A new study published in Case Studies in Chemical and Environmental Engineering tackles both problems at once. A research team from Khon Kaen University in Thailand, led by Siriporn Lunprom and Apilak Salakkam, combined a carefully optimized biomass-to-sugar pipeline with a pair of upgraded mathematical models that, for the first time in the Luedeking–Piret framework, explicitly account for the two phenomena that have quietly undermined previous PHB fermentation studies: cell death and the microbial consumption of the very polymer the cells have just made. The work demonstrates that sugarcane bagasse can yield record PHB titers when the process is run in fed-batch mode, and that modified kinetic models can pinpoint exactly when to stop a fermentation to avoid losing product.

The raw material preparation followed a two-stage strategy. First, bagasse was pretreated with alkaline hydrogen peroxide at pH 11.6 and 60 °C for six hours, a treatment that swells and delignifies the fibrous matrix. Composition analysis showed the pretreatment transformed the material from 36.5 percent cellulose, 23.1 percent hemicellulose, and 26.9 percent lignin into a fiber fraction containing 59.1 percent cellulose with lignin reduced to just 8.3 percent. Next, enzymatic saccharification using the commercial cellulase Cellic CTec3 together with a xylanase preparation released a hydrolysate containing 103 grams per liter of reducing sugar, dominated by glucose with a smaller xylose fraction. A fivefold concentrated version of the hydrolysate, containing nearly 499 grams per liter of sugar, served as the feeding solution for later fed-batch experiments.

The producing organism, Paracoccus sp. KKU01, was originally isolated from soil at a cassava starch plant and is known to convert sugars efficiently into PHB. In preliminary shake-flask fermentations on the bagasse hydrolysate, the bacterium grew steadily, consumed glucose over 108 hours, and accumulated up to 5.9 grams per liter of PHB. Interestingly, the xylose in the medium decreased only slowly, and genomic reasoning based on the KEGG database suggested that this strain, like other Paracoccus species, lacks the key enzymes for xylose metabolism, namely xylose isomerase and xylulokinase. The team therefore treated glucose as the true carbon source for growth and polymer synthesis, and confirmed that xylose played no role in either process.

When the process was scaled to a controlled 2-liter bioreactor, performance improved dramatically. The residual cell dry mass climbed rapidly to a peak of 32.9 grams per liter at 36 hours, and PHB production, first detected at 18 hours, reached 8.3 grams per liter by 42 hours — a 1.4-fold higher titer than in flasks, with productivity improving 3.3-fold thanks to precise control of pH, temperature, and dissolved oxygen. But then came the observation that motivated the entire modeling effort: after glucose was exhausted, both biomass and PHB concentrations fell. The polymer, stored intracellularly as a carbon reserve, was being mobilized by the starving cells as an energy source, a natural turnover response that classical fermentation models simply do not describe.

That limitation is rooted in the history of the models themselves. The logistic model, originally devised for human population growth, describes biomass rising smoothly toward a carrying capacity but can never predict a decline. The Luedeking–Piret model, developed in 1959 for lactic acid — a product its producers do not consume — treats product formation as the sum of a growth-associated and a non-growth-associated term and therefore predicts endless accumulation. To fix this, the researchers added a first-order cell death term to the logistic model and a first-order consumption term to the Luedeking–Piret equation, then reformulated both as piecewise change-point models. Growth and PHB accumulation follow classical kinetics until critical times t-crit and t-crit2, after which the models switch to exponential decay governed by a specific death rate and a specific PHB consumption rate. Crucially, the added complexity stays minimal: the AICc criterion confirmed that the extra parameters improved fit rather than chasing noise.

The numbers validated the approach. In the batch bioreactor, the modified logistic model raised the coefficient of determination from 0.9734 to 0.9927 and correctly captured the biomass decline beginning at a predicted critical time of 34.38 hours. The modified Luedeking–Piret model performed even more strikingly, lifting the fit for PHB from a poor 0.6960 to 0.9581 and predicting a PHB peak of 8.34 grams per liter at 41.75 hours with a consumption rate of 0.034 per hour. The fitted parameters also revealed the physiology of the process: the growth-associated coefficient approached zero, meaning essentially all PHB was synthesized during the stationary phase, consistent with nitrogen-limited conditions after the cells stopped dividing.

Fed-batch operation pushed the process further. By feeding concentrated hydrolysate twice, at 36 and 51 hours, whenever glucose dropped below about 15 grams per liter, the team raised the PHB titer to 14.1 grams per liter at 66 hours — roughly 70 percent above the batch result and the highest value yet reported for PHB production from sugarcane bagasse — while yield and productivity held steady at 0.14 grams per gram glucose and 0.21 grams per liter per hour. Again, the modified models proved their worth, tracking the biomass and polymer declines that began after 66 hours, likely driven by the depletion of nutrients other than carbon, since only glucose was being fed. The classical models, by contrast, predicted meaningless continuous accumulation and would have misled any attempt to time the harvest.

