Photoheterotrophs (Gk:photo = light,hetero = (an)other,troph = nourishment) areheterotrophicphototrophs—that is, they are organisms that use light for energy, but cannot usecarbon dioxide as their sole carbon source. Consequently, they use organic compounds from the environment to satisfy their carbon requirements; these compounds includecarbohydrates,fatty acids, andalcohols. Examples of photoheterotrophic organisms includepurple non-sulfur bacteria,green non-sulfur bacteria, andheliobacteria.[1] These microorganisms are ubiquitous in aquatic habitats, occupy unique niche-spaces, and contribute to global biogeochemical cycling. Recent research has also indicated that theoriental hornet and someaphids may be able to use light to supplement their energy supply.[2] Some recent research has even found hints of photoheterotrophy in a feweukaryotes, though it's still being studied.
Studies have shown that mammalianmitochondria can also capture light and synthesize ATP when mixed withpheophorbide, a light-capturing metabolite of chlorophyll.[3] Research demonstrated that the same metabolite when fed to the wormCaenorhabditis elegans leads to increase in ATP synthesis upon light exposure, along with an increase in life span.[4]
Furthermore, inoculation experiments suggest that mixotrophicOchromonas danica (i.e., Golden algae)—and comparable eukaryotes—favor photoheterotrophy in oligotrophic (i.e., nutrient-limited) aquatic habitats.[5] This preference may increase energy-use efficiency and growth by reducing investment in inorganic carbon fixation (e.g., production of autotrophic machineries such as RuBisCo and PSII).
Photoheterotrophs get energy from light and carbon from organic substances likecarbohydrates,fatty acids, oralcohols.
They're different from photoautotrophs, which use carbon dioxide for carbon, and fromchemoheterotrophs, which get both energy and carbon from organic compounds. Photoheterotrophy tends to be useful in places where light is available but carbon dioxide is in short supply—like some parts of the ocean or shallow water environments.
Photoheterotrophs generateATP using light, in one of two ways:[6][7] they use abacteriochlorophyll-based reaction center, or they use abacteriorhodopsin. Thechlorophyll-based mechanism is similar to that used in photosynthesis, where light excites the molecules in a reaction center and causes a flow of electrons through anelectron transport chain (ETC). This flow of electrons through the proteins causes hydrogen ions (protons) to be pumped across a membrane. The energy stored in thisproton gradient is used to driveATP synthesis. Unlike inphotoautotrophs, the electrons flow only in a cyclic pathway: electrons released from the reaction center flow through the ETC and return to the reaction center. They are not utilized toreduce any organic compounds.Purple non-sulfur bacteria,green non-sulfur bacteria, andheliobacteria are examples of bacteria that carry out this scheme of photoheterotrophy.
Other organisms, includinghalobacteria,flavobacteria,[8] andvibrios,[9] have purple-rhodopsin-basedproton pumps that supplement their energy supply. Thearchaeal version is calledbacteriorhodopsin, while theeubacterial version is calledproteorhodopsin. The pump consists of a single protein bound to aVitamin A derivative:retinal. The pump may have accessory pigments (e.g.,carotenoids) associated with the protein. When light is absorbed by the retinal molecule, the moleculeisomerises. This drives the protein to change shape and pump a proton across the membrane. The proton gradient can then be used to generate ATP, transport solutes across the membrane, or drive aflagellar motor. One particular flavobacterium cannot reduce carbon dioxide using light, but uses the energy from its rhodopsin system tofix carbon dioxide throughanaplerotic fixation.[8] The flavobacterium is still aheterotroph as it needs reduced carbon compounds to live and cannot subsist on only light and CO2. It cannot carry out reactions in the form of
where H2D may be water,H2S or another compound/compounds providing the reducing electrons and protons; the 2D + H2O pair represents an oxidized form.
However, it can fix carbon in reactions like:
where malate or other useful molecules are otherwise obtained by breaking down other compounds by
This method of carbon fixation is useful when reduced carbon compounds are scarce and cannot be wasted as CO2 during interconversions, but energy is plentiful in the form of sunlight.
Organisms that are known to be photoheterotrophic include:
Some other organisms—though not true photoheterotrophs—have interesting features that might be similar. For example, theOriental hornet can absorb light with pigments in its body and may use that light for energy. Certainaphids have also been shown to make light-sensitivecarotenoids that could help them get energy from sunlight. A few recent studies even suggest thatyeast cells can be modified to respond to light by inserting genes that allow them to userhodopsin.
