Seite - 194 - in Freshwater Microplastics - Emerging Environmental Contaminants?

[85–87]. This approach is based on taxonomically informative markers and pro- vides no direct information on metabolic activities. Overcoming this limitation could involveusingmetagenomicsormetatranscriptomics,with the formerprovid- ing informationonmetabolic capability [49] and the latter enabling investigations of functional gene expression [20] (Fig. 3). The origin of plastic-colonizing path- ogens could be determined by whole genome sequencing followed by genome comparisons or identification of single-nucleotide polymorphism (SNP), approacheswidely used in bacterial epidemiology. Thiswould result in important insights into the transfer of pathogens onplastics, provided that suitable databases are available for comparison [88, 89]. Several further developments could enable us tomovebeyond initial studies of biofilm formationonmicroplastics (Fig. 3). Stable isotope labeling is increasingly used to characterizemicrobial activity at the single-cell level, includingmethods such as heavy water labeling [90] and bioorthogonal noncanonical amino acid tagging (BONCAT) [91]. Heavy water labeling is compatible with Raman spec- troscopy and cell sorting using optical tweezers [90], and BONCAT has been combinedwithfluorescence-activated cell sorting (FACS) [91]. These approaches Fig.3 Movingbeyondinitial research into the taxonomyandformationofplastispheremicrobial assemblages.As investigationsof this topicmature,newtypesofexperimentsandanalytical tools are anticipated to improve our knowledge of topics including how plastisphere communities develop in several types of habitat, how they are affectedby transport from freshwater tomarine environments, and themetabolic functions of plastic-colonizingmicroorganisms 194 J.P.Harrison et al.