How do microplastics affect the marine microbial loop? Predation of microplastics by microzooplankton

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.