Page - 62 - in Freshwater Microplastics - Emerging Environmental Contaminants?

3. Themorphology of polymer particlesmust render the attachment ofmicroor- ganisms and the formation of a biofilm, while the structure of the polymeric substrate, e.g., chemical bonds, degree of polymerization, degree of branching, and parameter, such as hydrophobicity or crystallinity,must not hindermicro- bial actions. Since the size of synthetic polymers is generally too large to penetrate the cell membranesofmicroorganisms, thefirst stepofbioticdegradationis thecleavageof side chains or the polymer backbone and the formation of smaller polymer units (monomers, oligomers)byextracellular enzymes [64]. Inmost cases, thisfirst step of depolymerization involves an enzymatically catalyzed hydrolysis of amides, esters, or urethane bonds. These smaller molecules can then be absorbed by microorganisms andmetabolized. Of course, abiotic hydrolysis can also result in intermediates that are then furthermetabolizedbymicroorganisms [65]. The complete biotic degradations of poly(ε-caprolactam) and water-soluble polyethylene glycol are well described in literature [66]. However, most of the plasticsoccurringin theenvironmentarewater insoluble,andmanyof thesynthetic polymers present in the aquatic environment, such asPE, polypropylene (PP), PS, and PET, degrade very slowly or not at all. The degradation of these polymers is usually a combinationof abiotic andbiotic degradationpathways. Polyolefins, such as PE and PP, represent a class of substances with high industrial production volumes and are determined frequently in environmental samples. These polymers are usually not biodegradable, as the alkyl backbone is not accessible formicroorganismandmustundergoanabiotic transformation.The alkyl backbone of polyolefins offers a high resistivity against hydrolysis but is usually susceptible to oxidative degradation. To prevent this, additives are added during the production process, and the oxidative or photooxidative degradation of the polymer is delayed until the antioxidants are consumed. After the initial oxidation of the surface of polyolefins, the degradation could occur in several weeks but results in the formation of microplastics as possible intermediates [67]. These smaller and oxidized plastic fragments aremore susceptible tomicro- bialattack,e.g.,biodegradationofPEisdescribedforpre-oxidizedfragmentsof the originalmaterial byPseudomonas sp. [68]. 4 Conclusion Theresultsofstudiesworldwidehighlight thegreat importanceofmicroplastics for freshwater ecosystems, as they are present in high abundance. Microplastics are emergingcontaminants in theaquaticenvironment,andattentionshouldbefocused onaharmonizednomenclatureofmicroplasticparticleswithofficialguidelines for microplastic studies. The definition of microplastics often remains vague, and different size classes are investigated inmonitoring studies. For a thorough inves- tigation of microplastic pollution, standardized methods, especially for the 62 S.Klein et al.