Adverse effects of microplastics on the growth, photosynthesis, and astaxanthin synthesis of Haematococcus pluvialis

Due to the widespread pollution, microplastics (MPs) have garnered increasing attention. Research has shown that MPs negatively affect many organisms. Microalgae are primary producers in aquatic environments and play a crucial role in the stability of aquatic ecosystems. However, research on the effects of MPs on microalgae is relatively limited. Haematococcus pluvialis is known for its ability to produce astaxanthin, a powerful antioxidant, in response to environmental stress. MP exposure is also an environmental stressor, and we are curious whether MP stress will affect astaxanthin synthesis in H. pluvialis. To investigate the effects and mechanisms of MPs on H. pluvialis growth and astaxanthin synthesis, we exposed H. pluvialis to 5 μm polystyrene MPs at different concentrations (0.1, 1, and 10 mg/L) for 18 days, followed by high light induction of astaxanthin synthesis. Growth and photosynthesis-related indicators suggested that PS-MPs had a hormesis-like effect on H. pluvialis, with short-term exposure stimulating photosynthetic activity and growth, and long-term exposure inhibiting them. Morphological observations, oxidative stress markers, soluble proteins, and extracellular polymeric substance indicators showed that prolonged PS-MP exposure primarily disrupted the morphology and normal physiological functions of H. pluvialis by inducing oxidative stress. Although H. pluvialis actively resists the oxidative stress caused by PS-MPs, it cannot fully counteract the adverse effects. Prolonged PS-MP exposure ultimately resulted in reduced levels of photosynthetic pigments and inhibited photosynthetic activity, as well as the decreased expression of genes related to astaxanthin synthesis and reduced astaxanthin production. Integrated biomarker response analysis further indicated that the overall toxic effects of MPs on H. pluvialis exhibit a dose-dependent pattern. MP exposure potentially weakens the survival capability of H. pluvialis under adverse conditions. These findings highlight the impact of MP pollution on the stability of aquatic ecosystems and underscore the urgent need for policies and actions to mitigate MP pollution and protect aquatic environments.