{ "id": "epcm09ghv7", "slug": "photofermentation", "title": "Photofermentation", "updated": "2026-10-15", "topic_path": [ { "id": "life", "label": "Life and health", "api_url": "https://www.edgepedia.org/api/v1/topics/life" }, { "id": "life.production", "label": "Applied biology and nonhuman health", "api_url": "https://www.edgepedia.org/api/v1/topics/life.production" }, { "id": "life.production.biotechnology", "label": "Biotechnology and biological production", "api_url": "https://www.edgepedia.org/api/v1/topics/life.production.biotechnology" }, { "id": "life.production.biotechnology.bt-bioprocess", "label": "Bioprocess engineering and biomanufacturing", "api_url": "https://www.edgepedia.org/api/v1/topics/life.production.biotechnology.bt-bioprocess" }, { "id": "life.production.biotechnology.bt-fermentation", "label": "Fermentation and industrial microbiology", "api_url": "https://www.edgepedia.org/api/v1/topics/life.production.biotechnology.bt-fermentation" } ], "geo": [ { "id": "geo.nongeo.t1946.life.production", "label": "Non-geographic · 1946 to 2000: Applied biology and nonhuman health", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.life.production", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1946", "label": "Non-geographic · 1946 to 2000", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946" }, { "id": "geo.nongeo.t1946.life", "label": "Life and health", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.life" }, { "id": "geo.nongeo.t1946.life.production", "label": "Applied biology and nonhuman health", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.life.production" } ] } ], "excerpt": "Photofermentation is a light-driven microbial process in which purple non-sulfur bacteria oxidize organic substrates under anaerobic conditions, releasing high-purity hydrogen through the enzyme nitrogenase, studied for bioenergy.", "snippet": "Photofermentation is a light-driven microbial process in which purple non-sulfur bacteria oxidize organic substrates under anaerobic conditions, releasing high-purity hydrogen through the enzyme nitrogenase, studied for bioenergy.", "node": "life.production.biotechnology.bt-fermentation", "markdown": "# Photofermentation\n\nPhotofermentation is a light-driven microbial process in which purple non-sulfur bacteria (PNSB) oxidize organic substrates under anaerobic conditions and release the resulting electrons as hydrogen gas, using nitrogenase as the terminal electron-consuming enzyme. The main product is high-purity hydrogen, produced without oxygen generation, together with CO₂ from the oxidation of the organic matter.[1](https://www.mdpi.com/2076-3417/14/3/1191) The process is studied for bioenergy production and, in combined systems, for treating organic waste streams.\n\n| Key fact | Value |\n|---|---|\n| Main product | High-purity \\( H_{2} \\) (with CO₂), no \\( O_{2} \\) evolved[1](https://www.mdpi.com/2076-3417/14/3/1191) |\n| Theoretical glucose yield | 12 mol \\( H_{2} \\) per mol glucose[1](https://www.mdpi.com/2076-3417/14/3/1191) |\n| Nitrogenase ATP cost | 16 ATP per 4 \\( H_{2} \\)[2](https://www.mdpi.com/2076-2607/13/6/1386) |\n| Highest yield in continuous culture | 9.0 ± 1.2 mol \\( H_{2} \\)/mol glucose (*R. capsulatus* JP91, 48 h HRT)[3](https://www.sciencedirect.com/science/article/abs/pii/S0960852412015933) |\n| Volumetric rate, continuous *R. rubrum* culture | up to 180 mL \\( H_{2} \\)/L/h[4](https://escholarship.org/content/qt7wg6m017/qt7wg6m017.pdf?t=r9k3fb) |\n| Typical wild-type rate | ~15–25 mL \\( H_{2} \\)/L/h[4](https://escholarship.org/content/qt7wg6m017/qt7wg6m017.pdf?t=r9k3fb) |\n| Estimated current \\( H_{2} \\) cost | 1362 USD/kg \\( H_{2} \\)[2](https://www.mdpi.com/2076-2607/13/6/1386) |\n\n## How it works\n\nIn PNSB, \\( H_{2} \\) and CO₂ are produced from reduced organic compounds, which serve as electron sources, and from light, which provides energy in the form of ATP; the process is mediated by nitrogenase.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC218197/) Nitrogenase catalyzes hydrogen formation at a fixed ATP cost according to:\n\n\\[ 8\\,\\mathrm{H}^{+} + 8\\,\\mathrm{e}^{-} + 16\\,\\mathrm{ATP} \\rightarrow 4\\,\\mathrm{H}_{2} + 16\\,\\mathrm{ADP} + 16\\,\\mathrm{P_{i}} \\]\n\nThe enzyme is inhibited by oxygen and ammonium salts, so operation requires oxygen-free, ammonia-limited conditions.