Effect of microplastic particles on the population growth rate and clearance rate of selected ciliates (Protista, Ciliophora)

Abstract

Microplastics (MPs), due to their micro size, which overlaps with the typical food size of various aquatic organisms, can be ingested and move up the food chain, accumulating in the bodies of organisms at higher trophic levels. Few studies have focused on the uptake of MPs by ciliates, which are an important element of the microbial cycle. Three different ciliate species were used in this study: Blepharisma japonicum, Euplotes sp., and Spirostomum teres, as well as polystyrene beads with diameters of 1 and 2 µm at two concentrations (10⁶ and 10⁷ beads × mL^-1). The results of the experiments showed that MPs have a variable, species-specific effect on the population growth rate of ciliates, which is directly dependent on their concentration in the environment (P < 0.01). It was also observed that the number of MPs ingested changed over time depending on their concentration and size. On average, the highest number of ingested MPs (883.11 ± 521.47) was recorded at 60 min of exposure to a low concentration of small beads in B. japonicum. The lowest number of beads was ingested after 5 min of exposure to a low concentration of large beads in the same species. The rate of MP uptake by the ciliate species was significantly dependent on their concentration, exposure time, and size (P < 0.001). The highest clearance rate was observed in the fifth minute of the experiment in the environment with the lowest MP concentration.

Keywords: Ciliates; Clearance rate; Grazing; Microplastics; Population growth rate.

Fig. 1

Comparison of mean (± SD) population growth rates (PGRs) of ciliates at different microparticle exposures times (24, 48, 72 h), concentrations (10⁶ vs. 10⁷ beads × mL.⁻¹) and sizes (small, 1 µm vs. large, 2 µm)

Fig. 2

Effect of microparticle concentration, size, and exposure time on the population growth rate (PGR) of B. japonicum (a–c), Euplotes sp. (d–f), and S. teres (g–i). Plots show the least squares (LS) means of PGR (± SEs)

Fig. 3

Fluorescently labelled microplastic beads ingested by the ciliates Euplotes sp. (a), Blepharisma japonicum (b), and Spirostomum teres (c), visible under epifluorescence microscopy. The bar represents 50 µm

Fig. 4

Comparison of the mean number (± SD) of two sizes of microplastic beads ingested by ciliates at different concentrations and exposure times

Fig. 5

Effect of microparticle concentration, size and exposure time on the least squares (LS) mean number (± SDs) of beads ingested by Euplotes sp. (a), Spirostomum teres (b), and Blepharisma japonicum (c)

Fig. 6

Comparison of the mean clearance rate (± SD) of ciliates at different exposure times, concentrations and microparticle sizes

Fig. 7

Effect of microparticle concentration, size, and exposure time on the least squares (LS) mean (± SDs) clearance rate of Euplotes sp. (a), Spirostomum teres (b), and Blepharisma japonicum (c)

Fig. 8

Correlation between clearance rates (a) and the number of ingested beads (b) and cell area of each ciliate species. The plots show the least squares (LS) mean number of ingested beads and clearance rate (± SEs) a logarithmic scale