PHA Microplastic Aging Decreases N2O Sink Capacity: Released γ-Butyrolactone Decouples Denitrifying Electron Transfer and Oxidative Phosphorylation

Bacterial denitrification is a main pathway for soil N2O sinks, which is crucial for assessing and controlling N2O emissions. Biobased polyhydroxyalkanoate (PHA) microplastic particles (MPs) degrade slowly in conventional environments, remaining inert for extended periods. However, the impacts of PHA microplastic aging on the bacterial N2O sink capacity before degradation remain poorly understood. Here, the soil model strain Paracoccus denitrificans was exposed to 0.05-0.5% (w/w) virgin and aged PHA MPs. Although no significant changes in the molecular weights were observed, aged PHA MPs hindered cell growth and N2O reduction rates, leading to a surge in N2O emissions. 1H NMR spectroscopy and UPLC-QTOF-MS analysis identified γ-butyrolactone as the key component released from aged PHA MPs. Metabolic verifications at the cellular level confirmed its inhibition on the N2O sink and ATP synthesis. The γ-butyrolactone that protonated and hydrolyzed spontaneously in the periplasm would compete for protons with ATPase and destroy the coupling between denitrifying electron transfer and oxidative phosphorylation. Consequently, energy-deficient cells reduced the electron supply for N2O reduction, which did not contribute to energy conservation. This work unveils a novel mechanism by which PHA microplastic aging impairs the bacterial N2O sink and highlights the need to consider environmental risks posed by biobased microplastic aging.