[2](https://www.mdpi.com/2076-2607/13/6/1386) Electrons reach nitrogenase through the carriers NAD/NADH and ferredoxin, and ATP generated by the photosynthetic machinery is delivered together with protons and electrons.[2](https://www.mdpi.com/2076-2607/13/6/1386) Hydrogen evolution also serves redox balance: when no other reducible substrate is available, nitrogenase continues to turn over, reducing protons to dispose of excess reducing power.[3](https://www.sciencedirect.com/science/article/abs/pii/S0960852412015933) For glucose, the overall stoichiometry under photosynthetic bacteria is \\( C_{6} \\)\\( H_{12} \\)\\( O_{6} \\) + 6 \\( H_{2} \\)O → 6 CO₂ + 12 \\( H_{2} \\), a theoretical yield of 12 mol \\( H_{2} \\) per mol glucose, though the yield varies slightly with the microbe.[1](https://www.mdpi.com/2076-3417/14/3/1191)\n\n## How it is done\n\nCultures are grown anaerobically; in one described setup, *Rhodopseudomonas* cells were grown with 6.0 g/L acetate and 1.0 g/L glutamate (C/N ≈ 30) and incubated in the dark for 24 h before experiments to achieve anaerobiosis.[2](https://www.mdpi.com/2076-2607/13/6/1386) Glutamate is the most used nitrogen source because it inhibits nitrogenase weakly and is consumed rapidly.[2](https://www.mdpi.com/2076-2607/13/6/1386) A pilot-scale formulation used an enrichment medium of 1 g/L NH₄Cl, 2 g/L NaHCO₃, 0.2 g/L \\( K_{2} \\)HPO₄, 3 g/L CH₃COONa, 0.2 g/L MgSO₄·7H₂O, and 2 g/L NaCl.[6](https://www.nature.com/articles/s41467-024-48790-4) Most photofermentative microbes work well around pH 7, and temperature and bioreactor type strongly affect yield.[1](https://www.mdpi.com/2076-3417/14/3/1191)\n\nLight intensity and substrate and biomass concentrations are optimized jointly. [Response surface methodology](https://www.edgepedia.org/response-surface-methodology) with *R. capsulatus* DSM 1710 found the highest rate, 1.04 mmol/L-reactor·h, at 35.35 mM acetate, 0.27 g VSS/L initial biomass, and 263.6 W/m² (3955 lux); the optimum substrate-to-biomass ratio was 7.7 g acetate/g VSS.[7](https://open.metu.edu.tr/handle/11511/31573) PNSB consume pyruvate, acetate, amino acids, alcohols, organic acids, carbohydrates, and aromatic compounds such as benzoate.[1](https://www.mdpi.com/2076-3417/14/3/1191)\n\n## Origin\n\n[Howard Gest](https://www.edgepedia.org/howard-gest) and Martin D. Kamen reported photoproduction of molecular hydrogen by *Rhodospirillum rubrum* in *Science* in 1949.[8](https://doi.org/10.1126/science.109.2840.558) Historical scholarship records that Gest and Kamen discovered light-dependent \\( H_{2} \\) production and \\( N_{2} \\) fixation by *Rsp. rubrum*, and that later work showed many anoxygenic phototrophs fix nitrogen, with both light-dependent \\( H_{2} \\) formation and \\( N_{2} \\) reduction catalyzed by the same enzyme complex, nitrogenase.[9](https://www.life.illinois.edu/govindjee/Part3/7_TimeLineBacteria.pdf) Later milestones include continuous operation with immobilized *Rhodobacter sphaeroides* O.U.001, reported by R. Zagrodnik and colleagues in 2015 in the *International Journal of Hydrogen Energy*.[10](https://doi.org/10.1016/j.ijhydene.2015.02.079)\n\n## Variants\n\nTwo-stage systems couple dark fermentation with photofermentation in sequence, and co-culture processes mix dark-fermentative bacteria (DFB) and photofermentative bacteria (PFB); a single-stage hybrid variant augments photosynthetic bacteria with dark fermentative culture and has been evaluated with real-field wastewater.[11](https://biotechnologyforbiofuels.biomedcentral.com/counter/pdf/10.1186/s13068-014-0191-x.pdf) Two-stage coupling increases hydrogen yield relative to the individual processes, while single-stage photofermentation is more cost-effective because it uses a wide range of substrates.[1](https://www.mdpi.com/2076-3417/14/3/1191) In a sequential dark–photo process on fruit and vegetable waste and cheese whey powder, maximum overall yields of 793.7 and 695.4 mL \\( H_{2} \\)/g COD were obtained at 1:10 dilution and C/N ratios of 60 and 70, respectively.[12](https://ideas.repec.org/a/eee/renene/v147y2020ip1p924-936.html) The C/N ratio at the dark stage regulates both \\( H_{2} \\) production and the distribution of by-products, which in turn control \\( H_{2} \\) production during the photo stage; the dominant populations were *C. butyricum* and *Acetobacter lovaniensis* in the dark reactor and *Rhodopseudomonas palustris* in the photo reactor.