Photoheterotrophs are found in many different water-based environments like oceans, lakes, and even rice paddies. They tend to live near the surface of the water, where there's enough light but not much carbon dioxide.Photoheterotrophs—either 1)cyanobacteria (i.e. facultative heterotrophs in nutrient-limited environments likeSynechococcus andProchlorococcus), 2)aerobic anoxygenic photoheterotrophic bacteria (AAP; employing bacteriochlorophyll-based reaction centers), 3)proteorhodopsin (PR)-containing bacteria and archaea, and 4) heliobacteria (i.e., the only phototroph with bacteriochlorophyllg pigments, or Gram-positive membrane) are found in various aquatic habitats including oceans, stratified lakes, rice fields, and environmental extremes.[12][13][14][15]
In oceans' photic zones, up to 10% of bacterial cells are capable of AAP, whereas greater than 50% of net marine microorganisms house PR—reaching up to 90% in coastal biomes.[16] As demonstrated in inoculation experiments, photoheterotrophy may provide these planktonic microbes competitive advantages 1) relative to chemoheterotrophs inoligotrophic (i.e., nutrient-poor) environments via increased nutrient use-efficiency (i.e., organic carbon fuels biosynthesis, excessively, versus energy production) and 2) by eliminating investment in physiologically costly autotrophic enzymes/complexes (RuBisCo and PSII).[17][18] Furthermore, in Arctic oceans, AAP and PR photoheterotrophs are prominent in ice-covered regions during wintertime per light scarcity.[19] Lastly, seasonal turnover has been observed in marine AAPs as ecotypes (i.e., genetically similar taxa with differing functional trait and/or environmental preferences) segregate into temporal niches.[20]
In stratified (i.e., euxinic) lakes, photoheterotrophs—alongside other anoxygenic phototrophs (e.g., purple/green sulfur bacteria fixing carbon dioxide via electron donors such as ferrous iron, sulfide, and hydrogen gas)—often occupy the chemocline in the water column and/or sediments.[21] In this zone, dissolved oxygen is reduced, light is limited to long wavelengths (e.g., red and infrared) left-over by oxygenic phototrophs (e.g., cyanobacteria), andanaerobic metabolisms (i.e., those occurring in the absence of oxygen) begin introducing sulfide and bioavailable nutrients (e.g., organic carbon, phosphate, and ammonia) through upward diffusion.[22]
Heliobacteria are obligate anaerobes primarily located in rice fields, where low sulfide concentrations prevent competitive exclusion of purple/green sulfur bacteria.[23] These waterlogged environments may facilitate symbiotic relationships between heliobacteria and rice plants as fixed nitrogen—from the former—is exchanged for carbon-rich root exudates.
Observation studies have characterized photoheterotrophs (e.g., Green non-sulfur bacteria such asChloroflexi and AAPs) within photosynthetic mats at environmental extremes (e.g., hot springs and hypersaline lagoons).[14][24] Notably, temperature and pH drive anoxygenic phototroph community composition inYellowstone National Park's geothermal features.[14] In addition, various, light-dependent niches in theGreat Salt Lake's hypersaline mats support phototrophic diversity as microbes optimize energy production and combat osmotic stress.[24]
Photoheterotrophs influence global carbon cycling by assimilating dissolved organic carbon (DOC).[25][22] Therefore, when harvesting light-energy, carbon is maintained in themicrobial loop without corresponding respiration (i.e., carbon dioxide release to the atmosphere as DOC is oxidized to fuel energy production). This disconnect, the discovery of facultative photoheterotrophs (e.g., AAPs with flexible energy sources), and previous measurements taken in the dark (i.e., to avoid skewed oxygen consumption values due tophotooxidation, UV light, and oxygenic photosynthesis) lead to overestimated aquatic CO2 emissions. For example, a 15.2% decrease incommunity respiration was observed in Cep Lake, Czechia—alongside preferential glucose and pyruvate uptake—is attributed to facultative photoheterotrophs preferring light-energy during the daytime, given fitness benefits mentioned previously.[25]
Energy source Carbon source | Chemotroph | Phototroph |
|---|---|---|
| Autotroph | Chemoautotroph | Photoautotroph |
| Heterotroph | Chemoheterotroph | Photoheterotroph |
"Microbiology Online" (textbook). University of Wisconsin, Madison.[dead link]