The chemistry at the heart of the study is fermentation, the same family of microbial processes that turns sugars into alcohol or yogurt into existence. In dark fermentation, hydrogen-producing bacteria break down complex organic matter into hydrogen, organic acids, and alcohols without needing light or oxygen. Photo fermentation, the second stage, uses photosynthetic bacteria that capture light energy to generate ATP, which they then spend to ferment the organic acids left behind by the first stage into more hydrogen. Each stage alone is inefficient, converting only a fraction of the feedstock into gas, but the two-stage combination recovers substantially more hydrogen because the fatty acids that would otherwise accumulate as waste in dark fermentation become the fuel for the second act. This sequential logic formed the backbone of the experimental design.

The team collected fresh leachate from the Al Multaqa landfill in Al Amerat, Oman’s first engineered landfill, which receives roughly 200 cubic meters of leachate per day, along with household food waste and sewage sludge from a local treatment plant to serve as the microbial inoculum. Characterization of the leachate revealed a chemical oxygen demand of about 43,000 milligrams per liter and a biochemical oxygen demand of roughly 27,000 milligrams per liter, values indicating a highly biodegradable, organic-rich liquid drawn from a young landfill cell. Iron and magnesium concentrations fell within ranges previously reported as favorable for hydrogen-producing microbes. The food waste was composed of about 50 percent fruit and vegetables, 25 percent protein, and 20 percent carbohydrate, a composition typical of Omani kitchen discards, which collectively represent more than 50 million Omani rials in wasted value each year.

Eight experimental schemes probed the variables that matter most. Four tested different ratios of landfill leachate to food waste, from pure leachate to pure food waste, under dark fermentation at 30 degrees Celsius and a pH of 5.5. The sweet spot turned out to be a 75:25 leachate-to-food-waste blend, which yielded 2.507 milliliters of hydrogen per liter of medium at a rate of 0.021 milliliters per liter per hour, roughly double the output of either feedstock alone. The explanation lies in the carbon-to-nitrogen ratio: leachate brings abundant nitrogen that supports microbial growth, while a modest dose of carbon-rich food waste supplies the electrons that ultimately end up in hydrogen gas. Acetic and propionic acids dominated the fatty acid profile of the best-performing scheme, confirming that the microbial community was operating along productive metabolic pathways rather than being stalled by acid accumulation.

Pretreatment emerged as a make-or-break variable. When the researchers skipped the heat treatment of the inoculum and feedstock, hydrogen production fell to zero even though fatty acid concentrations soared, a signature of acid accumulation poisoning the hydrogen-producing metabolism. Similarly, omitting the centrifugation and sterilization of the dark fermentation effluent before feeding it to the photo fermentation stage cut hydrogen output and left a heavy load of unconverted acids. In the single-stage experiments using only food waste, smaller medium volumes outperformed larger ones: a scheme with 40 percent more substrate and inoculum produced 28.74 percent less hydrogen per liter, echoing earlier reports that smaller reactors avoid substrate inhibition. Hydrogen purity in the collected gas ranged from under 5 percent to over 60 percent depending on the blend and treatment regime.

With technical feasibility established at bench scale, the team scaled the concept to a hypothetical plant processing 50 cubic meters of leachate per day alongside collected food waste, using literature benchmarks of 200 to 400 milliliters of hydrogen per liter for dark fermentation and 700 to 1000 milliliters per liter for photo fermentation. Four plant configurations were modeled: two-stage fermentation with full pretreatment, two-stage with treatment only before the photo stage, two-stage with treatment only before the dark stage, and single-stage dark fermentation of food waste alone. Plant capacities ranged from 2.7 to 12.13 kilograms of hydrogen per day, and capital costs clustered around 25.5 to 25.8 million US dollars, dominated by bioreactors and gas compression equipment.

The economics told a striking story. All four scenarios produced a positive net present value over a ten-year lifetime at a 5 percent discount rate, but Scenario 2, the two-stage system with treatment applied only before photo fermentation, came out on top with an NPV of 45.06 million US dollars and cumulative revenue of roughly 80.65 million dollars. Revenue flows from four streams: hydrogen sold at approximately 12 dollars per kilogram, carbon dioxide captured during gas separation, biofuel derived from volatile fatty acids, and compost recovered from the microfiltration stage. Operating costs were driven primarily by energy and maintenance, with heating vessels alone consuming 45.6 percent of total energy demand. Notably, the feedstocks themselves cost nothing; in fact, facilities can collect tipping fees for accepting waste, a structural advantage over electrolysis plants that must purchase purified water and electricity.