[12](https://ideas.repec.org/a/eee/renene/v147y2020ip1p924-936.html) Mutant strains developed through optimization of carbon metabolism, electron transport, ATP synthesis, nitrogenase activity, and stress tolerance include a *R. palustris* nifA* draT2* mutant that was 25 times more tolerant to ammonium than the wild type and yielded 2744 ± 66 mL \\(H_{2}\\)/L on 20 mM acetate plus 20 mM butyrate at 30 °C and 39.5 W/m², and a transposon-mutagenized *R. capsulatus* mutant with reduced bacteriochlorophyll content that produces 50.5% more hydrogen.[1](https://www.mdpi.com/2076-3417/14/3/1191)\n\n## Applications\n\nPhotofermentation is applied to biohydrogen production from defined media and from waste streams. Continuous dark–photo systems have been run on simulated sugar-rich processing wastewater, inoculated with a mixed microbial consortium (5% v/v) in the dark reactor and a commercial photosynthetic bacterial culture in the photofermentation reactor, under suboptimal pH and temperature.[13](https://zaguan.unizar.es/record/177703/files/texto_completo.pdf) A 2025 review in *Green Chemistry* extends the frame to photo-fermentative production of both biohydrogen and biomethane from biowaste.[14](https://pubs.rsc.org/en/content/articlelanding/2025/gc/d4gc06079b) Operating photobioreactors at ambient conditions reduces energy input and process costs compared with thermochemical and electrolytic methods.[1](https://www.mdpi.com/2076-3417/14/3/1191)\n\n## Limitations and alternatives\n\nOxygen is the most important inhibitor of both biophotolysis and photofermentation, because hydrogenase and nitrogenase are oxygen-sensitive; systems must be anaerobic with oxygen content below 0.1%.[15](https://www.rees-journal.org/articles/rees/full_html/2021/01/rees210064/rees210064.html) [Nitrogen](https://www.edgepedia.org/nitrogen) inhibition of nitrogenase is reversible, and the enzyme can be reactivated when ammonium is consumed or removed; nitrogen limitation is achieved by sparging argon instead of molecular nitrogen, and experiments varying C/N ratios found maximum hydrogen production at an optimum C/N ratio of 25.[15](https://www.rees-journal.org/articles/rees/full_html/2021/01/rees210064/rees210064.html)\n\nMeasured hydrogen production rates are significantly lower than theoretical rates based on stoichiometric equations, which is the main drawback of photofermentation.[15](https://www.rees-journal.org/articles/rees/full_html/2021/01/rees210064/rees210064.html) Reported light conversion efficiencies in cylindrical photobioreactors were 0.58–0.72%, and 1.6% in a novel disk photobioreactor with *Rhodopseudomonas faecalis* RLD-53, which reached 2.68 mol \\( H_{2} \\)/mol acetate at 48 h HRT.[2](https://www.mdpi.com/2076-2607/13/6/1386) • [16](https://www.scientific.net/AMR.953-954.970) A critical review states that effective volumetric productivities of photofermentation are typically 0.5–3 mmol/L/h, an order of magnitude below dark fermentation, because dense suspensions self-shade and outdoor light conversion efficiencies rarely exceed 3–5% even in optimized flat-panel or fiber-illuminated bioreactors.[17](https://doi.org/10.3390/en19133152) [Dark fermentation](https://www.edgepedia.org/dark-fermentation) achieves rates up to 12 m³/d/m³ with no light requirement, and microbial electrolysis cells reach up to 72 m³/d/m³.[18](https://journal.hep.com.cn/ger/EN/10.1016/j.gerr.2025.100131) A techno-economic analysis by Genç and Koku determined an \\( H_{2} \\) cost of 1362 USD/kg \\( H_{2} \\) based on current photofermentative technology.[2](https://www.mdpi.com/2076-2607/13/6/1386) Uptake-hydrogenase-deficient mutants also raised rates, to 100 mL \\( H_{2} \\)/L/h for an *R. capsulatus* \\( hup^{-} \\) mutant and 82.6 mL \\( H_{2} \\)/L/h for *R. sphaeroides* \\( \\Delta hup \\) and \\( \\Delta PHB \\) mutants, against roughly 15–25 mL \\( H_{2} \\)/L/h for wild-type purple photosynthetic bacteria.