To show the framework was not tuned to one system, the team fitted the modified models to published datasets from five unrelated fermentations, including recombinant Ralstonia eutropha growing on glycerol, Bacillus megaterium on bagasse, Cupriavidus necator on corn hydrolysate, and two oleaginous yeasts producing microbial lipids from corn stover, one of them at 1000-liter pilot scale. Across these external systems the models achieved coefficients of determination as high as 0.997 and accuracy factors close to the ideal value of one, correctly identifying decay onsets where they occurred and reporting negligible decay where product levels never fell. This versatility matters because intracellular products such as PHB and microbial lipids share the same fundamental vulnerability: once carbon runs out, the producing cells eat their own reserves.

The practical implications reach beyond bioplastics. Because the modified models estimate the peak product concentration and the exact time it occurs, they offer a parsimonious engineering tool for scheduling harvests, minimizing losses, and designing feeding strategies, all while requiring nothing more than routine biomass and product measurements. The Thai team, supported by Khon Kaen University and the National Science, Research and Innovation Fund, emphasizes that further optimization of polymer content and productivity is still needed before bagasse-derived PHB becomes commercially viable. But the combination of a record titer from an abundant agricultural waste and a modeling framework that finally tells operators when the clock is ticking on their product brings the economics of bacterial plastic one significant step closer to reality.

Subject of Research: Kinetic modeling of PHB bioplastic production from sugarcane bagasse by Paracoccus sp. KKU01

Article Title: Poly(3-hydroxybutyrate) production from sugarcane bagasse by Paracoccus sp.: Kinetic modeling using modified logistic and Luedeking–Piret models incorporating cell death and PHB consumption

Article References: Lunprom, S., Jureemas, N., Komat, P., Moungprayoon, A., Khanpanuek, S., & Salakkam, A. (2026). Poly(3-hydroxybutyrate) production from sugarcane bagasse by Paracoccus sp.: Kinetic modeling using modified logistic and Luedeking–Piret models incorporating cell death and PHB consumption. Case Studies in Chemical and Environmental Engineering, 14, Article 101474. https://doi.org/10.1016/j.cscee.2026.101474

Image Credits: AI Generated

DOI: 10.1016/j.cscee.2026.101474

Keywords: PHB, bioplastic, sugarcane bagasse, Paracoccus sp. KKU01, kinetic modeling, Luedeking-Piret model, logistic model, fed-batch fermentation, PHB mobilization, biorefinery, cell death, lignocellulosic hydrolysate

The significance of the work lies in what it replaces. Virtually all industrial fermentation that yields nitrogen-rich products, from amino acids to nucleotides, depends on ammonium salts or urea as the nitrogen feedstock, and those inputs trace back to Haber–Bosch ammonia synthesis. That process, which combines atmospheric nitrogen with hydrogen under extreme pressures and temperatures, consumes an estimated one to two percent of global energy output and generates substantial carbon dioxide emissions as a byproduct of hydrogen production from natural gas. Biological nitrogen fixation, carried out by the nitrogenase enzyme complex in certain bacteria and archaea, performs the same chemical transformation at ambient temperature and pressure using ATP and electrons, offering a potentially far gentler route to usable nitrogen.

Azotobacter vinelandii has long served as the model organism for aerobic nitrogen fixation, a biologically awkward combination since oxygen both damages nitrogenase and competes for the electrons the enzyme needs. The bacterium survives this paradox through extraordinarily high respiratory rates that consume oxygen before it can reach the enzyme. Previous efforts had engineered A. vinelandii to excrete ammonium, the natural next step after nitrogen fixation, since the fixed nitrogen normally feeds the cell’s own biosynthesis. But the researchers behind the new study found that studies on actually using that excreted ammonium as a nitrogen source for other microbes remained limited, and that ammonium production levels were too low to be industrially interesting.

To push yields higher, the team took aim at NifA, the transcriptional activator that switches on the entire nif regulon encoding the nitrogen fixation machinery. They constructed A. vinelandii strains by integrating a nifA overexpression cassette, driven by the strong tac promoter, into the algU locus of the chromosome. This chromosomal integration strategy matters for stability: rather than relying on a plasmid that could be lost during cultivation, the engineered construct is inherited by every daughter cell, ensuring the nitrogen fixation program stays dialed up throughout a fermentation run. The resulting strain stably produced ammonium at a concentration of 1 gram per liter in simple flask cultures, a level the researchers describe as a solid baseline for the platform.

