use. Decomposers are examples of Chemoorganoheterotrophs which obtain carbon and electron reactions from dead organic matter. Herbivores and carnivores are examples of organisms that obtain carbon and electron reactions from living organic matter.

Chemoorganotrophs are organisms which oxidize the chemical bonds in organic compounds as their energy source. Chemoorganotrophs also attain the carbon molecules that they need for cellular function from these organic compounds. The organic compounds that they oxidize include sugars (i.e. glucose), fats and proteins.2

All animals are chemoheterotrophs (meaning they oxidize chemical compounds as a source of energy and carbon), as are fungi, protozoa, and some bacteria. The important differentiation amongst this group is that chemoorganotrophs oxidize only organic compounds while chemolithotrophs instead use inorganic compounds as a source of energy.3

Primary Metabolism Table.

Energy
Source
Oxidizing
Donor
Source
Carbon source Name Examples
Sun Light
Photo-
Organic
-organo-
Organic
-heterotroph
Photoorganoheterotroph
Carbon dioxide
-autotroph
Photoorganoautotroph
Inorganic
-litho-*
Organic
-heterotroph
Photolithoheterotroph
Carbon dioxide
-autotroph
Photolithoautotroph Embryophyta, algae
Breaking
Chemical
Compounds
Chemo-
Organic
-organo-
Organic
-heterotroph
Chemoorganoheterotroph Metazoa, Fungi
Carbon dioxide
-autotroph
Chemoorganoautotroph
Inorganic
-litho-*
Organic
-heterotroph
Chemolithoheterotroph
Carbon dioxide
-autotroph
Chemolithoautotroph Nitrobacter4
  • Some authors use -hydro- when the source is water.

Mixotrophs

Some, usually unicellular, organisms can switch between different metabolic modes, for example between photoautotrophy, photoheterotrophy, and chemoheterotrophy in Chroococcales 5 Such mixotrophic organisms may dominate their habitat, due to their capability to use more resources than either photoautotrophic or organoheterotrophic organisms. 6

Examples

All sorts of combinations may exist in nature. For example most cyanobacteria are photoautotrophic, since they use light as an energy source, water as electron donor, and CO2 as a carbon source. Fungi are chemoorganotrophic since they use organic carbon as both an electron donor and carbon source. Eukaryotes are generally easy to categorise. All animals are heterotrophic, as are fungi. Plants are generally photoautotrophic. Some eukaryotic microorganisms, however, are not limited to just one nutritional mode. For example, some algae live photoautotrophically in the light, but shift to chemoorganotrophy in the dark. Even higher plants retained their ability to respire heterotrophically on the starch at night which had been synthesised phototrophically during the day.

Prokaryotes show a great diversity of nutritional categories. For example, purple sulfur bacteria and cyanobacteria are generally photoautotrophic whereas purple non-sulfur bacteria are photoorganotrophic. Some bacteria are limited to only one nutritional group, whereas others are facultative and switch from one mode to the other, depending on the nutrient sources available.

See also

References

  1. ^ Brock Biology of Microorganisms Definitions of metabolic strategies to obtain carbon and energy
  2. ^ Kenneth Todar (2009). "Todar's Online Textbook of Bacteriology". Nutrition and Growth of Bacteria. Retrieved 2014-04-19. 
  3. ^ Kelly, DP; Julie Mason, Ann Wood. Energy Metabolism in Chemolithotrophs. Springer. pp. 186–187. 
  4. ^ Yafremava LS, Wielgos M, Thomas S, Nasir A, Wang M, Mittenthal JE, Caetano-Anollés G: A general framework of persistence strategies for biological systems helps explain domains of life. Front Genet 2013; 4:16; p.8
  5. ^ R. Rippka Photoheterotrophy and chemoheterotrophy among unicellular blue-green algae Archives of Microbiology; Volume 87, Number 1 / March, 1972; doi:10.1007/BF00424781
  6. ^ Alexander Eiler Evidence for the Ubiquity of Mixotrophic Bacteria in the Upper Ocean: Implications and consequences Appl Environ Microbiol. 2006 December; 72(12): 7431–7437; doi:10.1128/AEM.01559-06.







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