Approaches for Producing Fungal Cellulases Through Submerged Fermentation

Fungal cellulases are the most sought-after biological molecules produced from microbial sources in the last four decades. Owing to their emerging applications in the bioenergy industry for hydrolyzing cellulose, for which they are the most abundant source on this planet, research trends are shifting heavily toward adapting to submerged fermentation. However, filamentous fungal species, which are efficient cellulase producers, are well-adapted to low-moisture solid support as the substrate, such as in nature. Therefore, various fermentation strategies are currently being investigated to adapt them to submerged fermentation for large and high-quality production of cellulases. Emerging research trends, such as the use of inexpensive feedstocks, nutrient and/or culture optimization, innovative bioreactor designs, microparticle-assisted fungal growth, and innovative genetic engineering approaches, are some of the recent efforts by researchers to exploit the full potential of these biological molecules. This review discusses some of these strategies and their success rates in various research conditions. In addition, specific focus was provided to both increasing the market value of cellulases and the innovative strategies required to enhance their production on an industrial scale.


Introduction
Enzymes are protein-structured biocatalysts capable of catalyzing specific reactions in living systems.However, enzymatic applications in various industrial sectors have gained interest since their discovery outside of cells.Their major advantages over chemical catalysts are their specificity, mild condition requirements, and rate of acceleration.However, other benefits that make them preferable in various industries are their environmentally friendly nature, use in various applications, and lower energy requirements [1].Enzymes can be named according to their substrate and catalytic actions followed by the suffix-ase.For example, cellulases hydrolyze cellulose.Thus, they are called 1,4 beta-D-glucan 4-glucanohydrolase or simply cellulases.
Cellulose is the most abundant plant biomass on this planet [2].The lignocellulosic biomass comprises 40-60% cellulose, 10-40% hemicellulose, and 15-30% lignin and can provide 14% of the world's total energy requirement, which is currently met by fossil fuels [2][3][4].According to some speculations, if adapted to its full potential, bioenergy from biomass can be sufficiently generated to satisfy 27% of the world's transportation fuel by 2035.However, prominent problems in the current infrastructure remain troublesome in realizing the full potential of lignocellulosic biomass.
Cellulose also occupies the largest fraction of lignocellulosic biomass.However, it requires an extensive pretreatment step to release simple sugars (glucose) before they can be converted to transportation fuels, such as ethanol [5].Cellulases are a group of enzymes that can be produced from microbial sources and can help reduce the energy load and cost of pretreatment [6].As per the International Union of Biochemistry and Molecular Biology (IUBMB), cellulases are grouped into three categories: endocellulases, exocellulases (cellobiohydrolases), and cellobiases (β-glucosidases) [7].Endocellulases attack glycosidic bonds within the cellulose chain, while exocellulases hydrolyze terminal ends.Cellobiases act on cellobioses to convert them into glucose.Interestingly, cellobiose inhibits cellulases, while the readily ongoing conversion process to glucose reduces the inhibition of this product [5,8].Therefore, a cocktail of the three main enzymes yields efficient hydrolysis of cellulose compared to a single enzyme.Hence, microbial strains that can secrete all three types are preferable to single cellulase-producing microbial strains for the industry.
Cellulases are produced by a wide range of microbial species across the global niche [6].While some bacterial species, such as Bacillus subtilis, are being explored for cellulase production, the main industrial producers are filamentous fungal species belonging to Penicillium, Aspergillus, and Trichoderma [6,7,9,10].However, research is ongoing to identify the best strains that can produce various types of cellulases without the need for expensive feedstocks and culture conditions [5].Microbial production of cellulases and other products that add value is carried out by solid-state and submerged fermentations, as explained elsewhere [5,11,12].The filamentous fungi, which are also the top industrial producers of cellulases, prefer solid-state fermentation (SSF) as they have adapted to low-moisture environments, such as decaying wood [5,12].However, SSF is not the preferred mode of the industrial production of microbial bioproducts due to several underlying problems associated with expanding the process [5].Therefore, submerged fermentation (SmF) is more appropriate for industrial implementation.However, a major barrier with the fungal SmF mode is the low enzyme activity, which is still under extensive research [13].Therefore, this review focuses on novel approaches to produce fungal cellulases under submerged fermentation.These are namely the use of innovative or renewable feedstocks [14], media/culture optimization via statistical optimization techniques, such as response surface methodology (RSM) to enhance enzyme activities [10,13], the use of innovative additives such as microparticles [15], and genetic engineering [16].All these topics are discussed in detail, along with the market value of cellulases, which is expected to increase in the near future following the adaptation of greener energy options [7].This review also highlights some of the ideal approaches to increase the production of fungal cellulases on an industrial scale.

