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

Abstract

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. Three main findings underpinning microplastic feeding by a small ciliate. Microplastics are readily taken up by ciliates which then represent a reservoir for those pollutants. After egestion, microplastics form a pellet; hence the protists modulate the availability of the pollutant. Population growth declines with microplastic concentration. The photos were taken during the experiment at 200× magnification and show plastic microbeads inside the ciliates and after egestion. • The mechanisms underpinning microplastic uptake by a model freshwater ciliate and consequences for population growth were assessed. • Microplastic uptake increased in a saturating fashion with concentration. • Microplastic uptake by ciliates decreased through time, driven by the immobilisation of microplastics after egestion. • Ciliate population growth was compromised by feeding on microplastics. • Simple predator-prey models explained the results, providing a basis to understand the fate of microplastic uptake and transfer in food webs.

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