Page - 189 - in Freshwater Microplastics - Emerging Environmental Contaminants?

2.3 Potential forPathogenesis, ToxicantTransfer, andBiodegradation 2.3.1 Microplastics asVectors forPathogenTransfer andBiotoxins Gene sequencing analyses initially highlighted howmicroplasticsmay function as vectors for the transport ofpotential pathogens includingVibrioandArcobacter spp. [21,30,45,48,53].Ahighproportionof16SrDNAreads(24%)couldbeattributedto Vibrio spp.detectedonPPand, toa lesser extent, onpolyethylene (PE)collectedata station in North Atlantic waters [48]. Unfortunately, the widely used bacterial metabarcoding technique based on sequencing fragments of the 16S rRNAgene is limited in itsability toprovide therequired taxonomicresolutionfordetectinghuman pathogens[53].Usingoligotypingof16SrRNAgenedata,Schmidtetal.[54]obtained more specific results for taxa within the genus Vibrio indicating the presence of potential pathogensaffectinganimals includingfishes, corals, andbivalves inmarine ormixedsalineplastic samples.Thepresenceofpathogensonplastics sampled from seawaterwasalso impliedby increasedabundancesofgenes involved in typeIVand type VI secretion systems [49]. However, genes involved in these systems can be involved not only in virulence and infection [55] but also in conjugation [56] and interbacterial interactions [57] that are important in biofilms [58].Vibrio spp. were additionally isolated fromplastic collected fromaScottishbeach [59],butno further characterization of the isolates was performed. Only recently was the presence of Vibriospp.onmarineplasticsconclusivelyconfirmedbymatrix-assistedlaserdesorp- tion/ionization time-of-flight mass spectrometry (MALDI-ToF MS) [60]. In their study, Kirstein et al. [60] identified V. parahaemolyticus, V. fluviales, and V. alginolyticus onmicroplastics from theNorth Sea. Apart fromV. alginolyticus, these species were also found on plastics collected in the brackish Baltic Sea. In addition to bacteria, microplastics may transport microbial eukaryotes involved in disease transmission[12].Potentiallyharmfulalgae, includingOstreopsisandCoolia spp.,havebeendiscoveredonplasticintheMediterraneanSea[61].Todate,theonlyin situevidence formicroplastic-associatedpathogens inunmanagedfreshwaters identi- fied an increase inCampylobacteraceae attached tomicroplastics sourced from an urban river [45]. Specifically, 16S rRNAgene sequences related toArcobacter and Pseudomonasspp.wereenrichedonplasticincomparisonwithothersuspendedmatter andthesurroundingwater. In summary, current evidence indicates an important role of microplastics as vectors for opportunistic animal and human pathogens.Methodological advances are required to reliably detect viable pathogenic species, so that realistic distribu- tion patterns can be obtained and potential sources can be identified. This is particularly relevant with regard to waters used for recreational [13] but also for industrial purposes such as aquaculture. Relative abundances ofAeromonas spp. (a genus harboring fish pathogens) were increased on riverine plastics [45], implying that such species could take advantage ofmicroplastics as vectors. This possibility is reinforced by the presence of Aeromonas salmonicida, causing Microplastic-AssociatedBiofilms:AComparisonofFreshwater andMarine. . . 189