Sensitivity analysis showed the discount rate and hydrogen price exert the strongest influence on profitability, while carbon pricing barely moves the needle. A 30 percent increase in the discount rate dropped Scenario 2’s NPV to 41.63 million dollars, while a 30 percent decrease lifted it to 48.90 million. The Oman results sit within a global spectrum of biohydrogen economics, from a modest 4.7 million dollar NPV for rice straw fermentation in Malaysia to industrial-scale biomass systems in Bangladesh and Nigeria exceeding 480 million dollars, suggesting that revenue diversification through biorefinery integration is what separates marginal projects from compelling ones.

The authors are candid about limitations. The experiments were designed as a proof of concept without replicates, and the economic model relied on literature-based hydrogen prices rather than localized Omani market data. Pilot-scale validation will be essential before commercial deployment, since hydrogen productivity at industrial scale often diverges from laboratory performance. Still, the broader implications are hard to ignore: waste streams that cost municipalities money to manage can be converted into a zero-emission energy carrier, compost, and biofuel, aligning circular-economy practice with national climate goals. As global hydrogen strategies mature, the study argues that waste-derived biohydrogen deserves formal recognition within green hydrogen frameworks, supported by feedstock-based subsidies, carbon credit mechanisms, and standardized purity certification. If Oman’s landfills can become hydrogen factories, the model could travel well beyond the Gulf.

Subject of Research: Techno-economic feasibility of biohydrogen production from landfill leachate and food waste via dark and photo fermentation in Oman

Article Title: Economic feasibility and optimization of biohydrogen gas production- a case study in Oman

Article References: Barghash, H., Farai, S. A., AlRashdi, Z., & Okedu, K. E. (2026). Economic feasibility and optimization of biohydrogen gas production- a case study in Oman. Results in Engineering, 32, Article 113232. https://doi.org/10.1016/j.rineng.2026.113232

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113232

Keywords: biohydrogen, landfill leachate, food waste, dark fermentation, photo fermentation, techno-economic analysis, Oman Vision 2040, circular economy, waste-to-energy, net present value, renewable energy, Al Multaqa landfill

Pulp-paper mill sludge, or PPMS, is the concentrated solid or semi-solid residue generated by the wastewater treatment plants that serve paper mills. It is a heterogeneous material, a mixture of cellulosic pulp fibers that escaped the manufacturing process, inorganic fillers such as calcium carbonate, kaolin, and titanium dioxide, lignin byproducts, and organic compounds. Researchers have even detected microplastic fragments, with polyethylene and polypropylene the most prevalent polymers. The sludge comes in several distinct varieties: primary sludge settled out in the primary clarifier, secondary or biological sludge from the activated sludge process, deinking sludge from recycled paper processing, and mixed sludge combining the primary and secondary streams. Each type differs in composition, moisture, and ash content, and each presents its own challenges and opportunities for bioconversion.

The conventional disposal options for this waste are increasingly untenable. Sludge disposal can account for approximately 60 percent of the total operating expenses of wastewater treatment plants in numerous mills. Landfilling releases methane, a potent greenhouse gas, and produces leachate that can contaminate soil and water. Land application as a soil amendment risks introducing heavy metals and refractory organic contaminants into food chains. Composting demands energy inputs and can spread odors and pathogens. Incineration, meanwhile, is energy-intensive and frequently requires supplemental fuel, because the high moisture and ash content of the sludge depress its calorific value, promote equipment corrosion, and leave behind residual ash that itself requires disposal, all while emitting greenhouse gases along with sulfur and nitrogen oxides.

What makes sludge attractive as a fuel feedstock is its cellulose. Primary sludge from kraft pulp mills has been found to contain roughly 58 percent cellulose, 12 percent hemicellulose, and 20 percent Klason lignin by weight. Cellulose is a linear polymer of cellobiose units linked by beta-1,4-glycosidic bonds, and its chains are bound together by hydrogen bonds and van der Waals forces into strong microfibrils. Hemicellulose branches weave between the microfibrils, while lignin encases the whole structure in a rigid matrix. This recalcitrant architecture resists both chemical and biological attack, which is precisely why pretreatment sits at the heart of any strategy to unlock the fermentable sugars, chiefly glucose, trapped inside the fibers.

