Fate of Microplastics and Nanoplastics in the Environment
Tracking the “Missing” Plastic
Plastic debris is abundant and extensively spread throughout the marine habitat. Plastic pollution in marine environments has been recognized as a global environmental threat [45,47]. Microplastics have amassed on the shorelines from the poles to the equator, at the sea surface and in the deep sea and in terrestrial habitats . However, one of the crucial mysteries in the field of microplastic and nanoplastic research is that based on the amount of plastics entering the ocean, 99% of the plastics that should be present in the oceans is missing and cannot be accounted for . The surface load and size distribution of floating plastic debris has been found to be well below that expected from the generation of plastics and their input rates in oceans . However, no definite temporal trend in floating plastic concentration has been reported, perhaps because of confounded spatial and temporal variability in data [49,50]. A study carried out in 1999 discovered that the smaller the size of a microplastic particle, the lower was its abundance in the North Pacific Central gyre . These observations support the premise of substantial loss of plastic from the ocean surface. The following possible sinks may be proposed as the eventual fate of micro- and nano-sized plastics .
The gap in the distribution of plastic size below 1 mm could indicate a rapid breakdown of the plastic fragments from the millimeter to the micrometer scale. In some recent studies, the scanning electron micrographs of microplastic surfaces have indicated that in addition to solar-induced fragmentation, oceanic bacterial populations may also contribute to the degradation of plastics on the surface of oceans . Limited information on physical, chemical and biological processes driving plastic disintegration suggest that two-phase fragmentation, with accelerated breakdown of photo-degraded fragments, may also occur. Nanofragmentation can render the smaller plastic pieces undetectable to conventional sampling nets. The abundance of nano-scale plastic particles is still not quantified in the open ocean [30,49].
Small-sized plastic particles, with high surface-to-volume ratios, may preferentially submerge in the ocean waters by ballasting due to epiphytic growth. Biofouled plastic fragments are often incorporated in the sediment in shallow, nutrient-rich areas. However, this mechanism may not be applicable in the deep, open ocean [30,52]. Defouling of plastic in deep waters may occur from adverse conditions for epiphytic organisms, causing the plastic particle to return to the surface .
Zooplanktivorous organisms like epipelagic and mesopelagic fish are an abundant trophic assemblage in the ocean, and accidental ingestion of plastic is known to occur during their feeding activity . The most frequent plastic size ingested by these fish has been reported to range between 0.5 and 5.0 mm, matching the predominant size of plastic debris which experience global losses in oceans [53,54,55]. Small mesopelagic fish represent the most pervasive zooplanktivorous collection in the open ocean, with densities close to one individual per square meter, even in the oligotrophic subtropical gyres [49,56]. The plastic fragments ingested by smaller fish can be transferred to larger predators, sink with the bodies of dead fish or be defecated [49,54,55].