Response and adaptation of Chlorella pyrenoidosa to 6PPD: Physiological and genetic mechanisms

The extensive contamination of the tire antidegradant N-(1,3-dimethylbutyl)-N '-phenyl-p-phenylenediamine '-phenyl-p-phenylenediamine (6PPD) in aquatic environments have raised concerns about its potential threats to aquatic organisms. Here, the responses of green algae Chlorella pyrenoidosa (C. C. pyrenoidosa) to 6PPD exposure were investigated for the first time. The growth of C. . pyrenoidosa experienced three sequential phases, including inhibition, recovery and stimulation. Physiological and transcriptome analysis suggested that the growth inhibition was associated with the suppressed nitrogen assimilation and amino acid biosynthesis pathways, among which nitrate transporter (NRT) 2.1 was a key target of 6PPD. Molecular docking revealed the steadily binding of 6PPD to the substrate entry region of NRT 2.1 via hydrogen bonds and pi- cation interaction, blocking the acquisition of extracellular inorganic nitrogen. Along with the removal of 6PPD through abiotic processes and biodegradation, an adaptive metabolic shift in cells not only facilitated growth recovery but also triggered a compensatory stimulation phase. With regard to microalgal adaptation, upregulated DNA replication and repair pathways served to maintain the integrity of the genetic information, enhanced photosynthesis cascades and central carbon metabolism improved carbon flux and energy conversion to microalgal biomass, recovered amino acid biosynthesis produced essential proteins for multiple metabolisms. The results provide new insights into microalgal molecular responses to 6PPD exposure, facilitating a better understanding of ecological consequences of 6PPD in the environment.