The review highlights anaerobic digestion as one of the most mature routes for converting sludge into energy. In this oxygen-free microbial process, hydrolytic bacteria and fungi first break down complex macromolecules, acidogenic bacteria then convert the products into volatile fatty acids and hydrogen, acetogens generate acetate, carbon dioxide, and hydrogen, and finally methanogenic archaea produce biomethane, which makes up 50 to 75 percent of the resulting biogas. Yet industrial adoption has lagged because yields are low and digestion times long. In semi-continuous trials, primary sludge yielded 190 to 240 normal liters of methane per kilogram of volatile solids over 23 to 32 days, while blends with secondary sludge dropped to 150 to 170 liters over shorter retention times. Toxic compounds such as chlorolignin, mineral oil, and certain metals, together with high ash content, suppress methane potential, and hydrolysis remains the rate-limiting step.

Pretreatment can dramatically change that picture. When researchers applied thermal pretreatment at 170 degrees Celsius to secondary sludge from kraft and sulfite mills, the methane production rate from kraft sludge improved by 300 times and the yield by 280 percent. Autoclaving biosludge at 121 degrees Celsius for 20 minutes enabled stable digestion at a shortened 10-day hydraulic retention time, delivering 138 normal liters of methane per kilogram of volatile solids where untreated sludge failed. Thermal treatment at 140 degrees Celsius alone boosted methane yield by 170 percent, making added chemical steps unnecessary. Hydrothermal pretreatment at 150 degrees Celsius for 10 minutes raised yields by 31 percent, while alkali dosing with sodium hydroxide disrupted the floc structure and lifted soluble chemical oxygen demand by up to 83 percent. Even black liquor, a pulping byproduct, enhanced methane yields by 7 to 30 percent more cheaply than sodium hydroxide.

Biohydrogen offers another compelling pathway, prized because its combustion emits no pollutants. Under optimized conditions of a 32-hour solids retention time, anaerobic fermentation of paper mill sludge delivered 620.8 milliliters of hydrogen per gram of chemical oxygen demand. Co-digesting pulp-paper sludge with food waste in a two-stage mesophilic-thermophilic process achieved a hydrogen yield of 64.48 milliliters per gram of volatile solids fed alongside a methane yield of 432.3 milliliters per gram, without any accumulation of inhibitory volatile fatty acids. The extreme thermophile Caldicellulosiruptor saccharolyticus has also fermented paper sludge hydrolysate into hydrogen, although inhibitory substances in the hydrolysate limited production rates. Supplementing fermentation with the cellulolytic bacterium Clostridium thermocellum improved holocellulose degradation by nearly 33 percent and hydrogen yield by almost 97 percent under thermophilic conditions.

Bioethanol may be the most commercially advanced option. Production proceeds either through separate hydrolysis and fermentation, in which cellulase enzymes release sugars before yeast fermentation, or through simultaneous saccharification and fermentation, which merges the two steps and cuts enzyme use, energy demand, and cost. The central obstacle is ash: calcium carbonate filler binds enzymes more readily than fibers, with roughly 3 to 5 milligrams of enzyme lost per gram of acid-insoluble ash, and hydrolysis yields have been as low as 8 to 32 percent glucose per gram of cellulose. Washing sludge with dilute hydrochloric acid converts insoluble calcium carbonate into soluble calcium chloride, cutting ash from 27 percent to 0.5 percent in one study and raising sugar conversion from 20 to 88 percent in another. With surfactant-assisted enzymatic hydrolysis reaching 74.4 percent efficiency, Saccharomyces cerevisiae then produced 9.7 grams per liter of ethanol at a 92 percent yield, and techno-economic modeling suggests sludge-derived ethanol is about 20 percent cheaper to produce than ethanol from corn stover.

The portfolio extends further. The oleaginous yeast Cryptococcus vishniaccii converted ultrasonicated sludge extract into neutral lipids with 53.4 percent intracellular lipid content, suitable for biodiesel after transesterification. An engineered Clostridium tyrobutyricum strain co-fermented sludge sugars with corn steep liquor to yield 16.5 grams per liter of biobutanol, a fuel with higher energy density and lower corrosiveness than ethanol. Perhaps most striking is sustainable aviation fuel: a life-cycle assessment found that catalytic sugar upgrading of paper sludge yields fuel with a carbon intensity of 35.7 to 41.8 grams of carbon dioxide equivalent per megajoule, falling to 5.1 to 11.1 when ash is recycled as cement substitute, and that the approach could produce over 330 million gallons of aviation fuel annually while cutting emissions by 2 to 7 million tonnes of carbon dioxide equivalent, a far better climate outcome than landfilling.

