The plastic debris gets encrusted with foulants, increasing in density as fouling progresses. Once the density exceeds that of sea water it can sink well

below the water surface (Costerton and Cheng, 1987, Andrady and Song, 1991 and Railkin, 2003). Subsequent de-fouling in the water column due to foraging of foulants by other organisms or other mechanisms, can decrease its density causing CP-868596 chemical structure the debris to return back to the surface. A slow cyclic ‘bobbing’ motion of floating plastic debris attributed to this cyclic change in density on submersion below a certain depth of water, was proposed by Andrady and Song (1991) and later confirmed (Stevens and Gregory, 1996 and Stevens, 1992). Fouled debris may increase in density enough to ultimately reach benthic regions; plastics do occur commonly in the benthos (Stefatos and Charalampakis, IDH signaling pathway 1999, Katsanevakis et al., 2007 and Backhurst and Cole, 2000). Even an extensively weathered, embrittled plastic material (that falls apart on handling) still has an average molecular weight in the tens of thousands g/mol. The logarithmic plot of the tensile extensibility (%) versus the number-average molecular weight for LDPE that had undergone weathering shown in Fig. 3 illustrates this. Even for the data points at the very left of the plot (corresponding to extensively

degraded or embrittled plastic) the values of Mn ∼ 103–104 g/mol. Even at these lower molecular weights plastics do not undergo ready biodegradation. Ready microbial biodegradability has been observed in oligomers of about Mn ∼ 500 g/mol polyethylenes. Reduction in particle size by light-induced oxidation does is Thymidylate synthase no guarantee of subsequent biodegradability of the meso- or microplastic fragments. High molecular weight plastics used in common applications do not biodegrade at an appreciable rate as microbial species that can metabololize polymers are rare in nature. This

is particularly true of the marine environment, with the exception of biopolymers such as cellulose and chitin. Recent work, however, has identified several strains of microbes capable of biodegrading polyethylene (Sivan, 2011) and PVC (Shah et al., 2008). In concentrated liquid culture in the laboratory, Actinomycetes Rhodococcus ruber (strain C208) resulted in a reduction of ca. 8% in the dry weight of the polyolefin within 30 days of incubation ( Gilan et al., 2004). Laccases secreted by the species reduced the average molecular weight of polymer as demonstrated by GPC indicating degradation via scission of main chains. However, this process does not occur in soil or marine environments as the candidate microbes are not available in high enough native concentration and competing easily-assimilable nutrient sources are always present. There is virtually no data on kinetics of mineralisation of plastics in the marine environment. However, biopolymers such as chitins (Poulicek and Jeuniaux, 1991 and Seki and Taga, 1963), chitosan (Andrady et al.