Seite - 186 - in Freshwater Microplastics - Emerging Environmental Contaminants?

Each step in a spiral is likely to have implications for plastic-associated biofilm composition and activity, due to accompanying shifts in the surrounding environ- mental conditions (Fig. 1) [29, 31]. Studies ofmicroplastic spiralingmetrics will help estimate the spatial scales over which plastic particles move within lotic environments, informing how the associated microbial communities can be expected to change acrossmultiple downstream spirals. Rivers are also character- ized byflooding,which redistributesmaterials between riparian and aquatic com- ponentsof thefluvial landscape [37, 38]. Floodingmovesplastic from the riparian zone into aquatic habitats and increases stranding of plastic in debris dams [39]. Analogous processes in marine environments include tidal movements and storm surges which strand plastic on intertidal or wrack zones [2]. Despite their likely impacts on plastisphere communities (Fig. 1), the effects of movement between aquatic and terrestrial habitats on plastic-associated biofilms have not been studied. Hydrology in most lakes includes at least a single upstream inlet and down- streamoutlet,withwater andparticle residence times dependingonwater volume and currents. Little is known about plastisphere communities in lakes (Sect. 2.2), but research into this topic can be expected to benefit from a budgetary approach whichmeasures rates ofmicroplastic inflow,outflow, and retention.Thesemetrics will determine microplastic residence times, which are likely to influence microbial-plastic associations within several habitats, including the epilimnion, littoral, and benthic zones (Sect. 3.1).Wind andwave action are likely to further influence thedistributionofmicroplasticswithin lakes [2]. It is unclear how transport ofmicroplastics from freshwater tomarine environ- ments affects plastisphere assemblages, but theymay undergo a variety of taxo- nomic and physiological shifts during this transition (Sects. 2.2 and 2.3) [20, 40]. For example, subjecting Pseudomonas aeruginosa to salt stress (0.5M NaCl)was found to inhibit biofilm formation and reduce rates of benzoate degra- dationby this strain [41].Geographic and seasonal differences in the structure and composition of freshwater plastisphere communities are yet to be investigated. However, the spatiotemporal distribution of marine plastic-colonizing microbial consortiahas recentlybeenstudied [29,30,42].Basedon6-week insituexposures of polyethylene terephthalate (PET)bottles in theNorthSea,Oberbeckmannet al. [29, 42] found location-dependent and seasonal differences in the structure and compositionofplastispherecommunities.Similardifferenceswerealsoreportedby Amaral-Zettler et al. [30]. Further to distinct communities beingdiscovered in the NorthAtlantic andNorthPacific subtropical gyres, the authors reported latitudinal gradients in the species richness of plastic-colonizing assemblages [30]. While taxonomic differences were also observed between polymer types, the data suggested that geography is likely to be a stronger predictor of plastisphere com- munity composition at the scale of oceanbasins [29, 30, 42]. 186 J.P.Harrison et al.