The global energy crisis and escalating climate change necessitate urgent transitions from fossil fuels to sustainable alternatives. Biofuels, derived from organic biomass, offer a renewable solution with an 83% reduction in greenhouse gas (GHG) emissions compared to petroleum. ¹ They also bolster energy security, support rural economies, ² and exhibit a higher net energy balance than conventional gasoline. ³ However, scaling biofuel production faces hurdles, prompting innovations in microbial biotechnology.

However, scaling biofuel production remains a challenge. Researchers are advancing methods such as genetic engineering and microbial biomass optimization, ⁴ applying bio-refining techniques similar to petroleum processing. Among liquid biofuel, biodiesel is the second most widely produced globally, reaching 48 billion liters in 2021. ^{5, 6} As fossil fuel reserves decline and emissions rise, ⁷ optimizing biofuel efficiency and sustainability is imperative.

Fungi, with their unparalleled lignocellulose-degrading enzymes and lipid-accumulating capabilities, are emerging as game-changers in biofuel synthesis. This review analyzes fungi mechanistic advantages, recent progress, and persistent obstacles, providing a guide for development of sustainable fungal-based biofuel systems.

Biofuel production can be classified into four main generations, each defined by distinct feedstocks and technological innovations as following:

The first generation relies on edible sources such as corn, sugarcane, and animal fats. ⁸ However, their dependence on food crops has raised discussions about sustainability, particularly regarding food security and agricultural land allocation. ⁹

To overcome these drawbacks, Second-generation focus to non-food biomass, including agricultural waste and wood residues. ¹⁰ Although their potential environmentally favorable, the commercialization is constrained by high cost of processing procedures and technological barriers.

The third generation leverage microalgae, which grow rabidly, require minimal arable land, and produce fewer emissions than conventional crops. ^{11, 12} However, economical production and efficient methods remain key obstacles.

For that the fourth generation further refine this approach by using genetically engineered algal strains (e.g. Chlorella sorokiniana, Chlamydomonas reinhardtii) to enhance produce and carbon sequestration. ^{13, 14} While this approach is promising, these methods with ecological safety concerns requiring further study.

Some researchers propose a promising fifth advanced generation which combines synthetic biology with renewable energy (e.g. electro-fuels from green hydrogen) to accomplish a closed carbon cycle. ¹⁵ This cutting-edge approach aims to overcome the scalability and sustainability limitations of earlier biofuel generations.

Sustainable biofuel, also Known as non-conventional fuels, are derived from renewable biomass and can be used as a substitute for conventional fossil fuels including coal, petroleum, natural gas, and propane. By which methanol and bio-ethanol bends are frequently added to gasoline to improve its octane rating and lower greenhouse gas emissions. ¹⁶ In addition to reducing carbon emissions, biofuel also reduce Short-Lived Climatic Factors (SLCFs) which deteriorates air quality and has a negative impact on both agricultural production and human health. ¹⁷

Another key form of sustainable biofuel is biogas, produced through anaerobic fermentation. Biogas combustion generates a cleaner, hotter flame compared to conventional fossil fuel, making it an efficient energy source for heating and electricity. ¹⁶ However, certain biofuels can influence emissions of black carbon (BC) and organic carbon (OC), particularly during cooking. Studies indicate that while fossil fuel use increases BC and OC emissions, switching to biofuels can significantly reduce these pollutants. ¹⁷

A critical advancement in sustainable biofuels is Sustainable Aviation Fuel (SAF), which plays a pivotal role in decarbonizing the aviation sector. Before the COVID-19 pandemic, aviation contributed to approximately 10% of U.S. transportation emissions and 3% of total GHG emissions. ¹⁸ SAF is essential for reducing emissions in hard-to-electrify transport sectors, with targets aiming to scale domestic biofuel consumption to to3 billion gallons by 2030. Renewable biofuel (RB) further support the de-carbonization of media and heavy-duty vehicles, aligning with global climate mitigation strategies. ¹⁸

Fungi plays a pivotal role in various biological and ecological processes, making them vital in sustainable biofuel production ( Table 1). ¹⁹ Their unique metabolic capabilities offer several advantages over conventional biofuel production methods: 1.

