Page - 36 - in Freshwater Microplastics - Emerging Environmental Contaminants?

organisms and translocate into tissues are expected to have a stronger physical impact [45]. This is closely linked to particle size, as will be explained further below.Shapealsoplaysan important rolesince irregular, sharpfragmentsaremore likely to cause damage than round, smooth particles. Fibres are more likely to accumulate in the digestive system. The capacity of individual species to egest microplastics is also considered as an important factor because this process will determinehow long anorganism is exposed to the particles [45]. For nanomaterials, size-dependent changes in effects are of particular interest. Thewhole purposeof nanotechnology is to take advantage of thenovel properties that come with a smaller size. For engineered nanomaterials, this involves, for example, the novel catalytic effects of somematerials on the nanoscale including gold (Au), titanium dioxide (TiO2) and cerium dioxide (CeO2). As larger-sized (bulk) materials, these are relatively inert, but with decreasing particle size and increasingsurfacearea, theybecomereactive.Therefore,asparticlesizedecreases, there is a tendency for toxicity to increase, even if the samematerial is relatively inert in its correspondingbulk (micron-sized) form[85]. Inaddition, the small size of engineered nanomaterials may enable their uptake into tissues and cells [49].Observedbiological effectsofengineerednanomaterials inaquaticorganisms include oxidative stress, inhibition of photosynthesis, tissue damage, impaired growth anddevelopment, behavioural changes and increasedmortality (Table 1). Similarly, thequestion fornano-andmicroplastics is therefore: Is it likely thata decrease in sizewillmake themmorehazardous?Toanswer thisquestion,wewill examine the two main causes for concern: novel properties and ingestion by organisms (and potential subsequent transfer into tissues). The novel properties that would occur for smaller-sized polymer particles are linked to their increased surface-to-volume ratio. With decreasing particle size, a larger fraction of the molecules will be present on the surface of the particle. As the surface is where interactions with the surrounding environment take place, this can lead to an increase in chemical reactions and biological interactions. For example, smaller particlesmay(onamassbasis)havealargeradsorptioncapacitycomparedtolarger particles [86], which in turn is of relevance for the vector effects. The second concern relates to the potential to cross biological barriers. Nanosized particles, suchasnanoplastics, arepotentiallymorehazardousdue to their easier uptake into tissues and cells [2]. Depending on particle size, different uptake routes into organisms are also involved. For example, the freshwater crustacean Daphnia magna normally catches prey (mainly algae) in the size range 0.4–40 μm [87, 88]. For particles or agglomerates that arewithin this size range, uptake can occur through active filtration, and at the same time unwanted particles can be rejected. Particles smaller than the preferred size are not actively taken up by the animals, butmay insteadenter theorganisms through ‘drinking’of thesurrounding water, resulting in non-selective, uncontrolled uptake. Depending on the feeding strategiesofspecificaquaticorganismsand theirability toactivelyselect their food source, they may be able to regulate their uptake of microplastics, whereas nanoplasticsmay enter the organismsunintentionally. 36 S.Rist andN.B.Hartmann