Fungal Cellulases as a Key to Second-Generation Biofuels
The constant depletion of fossil fuels and the air pollution resulting from burning fossil fuels exacerbate the current world.It is envisioned that the consumption of the globally most abundant source of biomass is lignocellulose, while the eco-friendly hydrolysis by cellulases makes these enzymes highly desirable in bioenergy (biofuels, viz.methane and ethanol) research and industry.Biofuels generated using lignocellulosic biomass are second-generation biofuels, which are superior to the food crop-dependent first-generation biofuels [17].Conversely, the complexity of the lignocellulosic biomass due to cellulose, lignin, and hemicellulose is a challenge that manufacturers of secondgeneration biofuels face.The multistep transformation of the lignocellulosic biomass into usable and value-added commodities is not as easy as it sounds.The biggest challenge is to break down the raw biomass into simple sugars, followed by the bioconversion of these components into biofuels and value-added commodities [18].Environmentally hazardous chemical hydrolysis is fully replaceable by eco-friendly enzymatic hydrolysis [19,20].One eighty billion tons of raw lignocellulosic biomass on our planet makes it the most desirable inexpensive raw material, and the cellulases, in turn, have a plethora of appli-cations in industry [21].However, the high production prices of the cellulases are a bottleneck to making this commercially viable.It is envisaged that using lignocellulosic biomass for ethanol production in the transportation sector would make cellulases the most in-demand industrial enzyme.The greatest potential of fungal cellulolytic enzymes lies in ethanol production from biomass through the enzymatic hydrolysis of cellulose; however, low thermostability and low titer cellulase production results in high enzymatic costs [21,22].Nonetheless, merchantable production of ethanol from lignocellulosic biomass depends on the production of cellulases, whose finances require upgrading.

Other Industrial Applications of Cellulases
The importance of microbial cellulases in prospective applications to the lignocellulosic biomass started gaining attention in the early 1950s.The widespread need for cellulases is because the cellulose polysaccharide constitutes 50% of the globally most abundant source of biomass, i.e., lignocellulose, thereby making it an industrially desired raw material for manufacturing products that add value to the bioenergy sector.Annually, 100 billion tons of lignocellulosic biomass is accumulated from various agricultural and waste resources.Efficient and sustainable utilization of lignocellulosic biomass can be achieved by cellulolytic hydrolysis by cellulases.Indeed, Anselme Payen first achieved the discovery and isolation of cellulolytic enzymes from plants [23].Marine algae viz.Ulva and numerous prokaryotes, i.e., bacteria from the genus Glucanobacter and Agrobacterium, also contribute to the hydrolysis of lignocellulosic biomass [24,25].However, fungi were concluded to be the dominant and efficient cellulase producers globally.
Cellulases have played a significant role as biocatalysts for many decades, which has enlightened their possible manufacturing applications.Value-added products from lignocellulosic biomass by employing cellulases are a recent concept.Industrial demand for fungal cellulases has risen significantly over the last few decades, especially for strains capable of producing stable enzymes under harsh industrial conditions.The possibility of actively converting cellulose into simpler components for conversion to transportation fuel can make cellulases the most desirable industrial class of enzymes [26].
Widespread applications of cellulases in food are diverse, viz.cereal grains, polishing, feed supplements, and flavor enhancements.Moreover, cellulases are involved in improving the digestibility of animal feeds [27].Flavonoid extraction from the seeds and flowers is also carried out by cellulases.The food sector utilizes cellulases for bakery, juice, and wine processing, as cellulases provide enhanced wine filtration, improved pulp hydrolysis in the juice industry, and enriched texture and quality of bakery products [28].Cellulases are efficient and preferred in extraction because they ensure less heat damage and higher yields.
Biofuel production using lignocellulosic biomass hydrolyzed by cellulases has the potential to be the world's largest industry, yet requires these enzymes.Further, the pulp and paper industry has great market demand for stable cellulases.Cellulase cocktails are very effective in sustainable, environment-friendly detergents, and the utilization of cellulases in the textile, paper, and detergent industries is massive.Hence, demand has increased [31,32].In the textile industry, pumice stone washing of the jeans reduced the capacity of the jean load, while a fifty percent higher jean load can be achieved by replacing stone washing with cellulases [33].Additionally, acidic T. reesei endoglucanase II is employed in nontoxic fabric biopolishing [34].

