Key Takeaways & Executive Findings
- •• • APB exhibit anaerobic photophosphorylation and carbon fixation, enabling CO2 conversion to biomass; engineered Rhodopseudomonas palustris achieved enhanced CO2 capture for lycopene production (Green Chemistry, 2022). • • Extracellular electron transfer (EET) via exogenous mediators expands electron sources; CdS-photosynthetic hybrid systems achieved light-driven biodegradation of azo dyes without external electron donors (Applied Microbiology and Biotechnology, 2023). • • Syntrophic interspecies electron transfer drives carbon fixation and growth in R. palustris under dark, anoxic conditions, achieving significant CO2 reduction (Science Advances, 2021). • • APB-based microbial fuel cells and biohydrogen production are promising; addition of TiO2, ZnO, and SiC nanoparticles enhanced photo-fermentative hydrogen production in Rhodopseudomonas sp. strain A7 (International Journal of Hydrogen Energy, 2017).
Abstract
Anoxygenic photosynthetic bacteria (APB) are a phylogenetically diverse group of prokaryotes that perform photosynthesis without oxygen evolution. They possess versatile metabolic capabilities, including anaerobic photophosphorylation, carbon fixation, multi-substrate metabolism, and metal oxidation-reduction, enabling them to thrive in diverse environments such as lakes, rivers, soils, salt lakes, and hot springs. APB play a pivotal role in biogeochemical cycling of carbon, nitrogen, sulfur, and metals. This review systematically summarizes the metabolic diversity of APB, emphasizing their ability to utilize organic and inorganic compounds as electron donors and carbon sources. We highlight recent advances in understanding extracellular electron transfer (EET) mediated by exogenous electron shuttles and conductive materials, which expand the electron sources available for energy generation and reducing power. In environmental engineering, APB show promise in carbon sequestration, pollutant degradation (including azo dyes and heavy metals), biohydrogen production, and microbial fuel cells. For instance, Rhodopseudomonas palustris can fix CO2 under dark anoxic conditions via syntrophic interspecies electron transfer, achieving enhanced carbon fixation. Additionally, APB-based biohybrid systems incorporating CdS nanoparticles demonstrate light-driven degradation of azo dyes without external electron donors. Challenges remain in scaling up these technologies, optimizing reactor conditions, and understanding metabolic regulation. Future research should focus on genetic engineering to enhance APB performance and integrating APB into circular bioeconomy frameworks.
1. Introduction
Anoxygenic photosynthetic bacteria (APB) have long been recognized for their metabolic versatility, yet their industrial deployment in environmental engineering has been hampered by low conversion efficiencies and the need for light-driven processes. Traditional wastewater treatment and carbon capture technologies often rely on aerobic heterotrophs, which are energy-intensive and produce significant sludge. APB offer an alternative: they can couple light energy to the degradation of organic pollutants and heavy metals while simultaneously fixing CO2 and producing valuable byproducts such as hydrogen and bioplastics. However, the bottleneck lies in optimizing electron transfer pathways and reactor designs to achieve economically viable rates.
This review addresses these challenges by synthesizing recent advances in APB metabolism, particularly the role of extracellular electron transfer (EET) mediated by exogenous electron shuttles and conductive nanomaterials. By integrating these mechanisms, APB-based systems can overcome the limitations of slow electron transfer and low substrate utilization, enabling more efficient pollutant removal and energy recovery. The experimental protocols highlighted here, such as the use of CdS biohybrids for azo dye degradation and nanoparticle-enhanced hydrogen production, demonstrate tangible improvements in performance metrics, providing a roadmap for scaling up APB technologies from laboratory to field applications.
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BAI Xue, YANG Yue, DI Xinyu, CUI Daizong, ZHAO Min (2026). Metabolic Functions of Anoxygenic Photosynthetic Bacteria and Their Applications in Environmental Engineering. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2024113001
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Frequently Asked Questions
What are the main bottlenecks in scaling up APB-based wastewater treatment and how can they be overcome?
Key bottlenecks include light penetration in dense cultures, slow electron transfer rates, and competition with heterotrophs. Strategies include using photobioreactors with high surface area-to-volume ratios, immobilizing APB on conductive materials to enhance EET, and optimizing light wavelength and intensity. For example, CdS biohybrids have been shown to enable light-driven azo dye degradation without external electron donors, achieving high removal efficiencies under low-light conditions.
How does extracellular electron transfer (EET) contribute to APB's metabolic versatility and environmental applications?
EET allows APB to access insoluble electron donors/acceptors, such as minerals or electrodes, expanding their substrate range. This is crucial for bioremediation of heavy metals and for bioelectrochemical systems. For instance, syntrophic interspecies electron transfer enables R. palustris to fix CO2 under dark anoxic conditions, achieving carbon fixation rates comparable to photosynthetic conditions.
What are the comparative advantages of APB-based biohydrogen production over conventional dark fermentation?
APB can utilize a wider range of organic substrates and operate under lower light intensities, achieving higher theoretical hydrogen yields (e.g., 4 mol H2/mol acetate) compared to dark fermentation (typically 2-3 mol H2/mol glucose). Additionally, APB can produce hydrogen from waste streams without oxygen inhibition, and nanoparticle addition (e.g., TiO2) has been shown to enhance yields by improving light utilization and electron transfer.
What are the challenges in using APB for carbon capture and how can genetic engineering help?
Challenges include low CO2 fixation rates and competition with heterotrophic growth. Genetic engineering can enhance carbon fixation by overexpressing RuBisCO or introducing alternative CO2 fixation pathways. For example, engineered R. palustris with enhanced CO2 capture produced higher lycopene yields, demonstrating the potential for value-added chemical production from CO2.
How do APB-based microbial fuel cells (MFCs) compare to conventional MFCs in terms of power output and cost?
APB-based MFCs can generate power while treating wastewater, but power densities are often lower than those of heterotrophic MFCs due to lower metabolic rates. However, APB offer advantages in terms of substrate versatility and the ability to operate under light, potentially reducing aeration costs. Research is focused on improving electrode materials and reactor design to increase power output and reduce capital costs.
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