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

The global transition toward sustainable energy systems has intensified interest in biorefineries capable of converting renewable feedstocks and industrial residues into fuels and value-added chemicals. Bioethanol and biodiesel represent the two largest biofuel industries worldwide, collectively generating massive quantities of byproducts—including lignocellulosic biomass, vinasse, and crude glycerol—whose valorization remains both an economic and environmental challenge. Within this context, Clostridium species emerge as exceptionally versatile biocatalysts, capable of assimilating carbon sources ranging from C1 (CO 2 ) to C6 (glucose) and producing a broad spectrum of industrially relevant compounds. Despite considerable progress, a comprehensive assessment of how Clostridium -based bioprocesses can be integrated into bioethanol and biodiesel biorefineries—covering substrate diversity, metabolic pathways, and comparative bioprocess performance—remains needed. This review examines the role of Clostridium species as biocatalysts for valorizing the main byproducts of the bioethanol and biodiesel industries. For the bioethanol sector, we address the fermentation of lignocellulose-derived carbohydrates (C5 and C6 sugars from sugarcane bagasse and straw) and vinasse, detailing the acidogenic and solventogenic metabolic phases and the central biochemical pathways leading to hydrogen, butyric acid, acetic acid, and butanol. For the biodiesel sector, we review the fermentation of crude glycerol by Clostridium butyricum , Clostridium pasteurianum , and Clostridium beijerinckii , covering the reductive and oxidative metabolic branches that yield 1,3-propanediol, hydrogen, and butanol. Comparative analysis of yield and productivity data across species, reactor configurations, and operational modes shows that C. butyricum is the most efficient 1,3-propanediol producer from biodiesel-derived crude glycerol, reaching 44.16 mmol L −1 h −1 when the crude glycerol was pretreated with activated carbon, and 20.4–36.8 mmol L −1 h −1 with untreated crude glycerol. Hydrogen production follows a different pattern: yields from lignocellulosic hydrolysates remain well below the theoretical maximum of 4 mol mol −1 glucose, constrained by hydrogen partial pressure, by the competing reduction of NADH toward ethanol, lactate, and butyrate, and by inhibitors released during pretreatment. The mixotrophic metabolism of selected Clostridium species—enabling simultaneous assimilation of organic and inorganic carbon—is highlighted as a distinctive and underexplored asset for biorefinery integration. Clostridium species offer a versatile and industrially promising platform for closing material loops in the bioethanol and biodiesel production chains. The products with the greatest market potential—hydrogen, n -butanol, butyric acid, and 1,3-propanediol—can all be obtained from residual substrates generated by these industries. Whether this translates into economically viable integrated biorefineries remains to be established: the productivities reported to date are laboratory-scale, and technoeconomic assessment of the integrated configurations has not yet been performed. Advances in metabolic engineering, bioreactor design, and process optimization are prerequisites for any industrial-scale deployment of Clostridium -based bioprocesses.