A mixotroph or bitroph is an organism that can use a mix of different sources of energy and carbon. Possible combinations are photo- and chemotrophy, litho- and organotrophy, auto- and heterotrophy or other combinations of these. Mixotrophs can be either eukaryotic or prokaryotic. They can take advantage of different environmental conditions.
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. Some organisms have incomplete Calvin cycles, so they are incapable of fixing carbon dioxide and must use organic carbon sources.
Types of Mixotrophy
Trophic modes can be expressed on a continuum, with complete heterotrophy at one end, autotrophy at the other, and mixotrophy between the two extremes. In order to characterize the sub-domains within mixotrophy, several very similar categorization schemes have been suggested. (Jones, 1994; Jones, 1997; Stoeker, 1998)
Consider the example of a marine protist with heterotrophic and photosynthetic capabilities: In the breakdown put forward by Harriet JL Jones, there are four mixotrophic groups based on relative roles of phagotrophy and phototrophy.
- A: Heterotrophy (phagotropy) 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 injestion, phagotrophy is employed when light is limiting.
- D: Phototrphy is most common nutrition type, phagotrophy only used during prolonged dark periods, when light is extremely limiting.
In Stoeker's mixotrophic scheme, the role of nutrients and growth factors are included, as are mixotrophs who have a photosynthetic symbiont or who retain chloroplasts from their prey.
- Type I: "Ideal Mixotrophs" who utilize prey and sunlight equally well
- Type 2: Supplement phototrophic activity with food consumation
- Type 3: Primarily heterotrophic, use phototrophic activity during times of very low prey abundance.
- 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).
Organoheterotrophy can occur under aerobic or under anaerobic conditions; lithoautotrophy takes place aerobically.
- Many examples of the genus Euglena.
- Oriental hornet Vespa orientalis
- Venus Flytrap Dionaea muscipula
Amongst plants, mixotrophy classically applies to carnivorous, hemi-parasitic and partially hetero-mycotrophic species. However, this could be extended to a higher number of clades as research proves that organic forms of nitrogen and phosphorus such as DNA, proteins, amino-acids or carbohydrates also are part of a number of plants' nutrient supplies.
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