Current Market Value of Cellulases
Cellulases comprise approximately 3/4 of the total demand for industrial enzymes in many industries mentioned above, making them a frontrunner in global enzyme production and consumption [35].The most recent global market value of cellulases was estimated at 1.621 billion dollars in 2022, which is expected to increase at a compounded annual growth rate (CAGR) of 6.94% from 2022 to 2032 [36].In comparison, global biofuel enzymes are projected to increase from 905.2 million dollars (2020) to 1.3 billion dollars by 2026 with a CAGR of 7.3% [36], while the total enzyme market will grow from 10 billion dollars in 2019 to 14.7 billion in 2024 [37]; the CAGR of industrial enzymes is placed at 6.5% in terms of value [38].
The extensive research on cellulases by different manufacturers in the past two decades has led to the production of diverse blends of cellulases.The current market is dominated by Novozymes (Denmark) and Genencor, which have merged with DuPont (Netherlands), DSM (Netherlands), Solvay enzymes (Germany), and Dyadic (USA) [38].In fact, 75% of the entire industrial enzyme market, not just cellulases, is concentrated within the first three hosts listed [39].The indices used to indicate enzymatic activity are diverse; thus, the market value of cellulases is not standardized based on the quality of the product [40].The inconsistencies between commercial indices used in the market present another major hurdle: inappropriate production improvement and economic analyses of biofuel production from cellulases.
Owing to the diverse composition of the lignocellulosic biomass obtained from various sources, a specific cocktail of enzymes cannot provide a widespread solution.This allows us to identify crucial limitations in producing the ideal enzyme or ideal blend of enzymes that combine for a specific application.The Carbohydrate-Active En-Zymes (CAZy) database provides substantial information and plays a fundamental role in the improvisations of industrially needed enzyme preparations.The three approved approaches for preparing the desired mix of enzymes are (i) crude enzyme preparations from microbial consortia, (ii) defined cellulolytic enzyme preparation obtained from synergistic microbial interactions by fungal decomposers, and (iii) single component enzymes obtained from simple synergistic hydrolyzing systems for specific applications [41].Next-generation biological modification tools (-omics centered and high-throughput selection) are the driving force in characterizing these enzymes and promoting the progress in improved production of cellulases from diverse fungal strains along with studies on their synergism, the ideal cellulase cocktail, and relevant applications in the desired industry.
Due to the inducible expression of the cellulases, the chief expensive and demanding feature of industrial fungal cellulase production is delivering a suitable inducer.Additionally, bacterial cellulases are less desirable compared to fungal cellulases since bacterial cellulases mostly lack either of the three cellulolytic activities, particularly exoglycanase activity.From an economical and ease of process point of view, fungal cellulases are preferred due to simpler downstream processing.Fungal cellulases also have higher activities than their bacterial counterparts.

Cost Implications of Producing Cellulases From Renewable Resources
There are many economic challenges associated with producing cellulases, thereby making their applications expensive.Therefore, enzyme industries aim to reduce production costs.Hence, sustainable, economically reasonable, and readily procurable sources of nutrients for cellulase-producing microbes can significantly lower enzyme costs [42].However, the economic repercussions of producing cellulases from renewable biomass are manyfold and encompass multistep evaluations of the production process, from raw material selection to the market dynamics affecting the final product price.
Owing to its abundance and diversity, lignocellulosic biomass is an inexpensive and attractive substrate for cellulase production [43].However, sole dependence on conventional lignocellulosic biomass, such as forest residues, municipal waste, and agro-industrial resources, may eventually raise questions about land management and affordable availability due to their abundant applications [44].Additionally, lignocellulosic pretreatment is an energydemanding and expensive affair.Therefore, it is also advisable to explore unconventional resources, such as naturally growing unwanted weeds.For instance, Parthenium hysterophorus is among seven globally abundant and unwanted weeds growing in Australia, Asia, America, and Africa [45].Microbial fermentation of such renewable biomass would also allow environmental protection from these weeds and economic cellulase production.The environmental impact of SmF was higher than SSF [56] Evaluation of various fungal isolates 903.7 IU/mL and 800 IU/mL [57] *Filter Paper Cellulase Assay; SSF, solid-state fermentation; SmF, submerged fermentation.
Finally, Life Cycle Analysis (LCA) should be included to evaluate the environmental cost of utilizing various renewable resources.Life cycle assessment of cellulase production from solid-state fermentation of coffee husks has been reported [46].Similarly, a techno-economic evaluation of cellulase production by submerged fermentation was reported to compare the batch versus fed-batch process feasibility, whereby the fed-batch was concluded to be a more economically viable process [47].These studies imply that the feasibility of using renewable resources depends on overall process energy costs, fermentation time, and additional nutrient components.Cellulases are inducible enzymes, and the addition of lactose has been widely explored to induce their production, which also increases the final costs.There is no doubt that renewable resources and biomass management align with sustainability goals, yet the long-term availability and stability should also be considered.

