Page - 190 - in Freshwater Microplastics - Emerging Environmental Contaminants?

furunculosis in hatcheries, on several plastic types [62]. Recently, 16S rRNA gene sequences affiliated to Tenacibaculum spp. (another genus including fish pathogens) were detected on PET in seawater [42]. Research has only started to shed light on this issue, aswell as the ability of polymers to transport biologically produced toxins. 2.3.2 BiodegradationandPollutantTransport Several reviews of research into plastic biodegradation have been published (e.g., see [11–13, 24,63–65]).Therefore, onlyabrief overviewof this topic is provided. Plastic biodegradation involves several steps during which the polymer is enzy- matically cleaved into oligomers andmonomers that can be assimilated bymicro- organisms [65]. Many microbial taxa can degrade biopolymers2 including polyhydroxybutyrate (PHB) and polyhydroxybutyrate-polyhydroxyvalerate (PHBV). The biodegradation rates of biopolymers in freshwater have been found to exceed those inmarine environments, andhigher rates have also beenobserved insewage thanwithinnatural freshwaters [63,66,67].Evenso, thesematerialscan still persist for considerable periods of time in freshwaters, with a lifespan of ~10 years having been estimated for PHBVbottles deposited onto lake sediments at a depthof 85m[68]. In comparisonwith biopolymers, traditional plastics (such asPE,PET, andPP) will persist for even longer within aquatic environments (decades or centuries; [11, 63, 64]), with biodegradation typically preceded by abiotic weathering [24, 65]. Although it has been unclear whether plastispheremembers can biode- grade conventional plastics [11, 69, 70], a bacterial strain isolated from sediment near a Japanese bottle recycling facility (Ideonella sakaiensis)was recently found to assimilate PET [18]. The strainwas shown to employ two enzymes to degrade PET at a daily rate of 0.13mg cm 2 when incubated at 30 C [18]. This finding implies that other synthetic plastic-degrading taxa are likely to be present within aquatic environments. Indeed, colonization of plastics by potentially hydrocarbonoclastic bacteria has been observed in both marine and freshwater habitats [21, 45, 47–49]. However, due to a lack of research into plastisphere physiology, the long residence times of plasticwaste, and the ability of polymers to adsorbpolyaromatichydrocarbons [11, 12], themechanismsunderlying recruit- mentofhydrocarbondegradersonmicroplasticsareunknown.Theseandother taxa could mediate desorption and/or degradation of several plastic-associated com- pounds, including additives and diverse pollutants, with implications for the eco- logical impacts of microplastics. Indeed, Bryant et al. [49] already reported the presenceofdiversexenobioticdegradationgenes inassociationwithmarineplastic debris. Since organic contaminants and metals rapidly partition into biofilms 2Polymers derived from renewable biomass (as opposed tononrenewable fossil fuels). 190 J.P.Harrison et al.