Microplastic ingestion by an aquatic ciliate: Functional response, modulation, and reduced population growth

Microplastic particles are ubiquitous in aquatic environments and are considered a major threat to the large range of heterotrophic organisms that involuntarily consume them. However, there is current uncertainty around the mechanisms underpinning microplastic uptake by aquatic consumers and the consequences for both the fate of the microplastics and the growth potential of consumer populations. We performed a feeding experiment, exposing a model freshwater ciliate, Tetrahymena pyriformis, to six different microplastic concentrations and measured microplastic uptake and population growth over the course of several generations. Microplastic uptake increased in a saturating fashion with concentration, consistent with a Type II functional response, with a maximum feeding rate of 22 microplastic particles individual^-1 h^-1. Interestingly, microplastic uptake decreased through time and we observed that, after egestion, microplastic particles aggregated, rendering them too large for re-consumption. We built and tested a simulation model which matched rates of microplastic uptake when incorporating functional response parameters and assuming 50 % immobilisation of microplastics after egestion. Nevertheless, ciliate population growth was compromised by the presence of microplastics, decreasing by 43 % over the full microplastic concentration range. Taken together, our results demonstrate the potential for aquatic ciliates to play an important role in the uptake, transfer, and modification of microplastics in freshwater environments with associated negative impacts on population fitness.