Organism that can use a mix of different sources of energy and carbon
Amixotroph is an organism that uses a mix of differentsources of energy and carbon, instead of having a single trophic mode. Mixotrophs are situated somewhere on the continuum from completeautotrophy to completeheterotrophy. It is estimated that mixotrophs comprise more than half of all microscopicplankton.[1] There are two types ofeukaryotic mixotrophs. There are those with their ownchloroplasts – including those withendosymbionts providing the chloroplasts. And there are those that acquire them throughkleptoplasty, or throughsymbiotic associations with prey, or through 'enslavement' of the prey's organelles.[2]
A given trophic mode of a mixotroph organism is calledobligate when it is indispensable for its growth and maintenance; a trophic mode isfacultative when used as a supplemental source.[3] Some organisms have incompleteCalvin cycles, so that they are incapable of fixing carbon dioxide and must useorganic carbon sources.
Organisms may employ mixotrophyobligately orfacultatively.
Obligate mixotrophy: 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]
A mixotrophic plant using mycorrhizal fungi to obtain photosynthesis products from other plants
Amongst plants, mixotrophy classically applies tocarnivorous,hemi-parasitic andmyco-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.[6]
Mycoheterotrophic plants form symbiotic relationships withmycorrhizal fungi, which provide them with organic carbon and nutrients from nearby photosynthetic plants or soil. They often lack chlorophyll or have reduced photosynthetic capacity. An example isIndian pipe, a white, non-photosynthetic plant that relies heavily on fungal networks for nutrients.Pinesap also taps into fungal networks for sustenance, similar to Indian pipe. Certain orchids, such asCorallorhiza, depend on fungi for carbon and nutrients while developing photosynthetic capabilities (especially in their early stages).
The leaf of a carnivorous plant,Drosera capensis, bending in response to the trapping of an insect The floating fernAzolla filiculoides hosts a nitrogen-fixing cyanobacteria.
Hemiparasitic plants are partially parasitic, attaching to the roots or stems of host plants to extract water, nutrients, or organic compounds while still performing photosynthesis. Examples aremistletoe (absorbs water and nutrients from host trees but also photosynthesizes),Indian paintbrush (connects to the roots of other plants for nutrients while maintaining photosynthetic leaves), andYellow rattle (a root parasite that supplements its nutrition by tapping into host plants).
Someepiphytic plants, which are plants that grow on other plants, absorb organic matter, such as decaying debris or animal waste, through specialized structures while still photosynthesizing. For example, somebromeliads have tank-like leaf structures that collect water and organic debris, absorbing nutrients through their leaves. Also, some epiphytic orchids absorb nutrients from organic matter caught in their aerial roots.
Some plants incorporatealgae orcyanobacteria, which provide photosynthetically derived carbon, while the plant also absorbs external nutrients. For example,Azolla filiculoides, is a floating fern that hosts the nitrogen-fixingcyanobacteriaAnabaena in its leaves, supplementing nutrient intake while photosynthesizing. This has led to the plant being dubbed a "super-plant", as it can readily colonise areas of freshwater, and grow at great speed - doubling its biomass in as little as 1.9 days.[9]
Mixotrophy is less common among animals than among plants and microbes, but there are many examples of mixotrophicinvertebrates and at least one example of a mixotrophicvertebrate.
The spotted salamander,Ambystoma maculatum, also hosts microalgae within its cells. Its embryos have been found to havesymbiotic algae living inside them,[10] the only known example of vertebrate cells hosting anendosymbiont microbe (unless mitochondria is considered).[11][12]
Reef-buildingcorals (Scleractinia), like many othercnidarians (e.g. jellyfish, anemones), host endosymbioticmicroalgae within their cells, thus making them mixotrophs.
TheOriental hornet,Vespa orientalis, can obtain energy from sunlight absorbed by its cuticle.[14] It thus contrasts with the other animals listed here, which are mixotrophic with the help of endosymbionts.
TheLeaf sheep,Costasiella kuroshimae, retains chloroplasts from algae it consumes so it can supplement its diet with photosynthesis viakleptoplasty
Zooxanthellae is a photosynthetic algae that lives inside hosts likecoral.
