Nitric oxide release as a defense mechanism in marine microalgae against microplastic-induced stress

Nitric oxide (NO) has garnered significant attention as a critical regulatory factor and signaling molecule in plant growth. However, the effects of microplastic pollution on the release of NO by algae have not been reported. Thus, in this study, the release of NO by Skeletonema costatum and Gymnodinium sp. when exposed to different concentrations (5-50 mg/L) of polystyrene microplastics (PS, 0.1 μm) after a 15-day cultivation period was investigated. PS microplastics negatively impacted the photosynthetic processes of microalgae through several mechanisms, such as aggregation and adsorption, consequently affecting their physiological state. This physiological damage increased reactive oxygen species (ROS), which subsequently inhibited algal growth. However, the activity of nitrate reductase increased, and the released NO was able to scavenge ROS, alleviating oxidative stress and providing microalgae with a temporary adaptive advantage. This adaptability was only observed from day 3-7; beyond that period, microplastics continued to inflict irreversible toxicity on the microalgae. Notably, the toxicity of PS microplastics was less pronounced in S. costatum compared with Gymnodinium sp. Overall, these findings reveal that marine microalgae can release NO, mitigating the short-term toxic effects of microplastic pollution.