Significant hurdles remain before these laboratory successes translate into industry. The heterogeneity of sludge complicates standardized processing, pretreatment methods consume large quantities of chemicals and energy, and inhibitors such as phenolics, furfural, and ligno-carbohydrate complexes continue to hamper microbial fermentation. Almost all published work remains at laboratory scale, and the reviewers call for pilot and field studies, robust microbial strains tolerant of sludge variability, deeper understanding of sludge chemistry, and techno-economic and life-cycle analyses to guide commercialization. Still, the direction is clear: a waste stream that mills once paid to discard is emerging as a low-cost reservoir of fermentable sugars and a credible pillar of renewable energy, one that aligns with the United Nations Sustainable Development Goals and could help close the loop on one of the world’s oldest industries.

Subject of Research: Biofuel production from pulp and paper mill sludge through pretreatment and biorefinery methods

Article Title: A critical review on biofuels generation from pulp-paper mill sludge with emphasis on pretreatment methods: renewable energy for environmental sustainability

Article References: Kumar, V., Verma, P., de Freitas, F. A., Srivastava, P. K., Vashishth, A., & Américo‑Pinheiro, J. H. P. (2025). A critical review on biofuels generation from pulp-paper mill sludge with emphasis on pretreatment methods: renewable energy for environmental sustainability. BMC Environmental Science, 2(1), Article 2. https://doi.org/10.1186/s44329-024-00016-0

Image Credits: AI Generated

DOI: 10.1186/s44329-024-00016-0

Keywords: pulp and paper mill sludge, biofuels, pretreatment, anaerobic digestion, biomethane, biohydrogen, bioethanol, biobutanol, biodiesel, sustainable aviation fuel, circular economy, biorefinery

The process at the heart of the review is dark fermentation, a biotechnology in which anaerobic bacteria dismantle organic matter, sugars, and waste streams such as food waste, palm oil mill effluent, and agricultural residues, in the absence of light and oxygen. During this microbial dismantling, hydrogen gas is released as a byproduct of the bacteria’s energy metabolism. The appeal is obvious: the feedstock is often waste that would otherwise rot in landfills, the reaction proceeds at ambient to moderately warm temperatures without light energy, and the gas produced is pure energy carrier rather than a carbon-containing fuel. Yet the technology has struggled to escape the laboratory. Yields remain stubbornly low relative to the theoretical maximum, and the microbial communities that do the work are notoriously sensitive to disturbance, swinging between productive and unproductive metabolic states as conditions drift.

The review’s first pillar is a systematic account of the physicochemical and operational variables that govern microbial performance. pH emerges as perhaps the most decisive lever. Fermentative hydrogen producers operate best in a mildly acidic window, typically around pH 5 to 6, where the hydrogenase enzymes responsible for releasing hydrogen gas remain active and where competing microbes that channel carbon into methane or other reduced products are suppressed. Let the pH drift too low and acid accumulation crashes the culture; let it rise and methanogens invade, consuming the hydrogen that the fermenters worked to produce. Temperature is the second master variable, with mesophilic and thermophilic regimes each selecting for distinct microbial consortia and distinct metabolic end products. Thermophilic operation, the authors note, often supports higher hydrogen yields and faster kinetics, but demands more energy input and careful reactor management.

Beyond pH and temperature, the review examines the characteristics of the inoculum itself, the starting microbial community seeded into the reactor. Heat treatment of seed sludge is a common strategy to kill off hydrogen-consuming methanogens while sparing spore-forming hydrogen producers such as Clostridium species. Organic loading rate, the amount of substrate fed per unit reactor volume per day, must be balanced carefully: push it too high and volatile fatty acids accumulate faster than the microbes can process them, acidifying the reactor and stalling hydrogen production; run it too low and the process becomes economically unattractive. Reactor configuration adds another layer of control. Immobilized-cell systems, upflow anaerobic sludge blanket reactors, packed bed biofilm reactors, and membrane bioreactors all retain biomass more effectively than simple suspended-growth tanks, allowing higher cell densities, faster throughput, and greater resilience to washout during shock loads.

Into this well-mapped landscape, the review introduces its genuinely novel perspective: multifunctional green materials, with hydroxyapatite as the star. Hydroxyapatite, with the chemical formula reflecting its calcium, phosphate, and hydroxide components, is the mineral that gives bones and teeth their rigidity. In a fermenter, it performs several jobs at once. Its surfaces serve as attachment sites for bacteria, promoting cell immobilization and the formation of dense, stable granules that resist washout in continuous operation. Its mineral chemistry buffers pH, absorbing and releasing ions in ways that damp the acidification that so often kills hydrogen-producing cultures. And through the controlled release of calcium and phosphate ions, it appears to support microbial metabolic stability, keeping the community locked into hydrogen-producing pathways for longer stretches of operation.