Biomass degradation and Lignocellulose Breakdown

Fungi, especially white-root fungi, excel in decomposing complex organic compounds such as cellulose, and hemicellulose, and lignin-key component of plant biomass. ²⁰ This ability allows for efficient pretreatment of lignocellulosic feedstocks, which is crucial for bioethanol production. 2.

Utilization of Low-cost Row materials

Fungal biofuel production supports circular economy principles by utilizing abundant waste materials, including agricultural residues (e.g. straw, corn stover), forestry waste, and urban organic waste. ²¹ This reduces reliance on food crops, mitigating the “food vs. fuel” dilemma associated with first-generation biofuels. ^{22, 23} 3.

Enzymatic Efficiency in Biofuel Processing

Fungi produce a suite of lignocellulolytic enzymes essential for biomass conversion, including: •

Laccases, lignin peroxidase (LiP), manganese peroxidase (MnP), for lignin degradation. ²⁴

These enzymes enhance the efficiency of biofuel manufacturing by optimizing substrate breakdown. 4.

Fermentation and genetic modification potential

The yeast Saccharomyces cerevisiae is widely used in ethanol fermentation, and genetic modifications can further increase ethanol yield and substrate utilization. ²⁶ Mucor indicus and Trichoderma reesei can directly synthesize biofuel (e.g. biodiesel, biohydrogen) from biomass, offering alternative pathways for mycofuel production. ^{27, 28} 5.

Climate and environmental benefits

By decomposing organic materials, fungus biofuel system contribute to sequester carbon and decreased GHG-emissions when compared to fossil fuels. ²⁹ Moreover, by strengthening soil structure, nutrient cycling and lowering reliance on chemical pesticides and fertilizers, fungal mycelium improves soil health. ³⁰ 6.

Support for ecosystem and biodiversity

As fungal-based biofuel production increases soil biodiversity and decreased land-use conflicts, it consistent with sustainable agriculture, and offering sustainable energy source while promoting ecosystem resilience. ³⁰

While fungal-based biofuel production offers significant advantages, several technical, economic, and environmental challenges must be addressed before large-scale commercialization can be achieved ( Table 2). 1.

Substrate variability and enzymatic efficiency

The heterogeneity of lignocellulosic feedstocks (e.g. agricultural residues, forestry waste) affects fungal enzymatic performance, leading to inconsistent biomass degradation. ³¹ Variation in lignin content, cellulose crystallinity, and pretreatment requirements necessitate strain-specific optimization to maintain hydrolysis efficiency. ²⁰ 2.

Economic viability and process costs

Fungal biofuel production remains cost-prohibitive compared to conventional fuels due to: •

High expenses in enzyme production and purification. ³²

Large-scale fungal cultivation for biofuel may have unintended ecological consequences, including: •

Biodiversity disruption in ecosystems where fungal monocultures are introduced. ³³

In fact it is interesting to address some of these limitations in future perspectives, such as genetic engineering to enhance fungal enzyme secretion and substrate adaptability, ²⁴ integrated biorefineries to improve cost-efficiency and sustainability ¹⁸ and life-cycle assessments (LCAs) to evaluate environmental trade-offs. ³³

Lignocellulose represents Earth’s most abundant renewable carbon source, composed of carbohydrates polymers (cellulose and hymicellulose) and the aromatic polymers lignin. ³⁴ Fungi possess remarkable enzymatic capabilities that enable efficient lignocellulose deconstruction, making them indispensable for sustainable biofuel production. This section reported key fungal enzymes and their roles in biomass conversion. 1.

