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.

光合细菌（Photosynthetic bacteria, PSB）在全球氮循环中发挥关键作用，但微塑料（microplastics, MPs）对其氮代谢的影响仍有待深入阐明。本研究系统探究了聚氯乙烯（polyvinyl chloride, PVC）微塑料对水生系统中光合细菌介导的氮循环的影响。光合细菌易于在聚氯乙烯表面定殖，形成厚度约0.75微米的单层生物膜。研究结果显示，微塑料对光合细菌活性及氮转化的影响呈浓度依赖性：中等浓度（0.1%）的聚氯乙烯可显著提升氨氮（NH4-N）转化效率与气态氮生成量，相较于对照组，氮气（N2）生成量提升10.95%。功能基因分析表明，在0.1%聚氯乙烯暴露条件下，nirS与nosZ基因表达上调3.83%，提示反硝化作用增强；而GDH2基因表达小幅下调（-4.19%），表明氨同化作用减弱。与之相反，更高浓度（1%）的微塑料会引发潜在的氧化应激反应（超氧化物歧化酶（SOD）活性升高），并抑制微生物生长。偏最小二乘路径模型（PLS-PM）分析（拟合优度Goodness-of-Fit = 0.937）证实，聚氯乙烯主要通过改变微生物活性、诱导氧化应激来影响氮循环，而非直接促进一氧化二氮（N2O）释放。本研究首次定量证实了微塑料可通过生物膜形成与氧化还原调控，调节光合细菌介导的氮循环，为解析微塑料在水生氮循环中的生态效应提供了全新视角。