The evidence for this approach is not merely theoretical. The review’s own authors have published experimental work showing that eggshell-derived hydroxyapatite enhanced thermophilic hydrogen production, with the material promoting the formation of biogranules, compact microbial aggregates in which cells packed closely together exchange metabolites efficiently and maintain favorable microenvironments. Other studies cited in the review report that hydroxyapatite fabrication improved hydrogen output from glucose dark fermentation, and that graphene-hydroxyapatite composites boosted productivity from duckweed biomass. The mechanism is elegantly circular: the material that stabilizes the microbes is itself made from waste, so a discarded eggshell becomes the scaffold on which bacteria convert discarded food into fuel.

How the hydroxyapatite is made matters enormously, and the review devotes detailed attention to synthesis routes. Physical methods include pyrolysis and spray techniques that produce fine, well-defined particles. Chemical routes encompass wet chemical precipitation, sol-gel synthesis, hydrothermal treatment, sonochemical processing, and microwave-assisted reactions, each offering control over crystal size, morphology, and surface area, properties that directly determine how well the material buffers pH and hosts bacterial attachment. Biological and green synthesis routes use plant extracts, microbial activity, or biogenic templates to direct mineral formation under mild conditions, avoiding harsh reagents and high energy demands. Most compelling from a sustainability standpoint are the waste-based routes: eggshells, cow and fish bones, mussel and oyster shells, sea urchin spines, calcified algae, and even industrial byproducts such as phosphogypsum and carbide slag have all been converted into hydroxyapatite. Calcination temperature during processing, the review notes, strongly influences the crystallinity and purity of the final product, and higher specific surface area enhances the release of calcium ions, the very property that makes the material useful in fermentation.

This waste-to-material-to-energy chain is where the review’s conceptual contribution becomes clearest. By linking bioprocess engineering with materials science, the authors construct a framework for what they describe as more efficient, resilient, and environmentally friendly biohydrogen systems. The circular economy logic runs in both directions: organic waste feeds the fermenters, and inorganic waste supplies the additives that make the fermenters work better. Nothing in the chain requires virgin mined minerals or energy-intensive synthetic chemistry if biogenic sources are chosen. The review also situates hydroxyapatite within a broader family of additives, from carbonaceous materials and magnetite nanoparticles to nickel and cobalt ferrites, that have been explored to enhance fermentative hydrogen yields, but argues that hydroxyapatite’s combination of buffering capacity, immobilization support, biocompatibility, and waste-derived availability gives it a distinctive edge.

Challenges remain before the vision reaches industrial scale. The review is candid that low yield and process instability still limit the scalability of dark fermentation, and the integration of a solid additive introduces new variables: optimal dosing, particle size selection, long-term material stability, and the economics of recovering or disposing of spent mineral. Techno-economic assessments of dark fermentation systems generally conclude that profitability hinges on cheap feedstock, high productivity, and valorization of the fermentation effluent, which can itself be converted to biomethane, bioplastics, or microbial lipids in cascading processes. Hydroxyapatite strengthens two of those three pillars, feedstock flexibility and productivity, but the full chain must be demonstrated at pilot scale. The review’s framework, mapping process parameters onto material design, gives researchers a structured way to run those demonstrations, and it signals a broader shift in bioenergy thinking: the microbes may be the engine, but increasingly, the materials around them are the engineering that keeps the engine running.

Subject of Research: Enhancement of dark fermentative biohydrogen production using waste-derived hydroxyapatite additives

Article Title: Integrating process parameters and green hydroxyapatite (HAP) strategies for sustainable dark fermentative biohydrogen production

Article References: Zisha, S. Z., Che Man, H., Omar, R., Abdul Rashid, S., Tan, J. P., Abdul Manaf, S. F., Mumtaz, T., & Jamali, N. S. (2026). Integrating process parameters and green hydroxyapatite (HAP) strategies for sustainable dark fermentative biohydrogen production. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38274-x

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38274-x

Keywords: biohydrogen, dark fermentation, hydroxyapatite, waste valorization, circular economy, bioprocess optimization, cell immobilization, pH buffering, anaerobic digestion, sustainable energy, green synthesis, microbial consortium

The new study, published in Applied Microbiology and Biotechnology, builds on earlier work showing that EnfR acts as a crucial regulator of carboxydotrophic metabolism, the suite of reactions that lets T. onnurineus NA1 thrive on carbon monoxide. EnfR controls the expression of the codh-mch-mnh3 gene cluster, which encodes the carbon monoxide dehydrogenase and associated membrane complexes that funnel electrons from CO oxidation into energy conservation. But carbon monoxide is only half of the story. The archaeon also grows vigorously on formate, a one-carbon compound that is increasingly viewed as a promising liquid carrier for hydrogen and as a key intermediate in synthetic carbon-fixation pathways. Whether EnfR also governed formate metabolism was an open question.

