An overview of anoxygenic phototrophic bacteria and their applications in environmental biotechnology for sustainable Resource recovery

Abstract

Anoxygenic phototrophic bacteria (APB) are a phylogenetically diverse group of organisms that can harness solar energy for their growth and metabolism. These bacteria vary broadly in terms of their metabolism as well as the composition of their photosynthetic apparatus. Unlike oxygenic phototrophic bacteria such as algae and cyanobacteria, APB can use both organic and inorganic electron donors for light-dependent fixation of carbon dioxide without generating oxygen. Their versatile metabolism, ability to adapt in extreme conditions, low maintenance cost and high biomass yield make APB ideal for wastewater treatment, resource recovery and in the production of high value substances. This review highlights the advantages of APB over algae and cyanobacteria, and their applications in photo-bioelectrochemical systems, production of poly-β-hydroxyalkanoates, single-cell protein, biofertilizers and pigments. The ecology of ABP, their distinguishing factors, various physiochemical parameters governing the production of high-value substances and future directions of APB utilization are also discussed.

Keywords: ALA, 5-Aminolevulinic acid; APB, Anoxygenic phototrophic bacteria; Anoxygenic phototrophic bacteria (APB); BChl, Bacteriochlorophyll; BES, Bioelectrochemical systems; BPV, Biophotovoltaic; BPh, Bacteriopheophytin; Bacteriochlorophyll (BChl); Chl, Chlorophyll; CoQ10, Coenzyme Q10; DET, Direct electron transfer; DNA, Deoxyribonucleic acid; DO, Dissolved oxygen; DXP, 1 deoxy-d-xylulose 5-phosphate; FPP, Farnesyl pyrophosphate; Fe-S, Iron-Sulfur; GNSB, Green non sulfur bacteria; GSB, Green sulfur bacteria; IPP, Isopentenyl pyrophosphate isomerase; LED, light emitting diode; LH2, light-harvesting component II; MFC, Microbial fuel cell; MVA, Mevalonate; PH3B, Poly-3-hydroxybutyrate; PHA, Poly-β-hydroxyalkanoates; PHB, Poly-β-hydroxybutyrate; PNSB, Purple non sulfur bacteria; PPB, Purple phototrophic bacteria; PSB, Purple sulfur bacteria; Pheo-Q, Pheophytin-Quinone; Photo-BES, Photosynthetic bioelectrochemical systems; Photo-MFC, Photo microbial fuel cell; Poly-β-hydroxyalkanoates (PHA); Purple phototrophic bacteria (PPB); Resource recovery; RuBisCO, Ribulose-1,5-biphosphate carboxylase/oxygenase; SCP, Single-cell protein; SOB, Sulfide oxidizing bacteria; SRB, Sulfate reducing bacteria; Single-cell proteins (SCP).

Fig. 1

Diagram comparing the electron transport chains in purple bacteria and green sulfur bacteria adapted from Martin Rasmussen and Minteer [53] with (A) Type II reaction center present in purple bacteria and (B) Type I reaction center present in green sulfur bacteria. Pigment 870 (P870), bacteriochlorophyll (BChl), bacteriopheophytin (BPh), primary ubiquinone acceptor (Q_A), secondary ubiquinone acceptor (Q_B), ubiquinone pool (Q_P), cytochrome bc₁ (Cyt bc₁), cytochrome c₂ (Cyt c₂), hydrogen sulfide (H₂S), menaquinone (MQ), cytochrome c (Cyt c), pigment 840 (P840), chlorophyll a (A₀), phylloquinone (A₁), membrane bound iron sulfide (Fe-S) proteins (Fe-S_x, Fe-S_A and Fe-S_B), ferredoxin (Fd), ferredoxin-NADP+ reductase (FNR) and nicotinamide adenine dinucleotide (NAD)

Fig. 2

Cross section of microbial mat isolated from Purple Island mangroves, Qatar. Three distinct colored zones can be seen in the mat and are specific to the groups of organisms inhabiting them. Cyanobacteria and algae (oxygenic phototrophs) can be found in the green zone where oxygen concentrations are high and sulfide concentrations are very low. The red zone is inhabited by anoxygenic phototrophs while the black zone, with very low oxygen concentrations and high sulfide concentrations is dominated by sulfate reducing bacteria.

Fig. 3

Types of photosynthetic bioelectrochemical systems (A) Photosynthetic microbial fuel cell (Photo-MFC) and (B) Biophotovoltaic system (BPV) with phototrophic anode respiring bacteria (ARB) as the biocatalyst adapted from Jeuken [93].

Fig. 4

Outline of poly-β-hydroxybutyrate biosynthetic pathway in R. rubrum modified from Sagong et al. [124] and Fuller [113].