User:Chrishansen4/Photoheterotroph

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Photoheterotrophs (Gk: photo = light, hetero = (an)other, troph = nourishment) are heterotrophic phototrophs – that is, they are organisms that use light for energy, but cannot use carbon dioxide as their sole carbon source. Consequently, they use organic compounds from the environment to satisfy their carbon requirements; these compounds include carbohydrates, fatty acids, and alcohols. Examples of photoheterotrophic organisms include purple non-sulfur bacteria, green non-sulfur bacteria, and heliobacteria. These microorganisms are ubiquitous in aquatic habitats, occupy unique niche-spaces, and contribute to global biogeochemical cycling. Recent research has also indicated that the oriental hornet and some aphids may be able to use light to supplement their energy supply.

Ecology
Distribution and Niche Partitioning

Photoheterotrophs—either 1) cyanobacteria (i.e. facultative heterotrophs in nutrient-limited environments like Synechococcus and Prochlorococcus), 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 bacteriochlorophyll g pigments, or Gram-positive membrane—are found in various aquatic habitats including oceans, stratified lakes, rice fields, and environmental extremes.

In surface oceans, 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. As demonstrated in inoculation experiments, photoheterotrophy may provide these planktonic microbes competitive advantages 1) relative to chemoheterotrophs in oligotrophic (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). Furthermore, in Arctic oceans, AAP and PR photoheterotrophs are prominent in ice-covered regions during wintertime per light scarcity. 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.

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. 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), and anaerobic 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.

Heliobacteria are obligate anaerobes primarily located in rice fields, where low sulfide concentrations prevent competitive exclusion of purple/green sulfur bacteria. 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 as Chloroflexi and AAPs) within photosynthetic mats at environmental extremes (e.g., hot springs and hypersaline lagoons). Notably, temperature and pH drive anoxygenic phototroph community composition in Yellowstone National Park's geothermal features. In addition, various, light-dependent niches in the Great Salk Lake's hypersaline mats support phototrophic diversity as microbes optimize energy production and combat osmotic stress.

Biogeochemical Cycling

Photoheterotrophs influence global carbon cycling by assimilating dissolved organic carbon (DOC). Therefore, when harvesting light-energy, carbon is maintained in the microbial 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 to photooxidation, UV light, and oxygenic photosynthesis) lead to overestimated aquatic CO2 emissions. For example, a 15.2% decrease in community 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.