To answer it, the researchers took a two-pronged approach: they deleted the enfR gene entirely and, in a separate strain, cranked up its expression well beyond normal levels. The results were striking in both directions. When enfR was deleted, cells grown on formate suffered a clear growth defect, confirming that the regulator matters even for this non-CO substrate. When enfR was overexpressed, the opposite occurred. In pH-stat batch cultures, a controlled setup in which pH is held constant by automated addition of base as acids accumulate, the overexpression strain grew significantly faster than the parental strain and produced markedly more hydrogen gas. A single transcriptional tweak, in other words, was enough to boost one of the organism’s most industrially attractive traits.

The obvious hypothesis would be that EnfR simply switches on the genes that handle formate directly. Formate-dependent growth in T. onnurineus NA1 relies on a gene cluster called fdh2-mfh2-mnh2, which encodes formate dehydrogenase, the enzyme that splits formate into carbon dioxide and electrons, together with a membrane-bound hydrogenase complex that uses those electrons to make hydrogen while pumping ions to generate energy. But when the team measured transcript and protein levels for fdh2 in the overexpression strain, they found no significant differences from the parental strain. The formate-splitting machinery itself was not being induced. Something else was responsible for the growth advantage.

To find out what, the researchers turned to transcriptomics, sequencing the messenger RNA of both strains to build a genome-wide picture of gene activity. The comparison revealed 219 genes with differential expression between the EnfR-overexpression strain and the control. That is a substantial rewiring for a single regulatory protein, and the affected genes clustered into several functionally coherent groups. Among the upregulated players were genes encoding a ferredoxin-dependent membrane-bound hydrogenase, a family of enzymes that oxidize reduced ferredoxin, a small iron-sulfur protein that serves as a cellular electron shuttle, and channel the electrons into hydrogen production at the membrane.

Also among the differentially expressed genes were 2-ketoacid ferredoxin oxidoreductases, enzymes central to the archaeon’s central carbon metabolism. These enzymes catalyze the oxidation of 2-ketoacids, intermediates in amino acid and sugar breakdown, and in doing so they load electrons onto ferredoxin. Together with the ferredoxin-dependent hydrogenase, they form a pipeline: central metabolism generates reduced ferredoxin, and the hydrogenase drains those electrons off to produce hydrogen. By adjusting the expression of both ends of this pipeline, EnfR overexpression appears to change how much reducing power flows through the cell and how efficiently it is converted into hydrogen gas.

A third notable group of affected genes encoded flagellins, the protein subunits that assemble into the rotating filaments archaea use for motility. Changes in flagellin expression hint that the overexpression strain may alter its swimming behavior or surface properties as part of its overall physiological response, though the study’s central finding lies elsewhere. The authors propose that the collective effect of the 219 differentially expressed genes is to modulate the levels of reduced electron carriers, particularly ferredoxin, inside the cell. In this view, the growth and hydrogen-production gains do not come from making more formate dehydrogenase, but from reshaping the redox landscape so that the existing formate-oxidizing machinery operates under more favorable conditions.

This redox-balancing interpretation carries real weight for biotechnology. Microbial hydrogen production is often limited not by the abundance of the key enzymes but by the traffic of electrons between them. When reduced electron carriers accumulate faster than they can be consumed, metabolism stalls; when they are scarce, energy conservation suffers. Engineering a regulator that tunes the expression of ferredoxin-linked oxidoreductases and hydrogenases in concert offers a way to keep that electron traffic flowing smoothly. The fact that EnfR overexpression achieved this without touching fdh2 transcription suggests that indirect, systems-level tuning can be as powerful as direct induction of the canonical pathway genes, a lesson that may generalize to other hydrogen-producing microbes.

The work also deepens understanding of how archaea, the third domain of life, orchestrate their energy metabolism. T. onnurineus NA1 occupies a niche where formate and carbon monoxide are abundant, and its regulatory network must decide how to allocate resources between these alternative fuels. EnfR’s dual role in carboxydotrophic and formate-linked metabolism suggests it functions as a hub connecting carbon and electron flow across substrates. Because the organism grows at high temperatures and thrives in conditions mimicking hydrothermal vents, it is also a robust chassis for industrial processes that would challenge many conventional microbes, and regulators like EnfR provide natural control points for optimization.

