# Biodegradation of polyethylene terephthalate microplastics by Paenibacillus naphthalenovorans PETKKU2: Response surface optimization and genomic evidence for an alternative degradation mechanism

## TL;DR

A new soil bacterium from Thailand can efficiently break down PET microplastics using a novel mechanism that doesn't rely on known enzymes.

## Contribution

The first report of Paenibacillus naphthalenovorans degrading PET-MP with a non-classical enzymatic pathway confirmed by genomic and analytical evidence.

## Key findings

- PETKKU2 achieved 9.48% degradation of PET-MP after optimization, a 96% improvement over baseline.
- Alternative degradation mechanism confirmed by absence of MHET intermediates and presence of thermostable lipases and dioxygenases.
- Mesophilic degradation at 37°C eliminates need for high-energy heating while maintaining high efficiency.

## Abstract

This study establishes Paenibacillus naphthalenovorans PETKKU2, isolated from landfill soil in Thailand, as the first reported member of this species capable of degrading polyethylene terephthalate microplastics (PET-MP). Initial screening identified PETKKU2 as the most efficient degrader among ten isolates, achieving 6.07 ± 0.18% weight loss after 35 days at 37°C. Response surface methodology optimization of pH, nitrogen concentration, and PET-MP loading enhanced degradation to 9.48 ± 0.21%, closely matching the predicted maximum of 11.15% and representing 96% improvement over baseline conditions. Integrated analytical characterization (FTIR, SEM, GC-MS) revealed an alternative degradation mechanism distinct from classical PETase-MHETase pathways. FTIR analysis confirmed extensive polymer oxidation with 41% reduction in ester carbonyl groups, while SEM demonstrated progressive surface erosion. Critically, the absence of mono(2-hydroxyethyl) terephthalate (MHET) intermediates, combined with whole-genome analysis revealing thermostable lipases, carboxylesterases, and dioxygenases, but no PETase/MHETase homologs, indicates novel enzymatic routes. Operating under mesophilic conditions (37 °C), PETKKU2 eliminates energy-intensive heating requirements while achieving performance comparable to established thermophilic degraders. These findings establish a promising platform for sustainable PET-MP bioremediation and advance understanding of alternative microbial plastic degradation mechanisms.