Light availability modulates the responses of the microalgae Desmodesmus sp. to micron-sized polyvinyl chloride microplastics.

The vertical movement of large-size and high-density MPs in the water column is usually along with dynamic changes in light intensity. However, whether the change in light availability affects the bioeffects of MPs on surrounding microalgae is currently unknown. This study investigated the effects of micron-sized polyvinyl chloride (mPVC, 143.5 μm) microplastics, alone and in combination with light intensity (from 7.5 to 162.5 μmol·m-2·s-1) on the growth and physiology of Desmodesmus sp. Although mPVC did not impact microalgal growth under optimal light (40 and 93.8 μmol·m-2·s-1), it could induce a no-contact shading effect, thereby significantly affecting the physiology of Desmodesmus sp. The growth of Desmodesmus sp. exposed to mPVC was enhanced under a high light intensity of 162.5 μmol·m-2·s-1 which can induce growth inhibition but was retarded when under a light inadequacy condition (20 μmol·m-2·s-1), along with a dose-dependent effect. Significantly, the photosynthesis of Desmodesmus sp. was a highly sensitive metabolic pathway to mPVC stress and largely influenced by the plastic particles under different light conditions. Additionally, mPVC modulated the energy metabolism strategy of Desmodesmus sp., depending on exposure dose and external light availability. Our findings provided a critical basis for the risk assessment of MPs in fluctuating light conditions.