The research, funded by the Korea Institute of Ocean Science and Technology in-house program, was led by Bo Gyoung Choi and Sungjin Pyo, who contributed equally, together with Sung-Mok Lee, Ji-in Yang, Seong Hyuk Lee, Myeong-Eun Jegal, Hyun Sook Lee and Sung Gyun Kang, with correspondence from Hyun Sook Lee and Sung Gyun Kang. As hydrogen economies gather momentum worldwide, studies like this one show that some of the most useful engineering blueprints may already be written into the genomes of organisms living at the bottom of the sea. The next step will be to pin down exactly which of the 219 regulated genes deliver the biggest gains, and whether the same redox-tuning strategy can be transplanted into other production strains.

Subject of Research: Regulation of formate-dependent hydrogen production by the EnfR transcriptional regulator in the archaeon Thermococcus onnurineus NA1

Article Title: Physiological and transcriptomic changes by enfR overexpression in a formate- utilizing archaeon

Article References: Choi, B. G., Pyo, S., Lee, S.-M., Yang, J.-I., Lee, S. H., Jegal, M.-E., Lee, H. S., & Kang, S. G. (2026). Physiological and transcriptomic changes by enfR overexpression in a formate- utilizing archaeon. Applied Microbiology and Biotechnology. https://doi.org/10.1007/s00253-026-14047-x

Image Credits: AI Generated

DOI: 10.1007/s00253-026-14047-x

Keywords: Thermococcus onnurineus NA1, EnfR, archaea, formate metabolism, hydrogen production, formate dehydrogenase, ferredoxin, transcriptomics, gene regulation, redox balance, hydrothermal vents, biohydrogen

What makes Clostridium species so compelling as biocatalysts is the extraordinary breadth of carbon sources they can assimilate. The review emphasizes that these bacteria span the full range from C1 compounds such as carbon dioxide up to C6 sugars like glucose, a metabolic versatility few other industrial microbes can match. In the bioethanol sector, the relevant substrates are the C5 and C6 sugars liberated from sugarcane bagasse and straw—lignocellulosic materials left over after the sucrose has been pressed out—as well as vinasse itself. In the biodiesel sector, the star substrate is crude glycerol, the ten-percent fraction of every batch of biodiesel that plants often struggle to sell. Clostridium strains can ferment all of these, redirecting carbon that would otherwise be burned, landfilled, or discharged into waterways.

The metabolic machinery behind this flexibility is elegantly described in the review. Clostridium fermentation proceeds through two characteristic phases. During the acidogenic phase, cells grow rapidly and channel pyruvate—derived from glycolysis—into acids, chiefly acetic and butyric acid, via the enzyme pyruvate ferredoxin oxidoreductase, which also generates reduced ferredoxin that can be used to evolve hydrogen gas. As the culture acidifies and enters the solventogenic phase, the cells switch metabolic gears, re-assimilating acids and reducing them into solvents, most notably n-butanol, the flagship product of the classic acetone–butanol–ethanol fermentation. A further layer of sophistication comes from the NADH-ferredoxin oxidoreductase system, which links the cell’s NADH pool to ferredoxin and thereby determines how much reducing power flows toward hydrogen versus reduced end products such as ethanol, lactate, and butyrate.

For the biodiesel side of the biorefinery, the review focuses on three species in particular: Clostridium butyricum, Clostridium pasteurianum, and Clostridium beijerinckii. Crude glycerol fermentation splits along two branches. In the oxidative branch, glycerol is converted through glycolysis into pyruvate and then into the usual acidogenic and solventogenic products, including hydrogen and butanol. In the reductive branch, glycerol is dehydrated to 3-hydroxypropionaldehyde and then reduced to 1,3-propanediol, a molecule with real industrial cachet because it serves as a monomer for polytrimethylene terephthalate, a high-value polyester used in carpets and textiles. The balance between these branches depends on the species, the strain, and the operating conditions, which is precisely why the comparative analysis in this review matters for anyone designing a process.

The comparative data deliver some clear winners. When it comes to producing 1,3-propanediol from biodiesel-derived crude glycerol, C. butyricum emerges as the most efficient producer among the species examined. The review reports productivities reaching 44.16 millimoles per liter per hour when the crude glycerol was pretreated with activated carbon—a striking demonstration that impurity removal can pay dividends—while untreated crude glycerol still supported respectable productivities of 20.4 to 36.8 millimoles per liter per hour. Those numbers matter because crude glycerol is contaminated with methanol, fatty acid soaps, and catalyst residues, all of which can inhibit fermentation. The finding suggests that a modest purification step, rather than full refining, may be the sweet spot for industrial economics.

