Photoheterotroph

[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—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.

[14] 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).

[17] 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.

[18] 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.

[20] Heliobacteria are obligate anaerobes primarily located in rice fields, where low sulfide concentrations prevent competitive exclusion of purple/green sulfur bacteria.

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).

[12] In addition, various, light-dependent niches in the Great Salt Lake's hypersaline mats support phototrophic diversity as microbes optimize energy production and combat osmotic stress.

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.