Paracoccus pantotrophus is a bacterium that can live chemoorganoheterotrophically, whereby many organic compounds can be metabolized. Also, a facultativechemolithoautotrophic metabolism is possible, as seen in colorless sulfur bacteria (someThiobacillus), whereby sulfur compounds such ashydrogen sulfide, elementalsulfur, orthiosulfate are oxidized to sulfate. The sulfur compounds serve aselectron donors and are consumed to produceATP. The carbon source for these organisms can be carbon dioxide (autotrophy) or organic carbon (heterotrophy).[16][17][18] Organoheterotrophy can occur underaerobic or underanaerobic conditions; lithoautotrophy takes place aerobically.[19][20]
Traditional classification of mixotrophic protists
In this diagram, types in open boxes as proposed by Stoecker [21] have been aligned against groups in grey boxes as proposed by Jones.[22][23] DIN = dissolved inorganic nutrients
Several categorization schemes have been suggested to characterize sub-domains within mixotrophy.
Consider the example of a marine protist with heterotrophic and photosynthetic capabilities:In the breakdown put forward by Jones,[22] 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[21] 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" that use 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.[24]
Another scheme, proposed by Mitraet al., specifically classifies marine planktonic mixotrophs so that mixotrophy can be included in ecosystem modeling.[23] This scheme classified organisms as:
Constitutive mixotrophs (CMs): phagotrophic organisms that are inherently able also to photosynthesize
Non-constitutive mixotrophs (NCMs): phagotrophic organisms that must ingest prey to attain the ability to photosynthesize. NCMs are further broken down into:
Specific non-constitutive mixotrophs (SNCMs), which only gain the ability to photosynthesize from a specific prey item (either by retaining plastids only in kleptoplastidy or by retaining whole prey cells in endosymbiosis)
General non-constitutive mixotrophs (GNCM), which can gain the ability to photosynthesize from a variety of prey items
Pathways used by Mitraet al. to derive functional groups of planktonic protists [23]
Levels in complexity among those different types of protists, according to Mitraet al.[23] (A) phagotrophic (no phototrophy); (B) phototrophic (no phagotrophy); (C) constitutive mixotroph, with innate capacity for phototrophy; (D) generalist non-constitutive mixotroph acquiring photosystems from different phototrophic prey; (E) specialist non-constitutive mixotroph acquiring plastids from a specific prey type; (F) specialist non-constitutive mixotroph acquiring photosystems from endosymbionts. DIM = dissolved inorganic material (ammonium, phosphate etc.).DOM =dissolved organic material
The red arrows indicate additional trophic interactions that can occur when mixotrophy is present
Mixotrophs are especially common in marine environments, where the levels of energy from the sun and nutrients in the water can vary greatly. For example, in nutrient-poor (oligotrophic) waters, mixotrophicphytoplankton supplement their diet by consuming bacteria.[25][26]
The effects of mixotrophy on organic and inorganiccarbon pools introduce a metabolic plasticity which blurs the lines betweenproducers andconsumers.[27] Prior to the discovery of mixotrophs, it was thought that only organisms withchloroplasts were capable of photosynthesis and vice versa. This additional functional group of plankton, capable of bothphototrophy andphagotrophy, provides a further boost in the biomass and energy transfer to highertrophic levels.[28]
Arctic food web with mixotrophy: Yellow arrows indicate flow of energy from the sun to photosynthetic organisms (autotrophs and mixotrophs).[29] Gray arrows indicate flow of carbon to heterotrophs; Green arrows indicate major pathways of carbon flow to or from mixotrophs. HCIL, Strictly heterotrophicciliates; MCIL, Mixotrophic ciliates; HNF, Heterotrophicnanoflagellates; DOC,Dissolved organic carbon; HDIN, Heterotrophicdinoflagellates.
^Leles S G et al, (2017). Oceanic protists with different forms of acquired phototrophy display contrasting biogeographies and abundance,Proceedings of the Royal Society B: Biological Sciences.
^abStoecker, Diane 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.
Troost TA, Kooi BW, Kooijman SA (February 2005). "When do mixotrophs specialize? Adaptive dynamics theory applied to a dynamic energy budget model".Math Biosci.193 (2):159–82.doi:10.1016/j.mbs.2004.06.010.PMID15748728.
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