Table 1.

Distinguishing features of the various types of anoxygenic phototrophic bacteria.

GROUP	ORGANISM	REACTION CENTRE	PHOTOSYNTHETIC PIGMENTS	ABSORPTION WAVELENGTH	ELECTRON DONOR	TYPE OF METABOLISM	ENVIRONMENTS INHABITED	SPECIAL CHARACTERISTICS
Purple sulfur bacteria	Belong to the Gammaproteobacteria class and classified under Chromatiaceae and the Ectothiorhodospiraceae families [13]	Type II [22,23]	BChl a/b and carotenoid pigments such as spirilloxanthin, spheroidene, lycopene, and rhodopsin [22,24]	For bacteriochlorophyll a ∼ 800/ 815−960 nm. and bacteriochlorophyll b containing species have a range of 835−850 and 1010−1040 nm [23]	Inorganic sulfur compounds like sulfide, hydrogen and thiosulfate for photoautotrophic growth [22,24]	Mainly photoautotroph. Chromatiaceae can grow well under photoautotrophic, photoheterotrophic, chemoautotrophic and chemoheterotrophic conditions [13]	Found in alkaline and saline environments [13]	During their growth on thiosulfate, elemental sulfur or polysulfides, bacteria from the Chromatiaceae family can form sulfur globules within their cell wall whereas organisms that are a part of the Ectothiorhodospiraceae family accumulate sulfur globules extracellularly [25]
Purple non-sulfur bacteria	Belong to the Alpha and Betaproreobacteria class	Type II [22,24]	BChl a/b and carotenoid pigments such as spirilloxanthin, spheroidene, lycopene, and rhodopsin [22,23,24]	For bacteriochlorophyll a ∼ 800/ 815−960 nm. Whereas, bacteriochlorophyll b containing species have a range of 835−850 and 1010−1040 nm [23]	Photoautotrophic species use reduced sulfur and hydrogen compounds [13]	Mainly photoheterotrophs [22,24]. However, some species can grow photoautotrophically [13]	Thrive in conditions having low sulfide concentrations and can be found in sewage and waste lagoons [26,27]	Can grow in environments with low sulfide levels and during photoautotrophic growth, the reductive pentose phosphate cycle (Calvin cycle) is the path used for CO2 fixation [23] S0 formed by oxidation of sulfide is deposited extracellularly [28]
Green sulfur bacteria	These bacteria are from the Chlorobiaceae family [13]	Type I [22,29]	BChl c, d and e organized into chlorosomes (large light-harvesting organelles) [22]. Carotenoid pigments such as chlorobactene, γ-carotene isorenieratene and derivatives (OH- chlorobactene and β-isorenieratene) [30,31]	Bchl c 745–755 nm, Bchl d 715–745 nm and Bchl e 710–725 nm [22,29]	Reduced sulfur compounds such as sulfide and thiosulfate [13]	Obligate phototrophs that grow only in anoxygenic conditions and in the light by utilizing inorganic electron donors [13]	Exist as colored blooms in hyper saline water, can be found in marine lagoons, hypersaline sediments, freshwater lakes and even in marine sediments [32,33]	They play an important role in the biological sulfur cycle and deposit sulfur globules extracellularly; and use the reversed tricarboxylic acid (TCA) cycle to fix carbon dioxide [29].
Filamentous anoxygenic phototrophic bacteria	Chloroflexi phylum consists of primarily filamentous organisms and consist of the following three families: Chloroflexaceae, Oscillochloridaceae and Roseiflexaceae [19]	Type II [22]	BChl a or a and c/d [19]. Different carotenoid pigments such as carotene, β-carotene, OH-γ-caroteneglucoside ester, keto-OH-γ- carotene, keto- myxocoxanthin, myxobactene, methoxy-keto-myxocoxanthin, keto-myxocoxanthin glycoside ester are present depending on the family [34,35,36]	720−878 nm [34,35,36]	Sulfide or hydrogen used as electron donors [37]	Bacteria belonging to Chloroflexaceae family are mainly photoheterotrophs (aerobic conditions) whereas those belonging to Oscillochloridaceae are photolithoautotrophs or photolithoheterotrophs (anaerobic conditions) and bacteria from the Roseiflexaceae family are facultative phototrophs [13,19]	These bacteria can be found in marine and freshwater mesophilic environments [38] and particularly in microbial mats found in hot springs [39]	They exists as multicellular filamentous organisms in nature and have gliding motility [38].
Heliobacteria	These bacteria are classified under phylum Firmicutes and Heliobacteriaceae family [40]	Type I [22,29]	BChl g [13]. Carotenoid pigments such as 4,4’ -diaponeurosporene, OH-diaponeurosporene glucoside esters [41,42]	Optimum absorption is in the range of 786−792 nm [13]	Sulfate utilized by H. chlorum [40]	Photoheterotrophs that require light for their growth and depend on organic compounds as their carbon source [42]	Mainly thrive in agricultural and garden soils while only a few species survive in aquatic environments [42,43]	Some species that are found in agricultural soil exhibit a symbiotic relationship with rice plants where the plants act as a source of organic carbon for the bacteria [42,43]
Acidobacteria	Phylum Acidobacteria includes three families namely, Acidobacteriaceae, Holophagaceae and Acanthopleuribacteraceae [44]	Type I [45]	BChl c, BChl aP, Chl aPD and Zn-BChl a’P [46]. Carotenoid pigments such as Echinenone, canthaxanthin, lycopene, γ and β -carotene may be present [46]	Absorbs infrared light lying between 740–750 nm [47]	They use different sugars and organic compound such as acetate, succinate and propionate for their growth [44]	Mainly chemo-organotrophs but some species are photoheterotrophs [44]	Can be found in hot springs, metal contaminated soils and in marine sediments [44]	These bacteria mainly grow in aerobic environments and some grow under microaerobic conditions [45]
Gemmatimonadetes	Gemmatimonadaceae family [48]	Type II [49]	BChl a [49]	Absorption at 816 and 866 nm by BChl a [49,50]	Sulfide and thiosulfate utilized by some Gemmatimonas spp. [49,50]	Heterotrophs that can grow aerobically [48]	Found in soil [51] and fresh water lakes in arid conditions [49]	These aerobic organisms can accumulate polyphosphates within their cell [48]