Identification of bioplastic degrading fungi.

Since the middle of the last century, the demand for plastic materials have continuously increased reaching a global production of 368 million tons in 2019. Because of the large use of conventional plastics, their dispersion and accumulation into the environment have increased, causing the pollution of aquatic and terrestrial ecosystems and the release of micro- and nano- plastics. The persistence of plastics in the environment and the effects on flora and fauna have led researchers to focus on the development of new materials with a lower environmental impact: in this perspective, biodegradable plastics are emerging as an alternative to the traditional ones. The worldwide bioplastics production exceeded 2 million tons in 2019. However, although biodegradable and compostable plastics are designed to be mineralized by microorganisms in environments suitable for their disposal, if they are not properly collected or treated, they can accumulate in the ecosystems and cause harmful effects on the environment as well as non-biodegradable plastics. Therefore, it is necessary to investigate the mechanisms involved in their degradation by microorganisms, identifying the released by-products to evaluate the actual harm they may cause both in natural (soil and water) and in controlled environments, such as composting plants. In this context, the identification of fungal strains able to degrade different types of polymers may help strengthening the process. Indeed, in literature, several microorganisms are already known to be promising biopolymer degraders, fungi included. To this purpose, in this work, several fungi were isolated from a plastic-polluted landfill soil and their degradation ability against biodegradable polymers was explored. About one hundred of fungi, preserved at Mycotheca Universitatis Taurinensis, were grown in presence of a biodegradable aliphatic polyester, the poly-butylene succinate (PBS), as a representative of the biodegradable plastics. PBS was the sole carbon source present in the growth medium, and it was used in form of electrospinnated membrane that allowed a primary rapid selection of organisms able to use PBS as a source of nourishment. Fungal growth was monitored by measuring the colonies diameter and identifying possible halo zones on the PBS membrane. About forty fungal strains (belonging to the genera Alternaria, Aspergillus, Cladosporium, Fusarium, Penicillium and Purpureocillium) showed the capability to degrade the polymer and were assayed by a more selective screening, using PBS films as sole carbon source. The most performing fungi were observed under the low vacuum scanning electron microscope in order to assess the physical changes on the PBS surface due to the attack by microorganisms. One fungus, Purpureocillium lilacinum, with high capacity for colonization and degradation of PBS, was inoculated in liquid culture to study the polymer degradation mechanism: the HPLC-analysis studied the released monomers, revealing the presence of 1,4-butanediol whereas succinic acid was not detected. No oligomers were observed, suggesting that the fungus leads to the progressive hydrolysis of the ester bonds of PBS and to a fast utilization of succinic acid. Moreover, one of the main goals is to identify the enzymes involved in the process, in particular the presence of lipase, cutinase and esterase, primarily responsible for the degradation. The identification of the enzymatic pathway involved in the degradation process is under study since it would be useful to build up innovative enzymatic treatments to enhance the bioplastic transformation processes and to help the development of a plastic-based circular economy. Plastic waste may indeed become a resource of monomer recycling, ultimately enhancing the sustainability of the entire life cycle of bioplastic.