Types of Fungal Fermentations for Cellulase Productions
Fungi are a diverse group of microorganisms that contain several types of cells, ranging from yeast and molds to mushrooms.These saprophytic microorganisms are best known for their wide adaptability strategies across different niches on the planet, making them also one of the oldest domesticated microorganisms by humans [48].Roughly, fungi can be divided into filamentous (molds) or nonfilamentous microorganisms (yeasts).Cellulases are hydrolytic enzymes best known to be produced from woodrot fungi.Two of the best-known genera for this purpose are Aspergillus and Trichoderma, containing several species that can depolymerize the cellulosic materials effectively because of their ability to secrete the complete cellulase systems [6,49].Two types of fermentations are currently employed for producing cellulases by fungal strains: solid-state fermentation (SSF) and submerged fermentation (SmF).SSF is the application of a solid substrate that can be lignocellulosic in nature for the growth of mycelia.The SSF mode does not have free water in the fungal media but contains enough moisture to sustain growth across the solid substrate; the solid substrate can also provide nutrients for growth.Conversely, SmF contains free water, and the mycelial structures are suspended in the fungal media.Interestingly, both types of fermentation have their merits and demerits in terms of production efficiency, cost, and environmental impact.From an industrial point of view, SmF is the most widely used method due to its relative feasibility for scale-up [50].
The evaluation of fermentation type (SSF vs. SmF) for fungal cellulase production has been one of the most prominent research topics since realizing the importance of these enzymes in the biofuel industry [51].It has been widely accepted that while SmF is the most feasible method for controlling fermentation at industrial scales, SSF has produced higher enzyme activities in various comparative studies.Table 1 (Ref.[52][53][54][55][56][57]) shows some of these comparative studies conducted in the last five years.Each method has its advantages and challenges over the other, as explained below.However, the main production method used on an industrial scale must have some inherent properties, which should be improved via intensive research.Therefore, in this review, strategies to overcome the challenges of SmF are discussed in detail in later sections.

Challenges in the Production of Fungal Cellulases by SSF
The inefficient distribution of resources, such as nutrients, oxygen, and moisture, is the most notable challenge in the production and adaptation of fungal cellulases under SSF [49].Consequently, the fungal cells do not grow to their fullest extent, thereby limiting the production scale.In addition, successful industrial adaptation requires a large space and workforce to control fermentation parameters.To solve the problem of distribution, strategies such as rotary drums are employed [58].However, fungal cells are prone to shear, thereby rendering such strategies inefficient [49].Heat dissipation is another huge issue while operating SSF.The metabolic heat generated by the fungal cells during growth and enzyme production must be dissipated from the fermentation reactor.If the dissipation does not occur efficiently, the cells and the enzymes can become denatured.Therefore, small-scale bioreactors, which can be controlled efficiently, lead to larger yields from SSF than SmF (Ta- Mixed cultures Co-cultures of different fungal species [80] Microparticle-assisted enhancement Aluminum oxide, magnesium silicate, etc. [15] Thermophilic fungi Rasamsonia emersonii [16] ble 1) yet are still inadequate for industrial adaptations [59].
Hence, SmF has been widely investigated to overcome the ultimate challenge of extensive industrial adaptation.

