Effects of microplastics on nitrogen cycling in photosynthetic bacterial in aquatic system
Photosynthetic bacteria (PSB) play a crucial role in the global nitrogen cycle, yet the impact of microplastics (MPs) on their nitrogen metabolism remains poorly understood. This study systematically investigated the effects of polyvinyl chloride (PVC) microplastics on the nitrogen cycle mediated by PSB in aquatic systems. PSB readily colonized PVC surfaces, forming monolayer biofilms approximately 0.75 micrometers thick. The results demonstrated a concentration-dependent influence of MPs on PSB activity and nitrogen transformation: a moderate PVC concentration (0.1%) significantly enhanced both NH4-N conversion efficiency and gaseous nitrogen production, with a 10.95% increase in N2 generation compared to the control. Functional gene analysis revealed upregulation of nirS and nosZ genes (3.83%) under 0.1% PVC exposure, indicating enhanced denitrification, while the expression of GDH2 slightly decreased (-4.19%), suggesting weakened ammonia assimilation. In contrast, a higher MP concentration (1%) was associated with potential oxidative stress response (elevated SOD activity), and suppressed microbial growth. PLS-PM analysis (Goodness-of-Fit = 0.937) confirmed that PVC primarily affected the nitrogen cycle by altering microbial activity and inducing oxidative stress, rather than directly promoting N2O release. This study provides the first quantitative evidence that MPs modulate PSB-mediated nitrogen cycling through biofilm formation and redox regulation, offering new insights into the ecological effects of MPs in aquatic nitrogen cycles.