Abstract: Through a series of cultivation experiments, the effects of polystyrene (PS) microplastics of different particle sizes (1 µm, 3 µm, 10 µm) on the growth and the release of low molecular weight organic acids of Dunaliella salina were studied. The results showed that Dunaliella salina could release three types of low molecular weight organic acids, namely formic acid, acetic acid, and lactic acid, during its growth. The addition of PS microplastics could delay the accumulation of acetic acid and lactic acid in the culture system, reduce the amount of acetic acid, increase the amount of lactic acid, and cause a significant accumulation of formic acid. PS microplastics of different sizes had significantly different effect on the growth of Dunaliella salina and the release of low molecular weight organic acids. PS microplastics with a particle size of 3 µm could inhibit the conversion of NO₃-N to NO₂-N in the seawater by Dunaliella salina at the initial phase of its growth, and had significant effect to delay and reduce accumulation of acetic acid in the culture system. PS microplastics with a particle size of 1 µm could remarkably prolong the accumulation time of lactic acid in the culture system. PS microplastics with particle sizes of 1 μm and 3 μm both could significantly affect the production and consumption of formic acid by Dunaliella salina, leading to a noticeable accumulation of formic acid in the culture system. Releasing formic acid may be a common process in marine microalgae.