When Plastics Meet Microbes: Biodegradation in Aquatic Environments

In natural environments, the biodegradation of synthetic polymers depends on microorganisms that colonize polymer surfaces and secrete extracellular enzymes capable of depolymerizing these materials into low-molecular-weight compounds, which can subsequently be taken up and metabolized by the cells. To date, several microorganisms capable of hydrolyzing insoluble polymers have been identified. In contrast, limited information is available about aquatic microorganisms and the extracellular enzymes that mediate the biodegradation and mineralization of water-soluble polymers (WSPs). Water-soluble polymers are increasingly used in a wide range of applications such in cosmetics and home care products, and their incorporation into liquid formulations facilitates their entry into technical systems such as wastewater streams and wastewater treatment plants (WWTPs), as well as into natural aquatic environments.

In this study, we identified microorganisms and their enzymes that are capable of hydrolyzing WSPs. Hydrolases from various Pseudomonas species were identified and produced that hydrolyze structurally different ionic phthalic acid-based polyesters. In addition, the aerobic biodegradation of the polyesters in simulated fresh water with sewage sludge as inoculum was investigated. Beyond phthalic acid–based polyesters, synthetic poly(amino acids) represent another industrially relevant class of water-soluble polymers for which enzymatic biodegradation is still poorly understood. The most commercially successful synthetic poly(amino acid) is the water-soluble, anionic polymer poly(aspartic acid) (tPAA). Despite its widespread use, little is known about the biodegradability of tPAA. In this study, we investigated the interactions between tPAA and hydrolases derived from Sphingomonas sp. KT-1 and Pedobacter sp. KP-2, and assessed the effects of these enzymes on tPAA biodegradation. Detailed analyses using recombinant enzymes were conducted to characterize their activities and to elucidate potential synergistic effects during tPAA degradation. Finally, the individual and combined effects of the hydrolases were evaluated using an OECD 301F biodegradation test, demonstrating the potential of integrating specific enzymes into existing standardized tests to shorten testing times and to gain deeper insights into polymer biodegradation processes.