Fungal biosorption is an environmental biotechnology based on the ability of the fungal cell wall to concentrate harmful water pollutants. Among its advantages are its simplicity, high efficiency, flexibility of operation, and low cost. The biosorptive performance of fungal pellets is getting growing attention since they offer process advantages over the culture of disperse mycelia, such as an enhanced biomass separation, and a high resilience in severe environmental conditions. In this review, biosorption capacity of fungal pellets towards heavy metals, dyes, phenolic compounds, humic substances, pesticides, and pharmaceuticals was reviewed. Available data about the adsorption capacity of pellets, their removal efficiency, and the operational conditions used were collected and synthesized. The studies relying on biodegradation were discarded to present only the possibilities of fungal pellets for removing these concern pollutants through biosorption. It was found that the biosorption of complex mixtures of pollutants on fungal pellets is scarcely studied, as well as the interfering effect of anions commonly found in water and wastewater. Furthermore, there is a lack of research with real wastewater and at pilot and large scale. These topics need to be further explored to take full advantage of fungal pellets on improving the quality of aquatic systems.

Biosorption on fungal pellets constitutes a promising way of removing precious metals, which are often present at dilute concentrations in wastewater. Herein, we studied the Ag and Au biosorption by Aspergillus tabacinus and Cladosporium cladosporioides pellets. For A. tabacinus pellets the optimum pH values for the biosorption of Ag and Au were 5 and 4, respectively, while for C. cladosporioides granules, the best-suited values were 3 and 4, respectively. Biosorption kinetics of both metals were also studied at low adsorbate concentrations (1 mg/L) and the pH values mentioned above, and the contact times that allow maximum recovery of the two metals were defined. At the pH values estimated as optimum, A. tabacinus pellets adsorbed greater amounts of Ag than C. cladosporioides pellets, while for Au the opposite occurred. We found that the pseudo-second-order model adequately represents Ag and Au biosorption kinetics under the conditions tested. Due to the growing demand and limited availability of these metals, their recovery from aqueous residual solutions is economically attractive and desirable in the expanding circular economy scheme.

Solid-state fermentation (SSF) is a bioconversion process that utilizes a solid substrate that does not dissolve in an aqueous medium. Microorganisms grow on the surface of the substrate and penetrate its solid matrix to extract essential nutrients for their development. SSF is characterized by minimal free water, with a substrate moisture content maintained above 70%, and involves three interconnected phases -- gaseous, liquid, and solid. This protocol describes the use of wheat bran, an agro-industrial byproduct, as the base substrate for enzyme production in a rotary system. The substrate is supplemented with an inducer, such as chitin, chitosan, starch, or cellulose, to promote the synthesis of hydrolytic proteins. The system is highly adaptable, allowing the use of different fungal forms, including mycelium, spores, or pellets. In the methodology described, the inducer and substrate are mixed at a ratio of 1:100 (w/w), sterilized via autoclaving, and adjusted to the desired moisture level with sterile water. The fungal inoculum is then added, and the rotary system operates at 10 rpm to ensure adequate mixing and oxygenation. The system is incubated for 6-8 days under optimal growth conditions for mesophilic or thermophilic/thermotolerant fungi, enhancing its versatility. Following incubation, the enzyme is easily extracted using an appropriate cold buffer (e.g., acetate, citrate, or phosphate), depending on the type of enzyme. The extract is clarified through centrifugation and filtration to obtain a cell-free supernatant. The enzyme can then be further concentrated or purified as needed. The results demonstrated a 4-6-fold increase in enzyme activity compared to submerged fermentation (SmF), highlighting the effectiveness of the system. Its adaptability to different substrates, inducers, and fungal species makes it a valuable tool for various biotechnological applications.

Pellets are ultrastructural configurations of filamentous fungal biomass that form during growth in submerged culture. This growth pattern offers advantages for controlling and stabilizing bioprocesses through biomass immobilization, reduced medium viscosity, and facilitated compound extraction. These benefits are particularly valuable for bioremediation, synergistic applications with biomaterials, and industrial metabolite production. However, fungal pellets also present challenges, such as limited oxygen diffusion to the pellet core, inconsistent pellet homogeneity, and decreased productivity. Factors such as electrostatic interactions, hydrophobicity, and culture conditions influence pellet formation. Currently, optimization efforts for pellet production focus on evaluating parameters, such as: pH range, agitation rate, pellet formation time, carbon source, additive agents, trace metals, and inoculum concentration, among others. Fungal pellets are increasingly recognized as promising platforms in emerging environmental biotechnology due to their versatility in pollutant removal and sustainable processing. Unlike previous reviews, this work provides an integrated and up-to-date perspective that combines the fundamentals of pellet formation with recent advances in their environmental and industrial applications, highlighting strategies for optimization and scalability. This review focuses on analyzing the most relevant aspects of fungal pellets, including their formation, optimization, and biotechnological applications. Given the growing importance of fungi in contemporary biotechnology, a state-of-the-art review of fungal pellets is warranted. This includes presenting an updated overview of processes for generating fungal biomass with enhanced handling, based on the use of fungal granules, an essential component for the implementation of efficient biotechnological processes using model fungal pellets with relevant industrial applications.