Page - 39 - in Freshwater Microplastics - Emerging Environmental Contaminants?

nano-andmicroplastics.Agingiscurrentlynot incorporatedinstandardecotoxicity testprotocols,buthasbeenproposedforengineerednanomaterials [100].Thereare also indications that aging of plastic particles can influence biological effects [79]. This aspect should therefore generally be considered in the future develop- ment of ecotoxicological tests for particle testing. For testmethod developments, the field of ecotoxicity testing of nanomaterials has benefitted from the availability of reference materials (e.g. NIST Standard Reference Materials) and representative industrial nanomaterials (such as those fromtheJRCNanomaterialsRepository).Suchmaterialsarevaluableforanalytical methodvalidationandforconductingcomparableinter-laboratoryandinter-species studies.Thefieldof ecotoxicity testingofnano- andmicroplasticswould similarly benefit from the establishment of sources of well-characterised, industrially and environmentally relevantmaterials of various sizes and compositions. The applicability of current standard ecotoxicity tests has been questioned for engineered nanomaterials. Development of new test guidelines and guidance is underdiscussion, forexample,within theOECD[101].Thesameconcernsapplyto testingofnano- andmicroplastics:They represent a specific challengedue to their dynamic nature in environmental media, resulting in, for example, differences in relevant exposure routes (through food or other active uptake routes, grazing on sedimented materials, etc.), as well as potentially different effect mechanisms. Soluble molecules can be taken up into aquatic organism by diffusion and then distributedwithin theorganismbasedonpartitioning, e.g. to lipid tissues.Cellular uptake of soluble chemicals generally relates to passageof biologicalmembranes, mainly through passive diffusion or active uptake, such as transport through ion channels or carrier-mediated transport [102]. In the tissues, they can act non-specifically, leading to narcosis, or specifically by inhibiting or affecting certain biological processes. In comparison, particle distribution is not governed bydiffusion andpartitioning.Uptake of particles by organisms depends onmech- anismssuchasfeedingrather thanmoleculardiffusion.Onacellular level,particles may be taken up through processes such as phagocytosis. Effects will therefore most likelydiffer from thoseof soluble chemicals.Anessential aspect is therefore todeterminesensitivebiologicalendpoints for theexposure toparticles,potentially moving away from the current standard test organisms. A limited number of response variables and test species can be seen as a disadvantage of standardised tests.Basedon the argument above, itmay further be claimed that ‘no effect’ in a standard testdoesnot implya lackofecological impactofnano-andmicroplastics, as thesetestsmaynotcoverthemostsensitiveendpointsandtestspeciesforparticle exposure. As mentioned, effects of microplastics have been observed on a molecular, cellularandphysiological level (seeTable1).Whenperformingecotoxicity testing, the aim is to establish a dose-response relationship based on the underlying assumption that effects are strongly dependent on exposure dose/concentration and time. For engineered nanomaterials, however, an inverse relationship has been observed between concentration and agglomerate size, meaning that with higher particle concentrations, particles tend to form larger agglomerates AquaticEcotoxicity ofMicroplastics andNanoplastics: LessonsLearned from. . . 39