Pros and Cons of the Production of Fungal Cellulases by SmF
A prominent challenge of SmF is the decreased enzyme activities obtained in fungal enzyme productions (Table 1).The main reason behind this is the shear stress of agitation on mycelial cells [60], the formation of large mycelial clumps [13], and carbon catabolic repression [61].Cellulase production by filamentous fungal species is a natural phenomenon that has been adapted due to the absence of simple sugars in the environment.Cellulases are then secreted to break down the complex cellulosic material and convert them into simple sugar monomers.However, in the presence of simple sugars, the carbon catabolic repression causes a reduction in the levels of cellulase production by the fungal species.Therefore, complex yet biologically degradable material can induce enzyme production in fungal fermentation systems.Consequently, choosing a suitable feedstock that can induce enzyme production is imperative.Another challenge in the submerged fermentation systems is the variability in the reactions of different microbial strains toward different feedstock, media composition, and fermentation conditions.Therefore, it is essential to optimize fermentation conditions according to the specific strain and/or feedstock.Excessive foaming during the aeration and agitation of the aerobic fermentations is another major issue in SmF [62].While foaming can be controlled with the help of antifoaming agents, adding such chemicals can affect the viscosity of the culture media and decrease the oxygen hold-up due to the increased viscosity.Similar problems arise due to the excessive growth of the fungal mycelium in the media [62], while product inhibition is another problem in the liquid media.Cellulases can break down the cellulosic components in the feedstocks, releasing simple sugars, which can have inhibitory effects on the enzymes.Finally, a lower dissolved oxygen uptake due to the production of denser mycelial clumps can also decrease the overall productivity of fungal fermentation systems for cellulase production.Notably, all these challenges are currently being addressed in research studies, and some of the recently proposed solutions are provided in the subsequent sections.

Novel Approaches to Solving the Challenges in SmF for Fungal Cellulase Production
Fungal cellulase production by SmF is growthassociated [63], and filamentous fungal strains tend to adhere to surfaces for optimal growth.The enzymes are produced due to the scarcity of simple fermentable sugars in the environment, and fungal strains release the enzymes to break the complex lignocellulosic biomass around them and release simple sugars for growth.Therefore, the growth of fungal biomass in the submerged state can be considered unconventional or unnatural, making this process dif-ficult for microbial adaptation.However, denser mycelial growth also hinders further growth and enzyme production in the submerged bioreactors, presenting several problems related to process design and efficient adaptation on industrial scales [64].The denser mycelial growth is also associated with poor microbial performance due to the inefficient distribution of resources, such as nutrients and oxygen.However, despite all these challenges, submerged fermentation remains the preferred mode for large scales due to the relative ease in scaling up and the better control of process conditions.Therefore, researchers in the last few decades have dedicated many studies to solving the underlying problems associated with fungal cellulase production in submerged fermentation conditions.The most common strategies successfully implemented in fungal cellulase production under submerged fermentation are listed in Table 2 (Ref.[6,9,10,[13][14][15][16][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80]).

Non-Conventional, Economical, and Renewable Feedstocks
To manufacture value-added products via fermentation, the most common and simplest feedstocks are readily fermentable sugars, such as glucose, fructose, sucrose, galactose, alongside many other mono, di, or oligosaccharides [81][82][83][84].However, decades of research have shown that each product can be produced in higher amounts using certain types of feedstocks compared to others [81,83,85].For example, in the case of citric acid, simple sugars such as glucose and fructose have been shown to be ideal feedstocks compared to others [85].The definition of a microbial feedstock is also ambiguous as it sometimes contains all the nutrient requirements of the microbial species, whereas sometimes, additional macro or micronutrients are required in a prepared feedstock.As mentioned earlier, fungal cellulase production depends on the presence of cellulose in the media [5].However, cellulose is not soluble in the medium, creating an accessibility problem for the fungal cells under submerged fermentation.Conversely, simple sugars such as glucose are also not feasible for fungal cellulase production because they do not induce cellulase production in fungal cells.Therefore, media containing soluble cellulose fibrils can induce enzyme production.Indeed, Iram et al. [9] recently proved the theoretical basis of this concept.
Several low-cost feedstocks containing cellulose in the media are ideal to meet the above-mentioned conditions.Some famous examples include sugarcane bagasse [71][72][73], wheat or rice straw [66,69], corn cobs or husks [86], and many other agricultural waste products, as mentioned in Table 2.The main problem with all high cellulose feedstocks is that they need harsh pretreatments to release simple cellulose fibrils for enzyme induction and simple fermentable sugars for the initial growth of the fungal species.In addition, all such feedstocks also lack the essential nitrogen sources required to produce microbial proteins efficiently.Distillers' Dried Grains with Soluble (DDGS) are a byprod-uct of bioethanol production, which shows high potential to produce fungal cellulases with the help of optimization strategies mentioned below [14].DDGS contains 26-33% proteins that help produce fungal enzymes, although it also has fibers (33-40%) that induce cellulase production [14].The enzyme production (0.66 IU/mL) was highest in mildly acid-treated DDGS, compared to crystalline cellulose (0.4 IU/mL) or glucose (0.3 IU/mL) [9].Conclusively, DDGS is an ideal feedstock for fungal enzyme production under SmF.

