Mixotroph

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A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comprise more than half of all microscopic plankton.[1] There are two types of eukaryotic mixotrophs; those who have their own chloroplasts, and those with endosymbionts and others who require them through kleptoplasty or by enslaving the entire phototrophic cell.[2]

Possible combinations are photo- and chemotrophy, litho- and organotrophy, auto- and heterotrophy or other combinations of these. Mixotrophs can be either eukaryotic or prokaryotic,[3] they can take advantage of different environmental conditions.[4]

If a trophic mode is obligate, then it is always necessary for sustaining growth and maintenance; if facultative, it can be used as a supplemental source;[3] some organisms have incomplete Calvin cycles, so they are incapable of fixing carbon dioxide and must use organic carbon sources.

Types of mixotrophy[edit]

Organisms may employ mixotrophy obligately or facultatively.

  • Obligate mixotrophy: in order to support growth and maintenance, an organism must utilize both heterotrophic and autotrophic means.
  • Obligate autotrophy with facultative heterotrophy: Autotrophy alone is sufficient for growth and maintenance, but heterotrophy may be used as a supplementary strategy when autotrophic energy is not enough, for example, when light intensity is low.
  • Facultative autotrophy with obligate heterotrophy: Heterotrophy is sufficient for growth and maintenance, but autotrophy may be used to supplement, for example, when prey availability is very low.
  • Facultative mixotrophy: Maintenance and growth may be obtained by heterotrophic or autotrophic means alone, and mixotrophy is used only when necessary.[5]

In order to characterize the sub-domains within mixotrophy, several very similar categorization schemes have been suggested.

Consider the example of a marine protist with heterotrophic and photosynthetic capabilities: In the breakdown put forward by Jones,[6] there are four mixotrophic groups based on relative roles of phagotrophy and phototrophy.

  • A: Heterotrophy (phagotrophy) is the norm, and phototrophy is only used when prey concentrations are limiting.
  • B: Phototrophy is the dominant strategy, and phagotrophy is employed as a supplement when light is limiting.
  • C: Phototrophy results in substances for both growth and ingestion, phagotrophy is employed when light is limiting.
  • D: Phototrophy is most common nutrition type, phagotrophy only used during prolonged dark periods, when light is extremely limiting.

An alternative scheme by Stoeker[7] also takes into account the role of nutrients and growth factors, and includes mixotrophs that have a photosynthetic symbiont or who retain chloroplasts from their prey; this scheme characterizes mixotrophs by their efficiency.

  • Type 1: "Ideal mixotrophs" who utilize prey and sunlight equally well
  • Type 2: Supplement phototrophic activity with food consumption
  • Type 3: Primarily heterotrophic, use phototrophic activity during times of very low prey abundance.[8]

Examples[edit]

  • Paracoccus pantotrophus is a bacterium that can live chemoorganoheterotrophically, whereby a large variety of organic compounds can be metabolized. Also a facultative chemolithoautotrophic metabolism is possible, as seen in colorless sulfur bacteria (some Thiobacillus), whereby sulfur compounds such as hydrogen sulfide, elemental sulfur, or thiosulfate are oxidized to sulfate; the sulfur compounds serve as electron donors and are consumed to produce ATP. The carbon source for these organisms can be carbon dioxide (autotrophy) or organic carbon (heterotrophy).[9][10][11]
    Organoheterotrophy can occur under aerobic or under anaerobic conditions; lithoautotrophy takes place aerobically.[12][13]

Plants[edit]

Amongst plants, mixotrophy classically applies to carnivorous, hemi-parasitic and myco-heterotrophic species. However, this characterisation as mixotrophic could be extended to a higher number of clades as research demonstrates that organic forms of nitrogen and phosphorus — such as DNA, proteins, amino-acids or carbohydrates — are also part of the nutrient supplies of a number of plant species.[14]

See also[edit]

Notes[edit]

  1. ^ Beware the mixotrophs - they can destroy entire ecosystems 'in a matter of hours'
  2. ^ Microscopic body snatchers infest our oceans - Phys.org
  3. ^ a b Eiler A (December 2006). "Evidence for the Ubiquity of Mixotrophic Bacteria in the Upper Ocean: Implications and Consequences". Appl Environ Microbiol. 72 (12): 7431–7. doi:10.1128/AEM.01559-06. PMC 1694265. PMID 17028233.
  4. ^ Katechakis A, Stibor H (July 2006). "The mixotroph Ochromonas tuberculata may invade and suppress specialist phago- and phototroph plankton communities depending on nutrient conditions". Oecologia. 148 (4): 692–701. doi:10.1007/s00442-006-0413-4. PMID 16568278.
  5. ^ Schoonhoven, Erwin (January 19, 2000). "Ecophysiology of Mixotrophs" (PDF). Thesis.
  6. ^ Jones, H.J.L. (1997). "A classification of mixotrophic protists based on their behaviour". Freshwater Biology. 37: 35–43. doi:10.1046/j.1365-2427.1997.00138.x.
  7. ^ Stoecker, D.K. (1998). "Conceptual models of mixotrophy in planktonic protists and some ecological and evolutionary implications". European Journal of Protistology. 34 (3): 281–290. doi:10.1016/s0932-4739(98)80055-2.
  8. ^ Tarangkoon, Woraporn (29 April 2010). "Mixtrophic Protists among Marine Ciliates and Dinoflagellates: Distribution, Physiology and Ecology" (PDF). Thesis.
  9. ^ Libes, Susan M. (2009). Introduction to marine biogeochemistry (2 ed.). Academic Press. p. 192. ISBN 978-0-7637-5345-0.
  10. ^ Dworkin, Martin (2006). The Prokaryotes: Ecophysiology and biochemistry. 2 (3rd ed.). Springer. p. 988. ISBN 978-0-387-25492-0.
  11. ^ Lengeler, Joseph W.; Drews, Gerhart; Schlegel, Hans Günter (1999). Biology of the Prokaryotes. Georg Thieme Verlag. p. 238. ISBN 978-3-13-108411-8.
  12. ^ Bartosik D, Sochacka M, Baj J (July 2003). "Identification and Characterization of Transposable Elements of Paracoccus pantotrophus". J Bacteriol. 185 (13): 3753–63. doi:10.1128/JB.185.13.3753-3763.2003. PMC 161580. PMID 12813068.
  13. ^ Friedrich, Cornelius G.; et al. (2007). "Redox Control of Chemotrophic Sulfur Oxidation of Paracoccus pantotrophus". Microbial Sulfur Metabolism. Springer. pp. 139–150. PDF
  14. ^ Schmidt, Susanne; John A. Raven; Chanyarat Paungfoo-Lonhienne (2013). "The mixotrophic nature of photosynthetic plants". Functional Plant Biology. 40 (5): 425. doi:10.1071/FP13061. ISSN 1445-4408.

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