Seite - 187 - in Freshwater Microplastics - Emerging Environmental Contaminants?

2.2 Examples ofMicrobial-Microplastic Interactions inFreshwaterHabitats Despitemeasurementsofplasticdensityandcomposition in freshwaterecosystems [10, 43], little is known about microbial associations with plastic in unmanaged freshwaters. A limited number of publications have investigated polymer biodeg- radation in lakesand rivers (Sect. 2.3), and thereareat least three studies thathave experimentallycharacterizedthestructure,composition,and/oractivitiesofplastic- associated biofilms in these environments [44–46]. Because of differences in the study design and sites and the response parameters that were examined, there are fewfindings incommonamongthese threestudies.Thus, someof themajor results of each study are discussed and compared with insights into marine microbial- microplastic interactions. Hoellein et al. [44] comparedbacterial community composition andactivity on six substrate types (5 5 cm pieces of ceramic tile, glass, aluminum, PET, leaf litter, and cardboard) in a river, a pond, and recirculating laboratory streams. In contrast with McCormick et al. [45] and several studies of marine plastisphere communities [21, 29, 47], the authors found no differences in the composition of plastic-colonizing biofilms relative to those on other solid substrates. The plastic, tile, and glass samples also showed similar rates of gross primary production and respiration.Theprimary factors for determiningbacterial community composition and metabolic rates were the study site (river, pond, or artificial stream) and whether the substratewas hard (tile, glass, aluminum, andPET) or soft (leaf litter and cardboard).While the surface-colonizing assemblages onPETwere composi- tionallysimilar to thoseonothersurfaces, itwassuggestedthatdifferencesbetween substrate typesmaybestrongerduringearlystagesofbiofilmformation.Similarly, Oberbeckmann et al. [42] found PET- and glass-colonizing communities to be compositionally similar following up to 6 weeks of exposure to seawater; the authors noted that higher-resolution studiesmay be required to distinguish “plas- tic-specific” taxa from other biofilm members. Taken together, these studies emphasize how investigating the early-stage development of plastisphere commu- nities inmoredetailwillbenecessarynotonlyinmarineecosystems[21]butalsoin freshwater habitats. McCormick et al. [45] compared bacterial communities on microplastic, suspended organic matter (i.e., seston) and the water column downstream and upstreamof aWWTP.All habitats differed fromeach other, and themicroplastic community had a lower taxon diversity relative to seston and downstreamwater samples. Inmarine environments, plastic-associatedmicrobial communities have also been found to be taxonomically distinct from those in the surroundingwater [30, 47–49]. Genera selected for on plastic (relative to nonplastic habitats) in the study byMcCormick et al. [45] includedPseudomonas,Arcobacter,Aeromonas, Zymophilus, andAquabacterium. These genera contain species with the potential for plastic degradation andpathogenesis (Sect. 2.3).Aquabacteriumcommune is a commonmember of drinkingwater biofilms [50], and colonization of low-density Microplastic-AssociatedBiofilms:AComparisonofFreshwater andMarine. . . 187