Hydrogen production tells a more sobering story. The theoretical maximum yield from glucose is four moles of hydrogen per mole of glucose, but yields from lignocellulosic hydrolysates—the sugars obtained from sugarcane bagasse and straw—remain well below that ceiling. The review identifies three culprits. First, the partial pressure of hydrogen in the reactor headspace thermodynamically constrains further hydrogen evolution, which is why reactor design and gas sparging strategies feature prominently in the literature. Second, the competing reduction of NADH toward ethanol, lactate, and butyrate diverts electrons away from proton reduction. Third, and perhaps most insidiously, the harsh pretreatments needed to deconstruct lignocellulose release inhibitors such as 5-hydroxymethylfurfural and other furan and phenolic compounds that poison fermentative metabolism. Strategies to mitigate these effects, including the use of zero-valent iron to lower hydrogen partial pressure and adaptive laboratory evolution to build tolerance, are discussed as part of the optimization toolkit.

One of the most intriguing themes in the review is mixotrophy—the ability of certain Clostridium species to assimilate organic and inorganic carbon simultaneously. Through the Wood–Ljungdahl pathway, these organisms can fix carbon dioxide while fermenting sugars, effectively squeezing additional carbon out of every molecule of substrate and even improving product yields by rebalancing the cell’s redox state. The authors highlight this capability as a distinctive and underexplored asset for biorefinery integration, since bioethanol and biodiesel plants emit carbon dioxide streams that could, in principle, be fed directly into fermentation alongside vinasse or hydrolysate. In a future integrated facility, the same microbial platform could consume C5 and C6 sugars, organic acids in vinasse, glycerol, and CO2 all at once.

The review also surveys the process engineering landscape, comparing reactor configurations and operational modes across the literature. Continuous stirred tank reactors, anaerobic fluidized bed reactors, and upflow anaerobic sludge blanket systems each offer different trade-offs between biomass retention, mass transfer, and hydrogen partial pressure control, and the optimal choice depends on whether the target product is a gas, a solvent, or a diol. Immobilized-cell configurations and hydraulic retention time optimization appear repeatedly as levers for pushing productivities upward. What unites these engineering questions with the metabolic ones is that Clostridium processes are highly sensitive to operating conditions in ways that traditional ethanol fermentation is not, making careful bioreactor design a prerequisite rather than an afterthought.

The authors are refreshingly candid about the gap between laboratory promise and industrial reality. Every productivity figure in the review comes from laboratory-scale experiments, and no technoeconomic assessment of an integrated Clostridium-based biorefinery has yet been performed. Closing the material loops of the bioethanol and biodiesel chains—turning vinasse, bagasse hydrolysate, and crude glycerol into hydrogen, n-butanol, butyric acid, and 1,3-propanediol—will require advances in metabolic engineering, bioreactor design, and process optimization before any industrial-scale deployment can be justified. The review frames these as prerequisites, not optional refinements, and calls explicitly for rigorous technoeconomic analysis and life cycle assessment of the integrated configurations.

Even with those caveats, the vision is hard to dismiss. The products with the greatest market potential—hydrogen as a clean fuel and chemical feedstock, n-butanol as a drop-in fuel component and solvent, butyric acid for animal feed and plastics, and 1,3-propanediol for biopolymers—can all be obtained from residues that the biofuel industries already generate in enormous volumes. As the world’s ethanol and biodiesel capacity continues to grow, the economics of waste valorization improve with every new plant. Clostridium bacteria, once famous for their historical role in the first industrial acetone–butanol fermentation a century ago, may be poised for a second act: as the microbial workhorses that convert the biofuel economy’s leftovers into its next revenue stream. The review, funded by FAPESP, CAPES, and the Spanish Ministry of Science and Innovation, provides the roadmap—now the engineering has to catch up.

Subject of Research: Clostridium-based biorefineries for valorizing sugarcane bioethanol and biodiesel byproducts

Article Title: Clostridium-based biorefineries in sugarcane bioethanol and biodiesel production: metabolic pathways, substrate diversity, and bioproduct yields

Article References: de Moraes Altafini, R., Bortolucci, J., Monico, D. A., Coca, M., & Reginatto, V. (2026). Clostridium-based biorefineries in sugarcane bioethanol and biodiesel production: metabolic pathways, substrate diversity, and bioproduct yields. Biotechnology for Biofuels and Bioproducts. https://doi.org/10.1186/s13068-026-02818-7

Image Credits: AI Generated

DOI: 10.1186/s13068-026-02818-7

Keywords: Clostridium, biorefinery, bioethanol, biodiesel, crude glycerol, vinasse, 1,3-propanediol, biohydrogen, n-butanol, butyric acid, sugarcane bagasse, metabolic engineering