Cellulose-degrading enzymes

The cellulose breakdown system involves three primary enzymes working synergistically:

Rhizopus oryzae lipase shows exceptional methanol tolerance (>20% v/v), crucial for biodiesel synthesis. ⁴³

Form the above, the synergistic action of fungal enzymes enables complete biomass utilization, with modern metabolic engineering further enhancing catalytic efficiencies. Continued optimization of enzyme cocktails and fermentation processes will be pivotal for economically viable mycofuel production. ⁴⁴

There is a persistent disparity between global energy demand and the production capacity of both fossil fuels and biofuels. Fossil fuels originate from extensive geological processes in which organic biomass undergoes prolonged exposure to high pressure and temperature under anaerobic conditions. ⁴⁵ The precursor material, kerogen an insoluble organic polymer transforms into liquid hydrocarbons (petroleum) or natural gas, depending on the extent of carbon-chain breakdown. Petroleum primarily consists of hydrocarbons with 5–20 carbon atoms, whereas natural gas forms through further decomposition.

In contrast, biofuels are derived from renewable biomass (living or recently deceased organisms) and consist primarily of alkyl esters (propyl, ethyl, or methyl) with 16–24 carbon atoms. ⁴⁶ A key distinction between biofuels and fossil fuels is the higher oxygen content (10–12%) and negligible sulfur presence in biofuels, which influences their combustion properties and reduces pollutant emissions. ⁴⁷ The primary method for biofuel production is transesterification, a process that converts animal or vegetable oils into biodiesel by exchanging the R-group of an ester with that of an alcohol, typically catalyzed by acid or base. ⁴⁸ This reaction modifies triglycerides (saturated and unsaturated fatty acids) into semisolid fats, enhancing their suitability as fuel. Transesterification can be enzymatic, catalytic, or non-catalytic, with enzymatic approaches gaining prominence for sustainable biodiesel synthesis.

Fungi possess diverse metabolic pathways and robust enzymatic systems, making them promising candidates for alternative biofuel production. In order to improve lipid accumulation, which is a crucial component of biodiesel yield, fungal biotechnology has concentrated on metabolic engineering throughout the past two decades. When nitrogen is rare, oleaginous fungi exhibit high lipid storage. ^{49, 50} Lipid accumulation is then stimulated by a contribution of many factors including unbalanced carbon-nitrogen ratios, ⁵¹ low phosphorus availability and calcium shortage. ⁵²

Lipid production has also been optimized by genetic, changes i.e. fatty acid desaturation is increased by overexpression of the OLE1 gene in Saccharomyces cerevisiae, ⁵³ while lipid-producing pathways have been upregulated by the use of synthetic transcription factors and CRISPR-Cas9 activation systems. ⁵⁴ Their potential as biofuel is further increased by the production of long-chain polysaturated fatty acids by certain species. ⁵⁵

Increasing the production and efficiency of enzymes to improving lipid produce in fungus, transcription factors and lipid synthesis enzymes must be genetically modified. Genes like FAS (fatty acid synthase) and ACC (acetyl-CoA carboxylase) can be overexpressed or knocked off to greatly enhance lipid accumulation. ⁵⁶ Furthermore, it has been demonstrated that emerging techniques such as radio-frequency electromagnetic fields (RF-EMF) improve enzyme activity; Aspergillus oryzae produced 1.5-3 times as much α-amylase after being exposed to 2GHz RF-EMF. ⁵⁷ High-yield enzyme synthesis has been made possible by developments in molecular biology, including codon optimization, robust promoters, and genomic integration of numerous gene copies. ⁵⁸ In order to reduce production costs, bulk sample analysis has also been used to identify genetic variants such as SNPs connected to increased enzyme secretion in some fungal strains. ^{59, 60} Though, these advancements research on filamentous fungal gene regulation stile limited.

Fungi now days represent essential in the shift to sustainable biofuels, by offering genetic malleability, waste valorization and unparalleled enzymatic efficiency. Although scalability and cost issues still persist, these gaps are being rapidly filled by developments in integrated biorefineries, CRISPR-based metabolic engineering, and enzyme optimization. Future research can focus on strain enhancement by multi-omics methods, circular bio-economy models to cost-cutting, and maybe policy support for fungal biofuel commercialization. By addressing these points, fungal-based biofuel can become a cornerstone of worldwide renewable energy strategies, aligning with climate gals and supporting a carbon-neutral future.