Flask cultures, however, are a proving ground rather than a production environment. The team then scaled the process into a jar fermenter, where they could control oxygen transfer, pH and feeding with far greater precision. By combining fed-batch cultivation, in which fresh carbon source is supplied incrementally to avoid depletion or overflow metabolism, with antifoam treatment to keep the aerated broth from foaming over and lactic acid addition to manage the culture’s chemistry, they raised ammonium production to 1.5 grams per liter. Each of these process interventions addresses a practical bottleneck: fed-batch keeps the energy supply matched to the nitrogenase’s enormous ATP appetite, antifoam protects oxygen transfer and prevents contamination pathways, and acid addition stabilizes the pH as ammonium accumulates and shifts the broth’s acid-base balance.

With a reliable ammonium source in hand, the researchers turned to the second half of the partnership. Corynebacterium glutamicum is arguably the most successful amino acid production organism in industrial biotechnology, responsible for the bulk of the world’s several-million-ton annual L-glutamate output, the flavor-enhancing component of monosodium glutamate. In the co-culture scheme, the engineered A. vinelandii functions as a living nitrogen fertilizer, continuously fixing atmospheric nitrogen gas and releasing ammonium into the shared medium, while C. glutamicum assimilates that ammonium and channels it through its existing metabolic machinery into L-glutamate. The division of labor elegantly sidesteps the need to purify or concentrate the intermediate: the product of one microbe is the substrate of the other, delivered in situ.

The results demonstrated the concept convincingly. Co-cultivation of the ammonium-producing A. vinelandii strain with C. glutamicum enabled the production of 2 grams per liter of L-glutamate from nitrogen gas. While that titer remains well below the tens of grams per liter achieved in conventional glutamate fermentations fed with commercial ammonium, the demonstration establishes a complete biological pipeline from atmospheric dinitrogen to a finished amino acid in a single vessel. The researchers frame the strategy as a contribution to the development of environment-friendly fermentation processes for producing various nitrogen-containing compounds from nitrogen gas, suggesting the platform could extend well beyond glutamate to other amino acids, nucleotides and nitrogenous chemicals.

The engineering choices embedded in the study reveal a careful reading of nitrogenase regulation. NifA sits atop a hierarchy of control mechanisms that bacteria use to avoid wasting energy on nitrogen fixation when fixed nitrogen is already available, a regulatory logic that normally shuts the system down precisely when engineers want it running. By overexpressing NifA from a constitutive tac promoter, the team effectively overrides the ammonium-sensing feedback that would otherwise silence the nif genes as product accumulates. Placing the cassette at the algU locus, which governs stress responses in A. vinelandii, reflects a deliberate choice of a neutral genomic landing site that disrupts native function minimally while granting stable, high-level expression of the activator.

Scaling challenges remain before such co-cultures could challenge conventional plants. Nitrogenase is an enzyme of notorious fragility and metabolic cost, demanding roughly sixteen ATP per molecule of nitrogen reduced, and maintaining two microbial populations with different physiological optima in one fermenter requires balancing oxygen availability, carbon source preference and growth rates. The fed-batch jar fermenter results, with their combination of antifoam and lactic acid management, hint at the kind of process engineering refinement that will determine whether titers can climb toward commercial relevance. The involvement of Kikkoman Corporation, a company with deep roots in fermentation technology, alongside academic groups at The University of Tokyo’s Department of Biotechnology and Collaborative Research Institute for Innovative Microbiology, suggests industrial interest in closing that gap. Several of the authors have filed patent applications on the work, underscoring its perceived commercial potential.

For now, the study stands as a proof of concept with a compelling narrative: a flavor compound that seasons much of the world’s food, assembled in part from the air itself, by two bacteria cooperating in a fermenter. If the platform’s titers can be improved through further strain and process optimization, nitrogen-fixing co-cultures could offer fermentation industries a route to decouple amino acid production from synthetic fertilizer inputs, trimming both energy demand and carbon emissions. The researchers position their work as a step toward fermentation processes that draw their nitrogen directly from the atmosphere, converting a century-old industrial dependency into a biological partnership.

Subject of Research: L-glutamate production from nitrogen gas via co-culture of engineered Azotobacter vinelandii and Corynebacterium glutamicum

Article Title: L-Glutamate production from nitrogen gas by co-culturing Corynebacterium glutamicum with Azotobacter vinelandii

Article References: Ito, Y., Yoshidome, D., Araki, Y., Ito, K., Hidaka, M., Kosono, S., & Nishiyama, M. (2026). L-Glutamate production from nitrogen gas by co-culturing Corynebacterium glutamicum with Azotobacter vinelandii. Applied Microbiology and Biotechnology. https://doi.org/10.1007/s00253-026-14031-5

Image Credits: AI Generated

DOI: 10.1007/s00253-026-14031-5

Keywords: Azotobacter vinelandii, Corynebacterium glutamicum, nitrogen fixation, L-glutamate, co-culture, nifA overexpression, ammonium production, Haber-Bosch, metabolic engineering, fed-batch fermentation, nitrogenase, sustainable biotechnology