Protozoans play an integral role in the microbial loop, an important process of material and energy transfer in marine ecosystems. The number of microplastics in the marine environment has greatly increased, but the potential impacts of small nanoplastics and microplastics on marine organisms remain unclear. Here, we conducted a series of feeding experiments with various concentrations of microplastic beads (ca. 1 μm) to characterize the response of the planktonic ciliated protozoan Strombidium sulcatum to microplastics and a set of additional exposure experiments with four different particle diameters of microplastics to explore whether the feeding response exhibited size selectivity. As the microplastic concentration increased, the number, body size, and biomass of ciliates decreased sharply during the exposure period. Predator biomass in all microplastic treatments was markedly reduced relative to the microplastic-free control. For example, at 72 h of exposure, the biomass in the highest microplastic concentration treatment was observed to decrease by 96.59% relative to the control. There was no obvious difference in the biomass of ciliates exposed to various diameters of microplastics; however, compared with the free bead control, the biomass still significantly decreased. These findings suggest that microplastics in the ocean negatively affect the growth of protozoan microzooplankton that might have accidentally ingested these tiny particles during the feeding process. Generally, this study provides basic and novel data for understanding the effect of microplastics on the microbial loop in marine ecosystems.

原生动物在微生物循环中起着不可或缺的作用，微生物循环是海洋生态系统中物质和能量转移的重要过程。海洋环境中的微塑料的数量已大大增加，但是尚不清楚小型纳米塑料和微塑料对海洋生物的潜在影响。在这里，我们进行了一系列不同浓度的微塑料珠（约1μm）的饲养实验，以表征浮游纤毛原生动物Strombidium sulcatum的响应微塑料和一组其他暴露实验，分别使用四种不同直径的微塑料，以探讨进料反应是否表现出尺寸选择性。随着微生物浓度的增加，在暴露期间纤毛虫的数量，体型和生物量急剧下降。与无微生物塑料的对照组相比，所有微塑料处理中的捕食者生物量均明显减少。例如，在暴露72小时时，观察到最高塑料浓度处理中的生物量相对于对照减少了96.59％。暴露于不同直径的微塑料的纤毛虫的生物量没有明显差异；然而，与游离珠对照相比，生物量仍显着下降。这些发现表明，海洋中的微塑料会对原生动物微浮游动物的生长产生负面影响，而原生动物微浮游动物可能在进食过程中意外地摄取了这些微小的颗粒。通常，这项研究为理解塑料对海洋生态系统中微生物环的影响提供了基础和新颖的数据。