Evaluating the role of PET in fungal cellulase induction: Chemical signal or physical growth substrate?

The rapid expansion of synthetic textile production, particularly polyethylene terephthalate (PET) fibres, has intensified concerns over microplastic generation and the management of mixed-fibre textile waste. Fungal cellulases offer a low-impact route for biological upcycling of such materials; however, it remains unclear whether PET actively induces cellulase production or functions only as a physical support. This study explicitly tests the hypothesis that PET contributes to cellulase secretion through physical rather than biochemical mechanisms. Cellulase production by Aspergillus niger and Trichoderma reesei was evaluated using textile-derived substrates (100% cotton, 100% PET, and cotton-PET blends) and inert materials (glass, ceramic beads, sponge, and plastic flakes) under submerged (SmF) and solid-state fermentation (SSF). The highest enzyme titres were obtained on cotton-rich blends, with a 60% cotton/40% PET fabric yielding 0.546 ± 0.04 U mL⁻¹ by A. niger after 3 days, directly supporting the hypothesis that cellulase induction is governed by cellulose availability rather than PET chemistry. Pure PET did not stimulate cellulase synthesis, whereas PET-containing blends and inert supports enabled measurable enzyme release. Across non-cellulosic substrates, cellulase output increased systematically with available surface area (4-10 mm), reaching 0.415 ± 0.04 U mL⁻¹ on 10 mm ceramic beads, providing direct evidence that enzyme secretion under non-inducing conditions is driven by physical attachment rather than chemical signalling. By decoupling biochemical induction from surface-mediated effects, this work establishes PET as a structural scaffold rather than a biochemical inducer and highlights the scalability of surface-engineered, low-energy fungal fermentation strategies for valorising realistic blended textile waste streams.