Nutrient Optimization
Nutrient optimization means making a microbial medium recipe that is best for a particular microbial outcome or product.Thus, for fungal cellulase production, cellulase activity, yield, and stability are some of the crucial factors.Alternatively, growth is not a desired factor, as dense mycelial clumps in the media will hinder the effective distribution of nutrients and dissolved oxygen in the media [5,12].Any nutrient element such as nitrogen source, minerals, or vitamins can positively or negatively affect the desired outcome.Therefore, it is crucial to know the effect (positive or negative) of each element and the ideal concentration in the media.
Many statistical techniques, such as the Plackett-Burman design (PBD), can be used to screen the effectiveness of each nutrient element.In addition, a range of concentrations can be optimized for various media elements using statistical optimization techniques, such as the response surface methodology (RSM).RSM is a method used to set pre-determined variables in such combinations that the desired level (maximum, minimum, or a specific value) can be obtained throughout a low number of experiments.The method is a fractional factorial design, which also provides response values at corresponding values of the dependent variables [87].After an initial screening via PBD or other methods, RSM can be applied to optimize three or more variables for one or more response variables.This technique has been extensively used in the past few decades to optimize nutrient formulations to obtain maximum levels of specific fermentation products, as described in an example below.
In a study by the authors, statistical optimization of various minerals and nitrogen sources was evaluated using DDGS hydrolysate as the main feedstock.It was determined that additional minerals do not significantly positively affect fungal cellulase production under submerged fermentation [10].This could be because DDGS, a byproduct of a microbial fermentation process (bioethanol production), already has some minerals.In addition, the pretreatment and subsequent neutralization step can further enhance the salt concentrations in DDGS hydrolysates.On the other hand, RSM helped produce a 262% increase in cellulase production [10].
Many other studies show the positive effect of nitrogen source amendment and optimization on fungal enzyme production.For example, in a study by Acharya et al. [88], peptone and ammonium sulfate were optimized to maximize cellulase production.The maximum cellulase activity was obtained at 0.125% peptone and 0.14% ammonium sulfate.On the other hand, using only 0.03% urea produced 0.15 IU/mL of cellulase activity.The main feedstock in this study was sawdust, and the concentration of sawdust was also optimized.Similar results were also obtained by Ghori et al. [89], where urea showed significant enzyme production.All such studies show the efficacy of using inexpensive feedstocks to analyze and optimize nitrogen sources for maximum enzyme production.

Culture Parameters Optimization
Culture parameters are pH, temperature, aeration, agitation, and inoculum size.All such parameters directly influence growth rate, enzyme production, and the production of undesirable products.There are many studies where such parameters have been evaluated and optimized using RSM for maximum fungal cellulase production under submerged fermentation.The main difference between these studies is the choice of feedstock and fermentation scale (shake-flask or benchtop bioreactors or pretreatment method before the fermentation stage).All such parameters, however, show a direct influence on cellulase production.Using DDGS as the main feedstock, Iram et al. [13] optimized the effect of inoculum size (1-10%), aeration (0.5-2 vvm), and agitation (100-500 rpm) in benchtop bioreactors (1.5 L) for fungal cellulase production under the submerged condition [13].Cellulase production increased by 37% following fermentation parameter optimization.The optimum conditions were 6.5% inoculum size with 1.4 vvm aeration and 310 rpm agitation.
Several other studies have shown the effect of fermentation parameter optimization on cellulase production [5,88].In the study by Acharya et al. [88], pH, inoculum size, temperature, and agitation were evaluated for maximum enzyme production.An optimal pH of 4, a temperature of 28 °C, and an inoculum size of 10 discs were reported.
The effect of agitation is crucial for fungal fermentation under submerged conditions for several reasons.Agitation directly affects the distribution of nutrients and dissolved oxygen in the media [90].However, it also affects negatively by imposing shear stress on the fungal mycelial structures.Therefore, optimizing this culture parameter is crucial [12].In addition to the agitation rate, the selection and design of the impeller are also important as it can influence the shear stress on microbial cells [91].The impeller helps in the dispersal of oxygen bubbles in the media.There are several types of impellers, yet the two main types are Rushton (radial flow) impellers and axial flow impellers.Flat-blade or Rushton impellers have shown high shear stress to mycelial mass in submerged fermentation [92].Therefore, innovative impeller designs specifically for fungal fermentation have been reported to reduce the shear stress on fungal morphology [92].Fig. 1 (Ref.[92]) shows one of the innovative impellers designed specifically for submerged fungal fermentation.

