Insights into the Transcriptomic Response of Two Aspergillus Fungi Growing in the Presence of Microplastics of Polyethylene Terephthalate Residues Unveil the Presence of Fungal Machinery for Possible PET Bioconversion into High-Value Chemicals

PET biodegradation remains limited due to its intrinsic properties—high crystallinity, hydrophobicity, and strong chemical stability. These characteristics lead to extremely slow degradation rates and contribute to PET’s persistence in the environment. Understanding how microorganisms respond at the molecular level when exposed to such a recalcitrant polymer is therefore essential. Living organisms express genes in response to their needs during development. When microbes are under critical conditions, such as when contaminants are present, they express genes encoding specific enzymes that attack the pollutant. In this study, a fungus isolated from the infected fruit of the plant Randia monantha was identified as Aspergillus terreus. It was tested for polyethylene terephthalate (PET) degradation, and the fungus Aspergillus nidulans was evaluated due to its previously reported recombinant cutinases for PET degradation. A microplastic polyethylene terephthalate (PET-MP) particle size of <355 μm for degradation was established, and a PET weight loss of 1.62% for A. nidulans and 1.01% for A. terreus was found. Additionally, the degradation of PET was confirmed by FTIR and SEM. This study also compares the transcriptomic profiles of Aspergillus nidulans and Aspergillus terreus during cultivation with PET-MP residues, which serve as a replacement for the carbon source. We present the first evidence of chitinase overexpression during direct exposure of PET to Aspergillus fungi. Interestingly, chitinase expression was detected in the crude extracts of A. nidulans and A. terreus during culture in the presence of PET residues, which replaced the carbon source. The chitinase produced by each fungus has a similar molecular weight of approximately 44 kDa. Chitinase activity was monitored over a 14-day cultivation period; from day 2, chitinase activity was detected in both cultures and continued to increase until day 14, when the highest values reported in this work were 24.88 ± 4.17 U mg−1 and 10.41 ± 0.47 U mg−1 for A. nidulans and A. terreus, respectively. Finally, we proposed a pathway for PET degradation by Aspergillus fungi that involves mycelial adherence and the secretion of hydrophobins, followed by the production of intermediates and monomers via esterase hydrolysis, and ultimately, the entry of monomers to the ethylene glycol (EG) and terephthalic acid (TPA) pathways, further suggesting these Aspergillus as candidates to produce valuable compounds under these conditions, such as muconic acid, gallic acid, and vanillic acid.