Page - 40 - in Freshwater Microplastics - Emerging Environmental Contaminants?

[103]. High concentrations of engineered nanomaterials have also been linked to effects that are not due to an actual toxic response, but rather caused by an over- loadingof the testorganismswithengineerednanomaterials,causingphysical inhi- bition [8]. Testingof low, environmentally relevant particle concentrations during short exposure timesmay, however, not be sufficient to detect effectswhen using endpoints on a physiological level. Before an organism shows impairment to, for example, its reproduction or survival, multiple changes must take place on a cellular level. Cellular responsesmay therefore bemore sensitive tomicroplastic particle stress comparedwithwhole-organism responses. On this level, however, wearedealingwithacomplexnetworkandhugenumberofreactions,whichmakes it challenging tofindanddefineameaningful, reliable set of responsevariables. If cellular responses are to be used as indicators of the potentially hazardous prop- erties of nano- and microplastics, more research is needed to develop suitable (standard) test methods. Another option for testing the toxicity of relatively low concentrationsofparticles is chroniceffect studies, aschronicendpoints canprove more sensitive than acute ecotoxicity. An added benefit of testing lower concen- trations is that particle agglomeration/aggregation is reduced, leading to more stable exposure. Onemajorcriticismofcurrentnano-andmicroplasticecotoxicitystudies is their lack of realism and environmental relevance when selecting test parameters [104]. Pristine particleswith a clearly defined, homogenous chemical composition aremost often applied in laboratory tests. This is in sharp contrast to the particles present in the environment, which undergo transformation processes, potentially influencing their morphology, and, in the case of plastic, often contain various additives. This trade-off between environmental realism and standardised test conditions is not a dilemma that is unique to testing of particles [105]. It should be kept in mind that different testing paradigms inform different scientific and regulatory questions. In standard ecotoxicity, applying simplified test systems and often syntheticmedia, test parameters canmore easily be controlled andmodified one by one in order to gain deeper insight into the mechanisms of toxicity and particle uptake [89]. They are also developed to ensure data comparability and study repeatability. For example, data generated followingOECDTestGuidelines and Good Laboratory Practice are considered to satisfy the criteria for Mutual Acceptance of Data and can be used for regulatory assessment purposes in all OECDmember states, ideally minimising testing efforts and use of test animals [106].Moreenvironmentally realistic studies can, on theotherhand, providecase- and site-specific information on the effects of particle pollution under specific environmental conditions. Theymay also providemore realistic informationwith regard to the combined effects ofmultiple environmental stressors and their inter- actionswithplastic particles. Standard ecotoxicity tests andmore environmentally realistic studies should therefore be seen as complementary tools of equal importance but potentially addressing different questions of scientific and regulatory relevance. 40 S.Rist andN.B.Hartmann