Seite - 164 - in Freshwater Microplastics - Emerging Environmental Contaminants?

3.1.1 Algae So far, the majority of studies focused on the effects of MPs on consumers of aquatic food webs, and information on primary producers is limited. However, there are some indications thatMPs adversely affect algae in a concentration and size-dependentmanner [41–43]. For instance, 1 μmPVC fragments inhibited the growth and negatively affected photosynthesis (50 mg L 1) of the marine algae Skeletonema costatum [43], while 1 mm PVC fragments did not induce such alterations. The underlying mechanisms are still unknown, whereby the direct interactionbetweenMPsandalgaeandformationofaggregatesseemtobestrongly related.Sincealgaeareusedasafoodsource inecotoxicologicalexperiments,MPs may induce direct and indirect (quality and quantity of the algae) effects in the consumer. 3.1.2 Daphniamagna In contrast tomarine studies, onlyonefilter-feeding freshwater species,D.magna, has been tested thoroughly in chronic and acute exposure regimes.Acute toxicity testingover96h resulted in anelevated immobilizationat extremelyhighconcen- trations of 1 μmpolyethylene (PE) particles [34].With a median lethal concen- tration (LC50) of 75.3 mg L 1, these acute effects are (presumably) not environmentally relevant. Compared to this, chronic exposure to nanoscale PS over21days(0.22–150mgL 1, [41])wasnot lethal.However,highconcentrations of nano-PS (>30mgL 1) induced neonatalmalformations and slightly decreased the reproductive output. Interestingly, the mortality as well as the amount of malformations increased when the daphnids were fed with nano-PS incubated algae (5 days). Since nano-PS particlesmight be too small for a direct ingestion, the formationofparticle-algae aggregatesmayhave resulted in ahigher exposure. Furthermore, nano-PS reduced the growth and the chlorophyll a content of algae (Scenedesmus obliquus) indicating a reduced nutritional value of algae cultured with polymer particles. Ogonowski et al. [35] conducted life-history experiments with D. magna exposed to primaryMPs (spherical beads, 1.3 g cm 3, 4.1 μm), secondaryMPs (PE fragments, 1.0 g cm 3, 2.6μm), andkaolin (2.6 g cm 3, 4.4μm)under food- limited conditions. They observed an increased mortality and slightly decreased reproduction of daphnids for the highest concentration of secondary MPs (105 P mL 1). However, incoherent exposure regimes (different particle sizes, concentrations, and exposure durations, amongothers) limit a general com- parability and conclusion. In fact, the strongest response was driven by the low amountoffood(reproductionfarbelowvalidationcriteria,OECD).However, these studies illustrate that (a)adverseeffectsdependonseveral factors, e.g., thesizeand shapeofprimaryvs. secondaryMPs, particle concentrations, polymerdensities, as 164 C.Scherer et al.