PHA Microplastic Aging Decreases NO Sink Capacity: Released γ-Butyrolactone Decouples Denitrifying Electron Transfer and Oxidative Phosphorylation.

Bacterial denitrification is a main pathway for soil NO sinks, which is crucial for assessing and controlling NO 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 NO sink capacity before degradation remain poorly understood. Here, the soil model strain 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 NO reduction rates, leading to a surge in NO emissions. H 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 NO 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 NO reduction, which did not contribute to energy conservation. This work unveils a novel mechanism by which PHA microplastic aging impairs the bacterial NO sink and highlights the need to consider environmental risks posed by biobased microplastic aging.