[4](https://escholarship.org/content/qt7wg6m017/qt7wg6m017.pdf?t=r9k3fb)\n\n## References\n\n1. [Photo-Fermentative Bacteria Used for Hydrogen Production (Applied Sciences, 2024)](https://www.mdpi.com/2076-3417/14/3/1191)\n2. [Comparison of Photofermentative Hydrogen Production in Cylindrical Photobioreactors Using Different Mixing Systems (Microorganisms, MDPI)](https://www.mdpi.com/2076-2607/13/6/1386)\n3. [High yield single stage conversion of glucose to hydrogen by photofermentation with continuous cultures of Rhodobacter capsulatus JP91 (Bioresource Technology, 2013)](https://www.sciencedirect.com/science/article/abs/pii/S0960852412015933)\n4. [Photobiological hydrogen production: Recent advances and state of the art](https://escholarship.org/content/qt7wg6m017/qt7wg6m017.pdf?t=r9k3fb)\n5. [Hydrogen formation in nearly stoichiometric amounts from glucose by a Rhodopseudomonas sphaeroides mutant](https://pmc.ncbi.nlm.nih.gov/articles/PMC218197/)\n6. [Biological fermentation pilot-scale systems and evaluation for commercial viability towards sustainable biohydrogen production (Nature Communications, 2024)](https://www.nature.com/articles/s41467-024-48790-4)\n7. [Investigation of the effects of initial substrate and biomass concentrations and light intensity on photofermentative hydrogen gas production by Response Surface Methodology (Int. J. Hydrogen Energy, via METU repository)](https://open.metu.edu.tr/handle/11511/31573)\n8. [Howard Gest, Martin D. Kamen (1949). Photoproduction of Molecular Hydrogen by Rhodospirillum rubrum. Science.](https://doi.org/10.1126/science.109.2840.558)\n9. [Timeline of discoveries in anoxygenic phototrophic bacteria (Blankenship et al., historical review chapter)](https://www.life.illinois.edu/govindjee/Part3/7_TimeLineBacteria.pdf)\n10. [R. Zagrodnik and colleagues (2015). Continuous photofermentative production of hydrogen by immobilized Rhodobacter sphaeroides O.U.001. International Journal of Hydrogen Energy.](https://doi.org/10.1016/j.ijhydene.2015.02.079)\n11. [Simultaneous hydrogen and ethanol production from cascade utilization of mono-substrate in integrated dark and photo-fermentative reactor (Biotechnology for Biofuels)](https://biotechnologyforbiofuels.biomedcentral.com/counter/pdf/10.1186/s13068-014-0191-x.pdf)\n12. [Enhanced hydrogen production by a sequential dark and photo fermentation process (Renewable Energy, 2020)](https://ideas.repec.org/a/eee/renene/v147y2020ip1p924-936.html)\n13. [Performance of a Continuous Dark–Photo Fermentation System to Produce Hydrogen from Simulated Sugar–Rich Processing Wastewater Under Suboptimal pH and Temperature Conditions (University of Zaragoza repository)](https://zaguan.unizar.es/record/177703/files/texto_completo.pdf)\n14. [A comparative review of biohydrogen and biomethane production from biowaste through photo-fermentation (Green Chemistry, RSC, 2025)](https://pubs.rsc.org/en/content/articlelanding/2025/gc/d4gc06079b)\n15. [Waste to Sustainable Biohydrogen Production Via Photo-Fermentation and Biophotolysis − A Systematic Review](https://www.rees-journal.org/articles/rees/full_html/2021/01/rees210064/rees210064.html)\n16. [Continuous Hydrogen Production in a Novel Photo-Bioreactor with High Light Conversion Efficiency (Advanced Materials Research)](https://www.scientific.net/AMR.953-954.970)\n17. [Carbon and Electron Recovery in Integrated Biohydrogen Systems: A Critical Review of Dark Fermentation, Photo-Fermentation, and Microbial Electrolysis Cells (Energies, MDPI)](https://doi.org/10.3390/en19133152)\n18. [Microbial biomass conversion for hydrogen production: A review (Green Energy and Resources, 2025)](https://journal.hep.com.cn/ger/EN/10.1016/j.gerr.2025.100131)\n\n---\n*Topic: Encyclopedia › Life and health › Applied biology and nonhuman health › Biotechnology and biological production › Bioprocess engineering and biomanufacturing › Fermentation and industrial microbiology*\n\n*Initially written Sep 29, 2026 · Edited: Oct 15, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/photofermentation", "markdown_url": "https://www.edgepedia.org/photofermentation.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Photofermentation\", Edgepedia (EdgeChat), https://www.edgepedia.org/photofermentation. Edgepedia Community License 1.0.", "credit_md": "\"[Photofermentation](https://www.edgepedia.org/photofermentation)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/photofermentation](https://www.edgepedia.org/photofermentation). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Photofermentation\", Edgepedia (EdgeChat), https://www.edgepedia.org/photofermentation. Edgepedia Community License 1.0.", "speakable": "Photofermentation is a light-driven microbial process in which purple non-sulfur bacteria oxidize organic substrates under anaerobic conditions, releasing high-purity hydrogen through the enzyme nitrogenase, studied for bioenergy." }