Page - 37 - in Freshwater Microplastics - Emerging Environmental Contaminants?

5.1 Nano- andMicroplastics inStandardEcotoxicityTests In thequest todetermine theenvironmental riskposedbynano- andmicroplastics, laboratory-basedexperimentsneedtobecarriedoutwhichanalyse theeffectsof the particles underwell-definedconditions.Thenumberof controlled laboratory stud- ies investigating the effects of nano- andmicroplastics on freshwater organisms is steadily increasing, andmany different impacts have been observed – extending from the molecular and cellular to the physiological level (see Table 1). These include inflammation,disruptionof lipidandaminoacidmetabolism, lowergrowth rates, decreased feeding rates, behavioural changes, impairment of reproduction and increasedmortality [62, 64, 75, 79, 81, 82].When studies involvingmarine organismsare also taken intoaccount, thenumber andvarietyofbiological effects of nano- andmicroplastics that havebeen found are evengreater. However,most effect studies differ greatlywith respect to theparameters used, for example, particle type (different polymers, sizes, shapes, presence of chemicals), test species, exposure duration, exposure concentration and response variables. Thismakes it difficult to compare results between studies and hampers reproducibility. It can, therefore, be advantageous to apply standardised tests, which comewith a number of benefits as they ensure controlled and reproducible test designs and inter-laboratorycomparability.Another advantageof standardised ecotoxicity tests is the extensiveknowledgebase resulting fromdecadesof testing the effects of chemicals on selected model organisms. For ecotoxicology of nanomaterials, this has been highlighted as a motivation for using standardised short-term tests as a startingpoint for gainingan insight into the fate andbioavail- ability of engineered nanomaterials in the environment [89]. By using a well- defined test system and a fully defined synthetic medium, other test parameters canbevaried individuallyand inacontrolledmanner, therebyprovidingan insight into specificprocesses andmechanisms [90]. However, the use of standard test guidelines also comes with some potential disadvantages, especially for testing of particles. For freshwater systems, a com- monlyusedspecies is the freshwaterfleaDaphniamagna, forwhich theOECDhas developed twostandard tests: anacute immobilisation test (48h) (OECDTG202) and a chronic reproduction test (21 days) (OECD TG 211). These tests were originally developed for soluble chemicals. Since particles show very different behaviours to soluble chemicals, it is challenging to apply the same test set-ups. Even so, some studies have used these standard tests to investigate the effects of nano- andmicroplastics.Casadoet al. [91] conductedanacute immobilisation test with 55 and 110 nm polyethyleneimine PS beads and reported EC50 values of 0.8mg/land0.7mg/l, respectively.Thesametestwith1μmPEbeadsresultedinan EC50 value of 57.4mg/l [83]. This huge difference could be a consequence of the different polymer typesandsizesused in the studies, but itmight also indicate that mortality is not a very sensitive biological response when it comes to plastic particles. Finally, it may be indicative of a problem that has been highlighted for testswithengineerednanomaterials:That reproducibilityanddata interpretation in AquaticEcotoxicity ofMicroplastics andNanoplastics: LessonsLearned from. . . 37