Study and Modeling of Enzyme and Growth Kinetics
Among various techniques to increase the productivity of the enzymes for a particular industrial product, enzyme kinetics can help better interpret results and optimize the production process.Enzyme kinetics involves modeling the rate of enzymes with respect to substrate conversion.Furthermore, quantifying enzyme-substrate interactions can contribute to a better understanding of hydrolysis, yield, and any reasons that may reduce the reaction rate.Due to the heterogeneous nature of the lignocellulosic biomass, the study of cellulase kinetics has always been simplified with different assumptions [93].Different sets of assumptions result in varying degrees of model accuracy, yet all ensure different strategies to improve the degree of hydrolysis.For example, in a study by Paulraj et al. [94] enzyme kinetics were used to assess the effect of inorganic salts on cellulase activity.
In fermentation process improvement, growth kinetics also play an important role in defining the quality of the desired product.Predominantly, enzyme kinetics (e.g., substrate consumption) are closely correlated with the growth of the microorganism.In a study by Sasikumar and Viruthagiri [95] growth kinetics were used to predict the constants in the polysaccharide fermentation process.Similarly, in a study by these authors, substrate conversion was correlated to cellulase production trends, and it was concluded that higher enzyme activity results from low substrate concentration in the fungal media [12].Therefore, it can be supposed that modeling enzyme and growth kinetics can provide useful insights into the control of the cell mass and enzyme activity in fungal fermentation systems.

Use of Suitable Fungal Strains or Mixed Cultures
Cellulase is a set of several different enzymes that act in synergy for the effective hydrolysis of cellulose.However, not all fungal species or their specific strains can produce all these enzymes, with most strains only producing a subset [6].Therefore, for effective adaptation at industrial scales, microbial strains that can produce many different types of cellulases are preferred due to the production of an efficient enzyme cocktail.For example, Trichoderma reesei RUT-C30, which is a top producer of cellulase enzymes, produces higher amounts of exoglucanases [6].Therefore, screening the best fungal strains for the desired cellulase is extremely important.With DDGS as the main fermentation feedstock, several A. niger and T. reesei strains showed promise for the production of many different types of fungal cellulases [6].
In addition to the ideal strains, a mixture or a coculture of several fungal strains has also been explored for maximum cellulase production under submerged fermentation [80,96].While the efficacy of using multiple microbial strains for a specific outcome has been proven many times for maximum product formation, undesirable microbial metabolic interactions can also hinder optimal production.Therefore, co-cultures should be designed by their synergetic effect on each other to produce the maximum output.

Use of Biofilm Reactors
Biofilm reactors are a special type of bioreactors, whereby the microbial species are facilitated with solid or semi-solid surfaces where they can form biofilms for the growth and production of desired products [97].Several microbial species can form biofilms following mutual interactions to create the desired product formation.However, in the case of filamentous fungal species, the main incentive is to give fungus support for mycelial growth while tuning its environment via fermentation parameter optimization for maximum productivity.A recent example of this is shown in the study by Xiros and Studer [79], where they used multiple fungal species for cellulase activity.How-ever, the concept is still in its infancy and should be explored further to assess the full potential of this technique for fungal submerged cellulase production.

