In recent years, increasing concerns have been raised about the environmental risk of microplastics in freshwater ecosystems. Small microplastics enter the water either directly or accumulate through disintegration of larger plastic particles. These particles might then be ingested by filter-feeding zooplankton, such as rotifers. Particles released into the water may also interact with the biota through the formation of aggregates, which might alter the uptake by zooplankton. In this study, we tested for size-specific aggregation of polystyrene microspheres and their ingestion by a common freshwater rotifer Brachionus calyciflorus. The ingestion of three sizes of polystyrene microspheres (MS) 1-, 3-, and 6-μm was investigated. Each MS size was tested in combination with three different treatments: MS as the sole food intake, MS in association with food algae and MS aggregated with biogenic matter. After 72 h incubation in pre-filtered natural river water, the majority of the 1-μm spheres occurred as aggregates. The larger the particles, the higher the relative number of single particles and the larger the aggregates. All particles were ingested by the rotifer following a Type-II functional response. The presence of algae did not influence the ingestion of the MS for all three sizes. The biogenic aggregation of microspheres led to a significant size-dependent alteration in their ingestion. Rotifers ingested more microspheres (MS) when exposed to aggregated 1- and 3-μm MS as compared to single spheres, whereas fewer aggregated 6-μm spheres were ingested. This indicates that the small particles when aggregated were in an effective size range for Brachionus, while the aggregated larger spheres became too large to be efficiently ingested. These observations provide the first evidence of a size- and aggregation-dependent feeding interaction between microplastics and rotifers. Microplastics when aggregated with biogenic particles in a natural environment can rapidly change their size-dependent availability. The aggregation properties of microplastics should be taken into account when performing experiments mimicking the natural environment.

中文翻译：

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小型微塑料的生物聚集改变了普通淡水微型无脊椎动物对它们的摄取

近年来，人们越来越关注淡水生态系统中微塑料的环境风险。小的微塑料会直接进入水中，或者通过较大的塑料颗粒分解而积聚。然后这些颗粒可能会被滤食性浮游动物（例如轮虫）摄取。释放到水中的颗粒也可能通过形成聚集体与生物群相互作用，这可能会改变浮游动物的吸收。在这项研究中，我们测试了聚苯乙烯微球的尺寸特异性聚集及其被普通淡水轮虫 Brachionus calyciflorus 摄取的情况。研究了三种尺寸的聚苯乙烯微球 (MS) 1-、3- 和 6-μm 的摄入。每种 MS 大小都结合三种不同的处理进行了测试：MS 作为唯一的食物摄入量，MS 与食用藻类相关，MS 与生物物质聚集。在预过滤的天然河水中孵育 72 小时后，大多数 1-μm 球体以聚集体的形式出现。颗粒越大，单个颗粒的相对数量越高，聚集体也越大。所有颗粒都在 II 型功能反应后被轮虫摄取。藻类的存在不影响所有三种尺寸的 MS 的摄入。微球的生物聚集导致其摄入发生显着的大小依赖性改变。与单个球体相比，轮虫在暴露于聚集的 1 和 3 微米 MS 时摄入更多的微球 (MS)，而摄入的聚集的 6 微米球体较少。这表明聚集时的小颗粒对于腕轮属有效尺寸范围，而聚集的较大球体变得太大而无法有效摄取。这些观察提供了微塑料和轮虫之间依赖大小和聚集的进食相互作用的第一个证据。微塑料在自然环境中与生物颗粒聚集时，会迅速改变其大小相关的可用性。在进行模拟自然环境的实验时，应考虑微塑料的聚集特性。