Herbicide atrazine impairs metabolic plasticity of mixotrophic organisms: Evidence in photochemistry, morphology, and gene expression

Abstract

Herbicide pollution is a main form of water pollution. As a result of additional harms to other non-target organisms, it threatens the function and structure of ecosystems. Previous researches mainly focused on the assessment of the toxicity and ecological effect of herbicides on monotrophic organisms. Responses of mixotrophs as an important component of functional groups are rarely understood in contaminated waters, although their metabolic plasticity and unique ecological functions in ecosystem stability are a major concern. This work aimed to investigate the trophic plasticity of mixotrophic organisms in atrazine-contaminated waters, and a primarily heterotrophic Ochromonas was used as the tested organism. Results showed that the herbicide atrazine significantly inhibited the photochemical activity and impaired the photosynthetic machine of Ochromonas, and photosynthesis activated by light was sensitive to atrazine. However, phagotrophy was unaffected by atrazine and closely correlated with growth rate, indicating that heterotrophy helped population maintenance during herbicide exposure. Mixotrophic Ochromonas upregulated the gene expression level involved in photosynthesis, energy synthesis, and antioxidation to adapt to increasing atrazine after long-term exposure. Compared with bacterivory, herbivory increased atrazine tolerance of photosynthesis under mixotrophic status. This study systematically illustrated the mechanism by which mixotrophic Ochromonas respond to the herbicide atrazine at population, photochemical activity, morphology, and gene expression levels and demonstrated the potential effect of atrazine on the metabolic flexibility and ecological niches of mixotrophs. These findings will provide important theoretical reference for governance and management decision-making in contaminated environments.