Microparticle-Enhanced Fermentation
Recently, a newer technique where the microbial medium is supplemented with inert microparticles, such as aluminum oxide, magnesium silicate, or titanium oxide, to control the morphology of filamentous fungi under submerged fermentation has been explored [15].Several studies have been shown to increase microbial product formation, while decreasing the mycelial clumps in liquid media.Some examples include the studies conducted by Coban et al. [98] on phytase production and Coban and Demirci [99] on lactic acid production.Iram et al. [100] assessed the effect of microparticles (e.g., aluminum oxide and magnesium silicate) on fungal cellulase production using a shakeflask and 1.5 L benchtop bioreactor scales.The addition of microparticles increased cellulase production by several folds while decreasing the size of mycelial clumps in the media, resulting in a better distribution of nutrients and dissolved oxygen.These results show the positive effect of microparticles on fungal cellulase production under submerged fermentation.

Genetic Engineering
Wild-type strains of fungi have been studied tremendously for cellulase production.The research studies underline the major drawbacks of wild-type strains, such as the unavailability of hyper-producing efficient strains, incapacity of industrial-scale production, and limited tolerance to extreme conditions in industrial settings.Improvement in the catalytic properties of cellulases can withstand industrial-scale requirements.The high demand for improved production and desired properties of cellulases underlines the importance of tailored genetic manipulations to construct novel fungal strains that can be exploited industrially.Time-consuming and tedious conventional genetic manipulations are also not viable solutions.
Innovative sequencing methods and molecular-level tools in the present times reinforce the efficacy of genetic manipulations of filamentous fungi for improved properties and industrial-scale production.Moreover, next-generation sequencing has enabled access to the genome sequences of several industrially appropriate filamentous fungal strains to design relevant studies.The most rigorously studied and the major cellulase-producer filamentous fungi include Trichoderma reesei [101], Aspergillus niger [22,102], Neurospora crassa [103], and Penicillium oxalicum [104].Natural processes are slow owing to the scarcity of bioavailable nutrient components; therefore, industrial scale-up is required to speed up the processes and cell factory manipulations to enhance their enzyme production speeds and capacities.T. reesei, A niger Genetic engineering fermentation friendly morphology Enhanced agitation speed results in higher pellet diameter and higher cellulase productivity [102,114,115] A. fumigatus, P. chrysogenum CRISPR-Cas9-based genome editing system Targeted gene editing (deletion, addition, or inhibitions) [116][117][118][119] CRISPR-Cas9, clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9.
Implementation of metabolic engineering for efficient production and improved catalytic properties of cellulolytic enzymes by fungal strains is linked to understanding the mode of action of cellulases and their transcription level expression.As opposed to traditional genome editing tools, CRISPR/CAS has been exploited to test multiple target sites (Table 3, Ref. [102,[105][106][107][108][109][110][111][112][113][114][115][116][117][118][119]).Clustered, regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR-CAS) technology has emerged for developing competent genetically modified cellulolytic fungi strains.The preferred level of cellulase production and the futuristic approaches require a better understanding of the mode of action of cellulases and transcription level regulation of enzyme expression, which has progressed by utilizing technology such as CRISPR-CAS.Genetic manipulations are also employed to improve the enzymes' properties, namely stereospecificity, enantioselectivity, substrate specificity, enzyme activity, enzyme stability, and tolerance to environmental factors.

Conclusions
Cellulases are a group of enzymes that are crucial in biomass hydrolysis prior to their subsequent use in the generation of biofuels.Compared to other microbial species, fungal species produce high-quality cellulases.However, their effective adaptations at industrial scales require improvements in submerged fermentation.Fungal species produce low-quality enzymes with lower activities in submerged fermentations.New and innovative approaches are being developed to improve the activity of such enzymes under submerged fermentation methods.Some highly cited examples are innovative feedstocks, nutrient or culture optimization, innovative bioreactor designs, such as the use of biofilms, microparticle-enhanced fermentation modes, the use of thermophilic fungi, and genetic engineering.

Future Perspectives
Future studies should also focus on analyzing the potential of such approaches individually and assessing the combined effect of different approaches.For example, the choice and concentrations of microparticles should be assessed with a specific feedstock (e.g., DDGS) alongside media and culture optimization strategies.Similarly, genetically modified producers can be used to assess the effect of a particular feedstock, or a microbial strain can be genetically tuned to use a particular feedstock.All such research trends will surely provide new ways to reduce the excessive utilization of cellulases on industrial scales.