One of the major potential threats of microplastics to marine fauna is ingestion. Figure 1 shows potential transport pathways of plastic particles, as well as their interaction with the biota (Wright et al. 2013).
Among the different methods of ingestion by aniamls, microplastics may be taken up either actively and passively. The former describes the voluntary selection of microplastics due to their similar appearance in both size and shape to the food normally consumed by the animal. Passive ingestion refers to indiscriminate feeding, whereby the animal doesn't differentiate between types of food. Animals may also ingest microplastics through scavenging or indirectly when they consume prey that had previously ingested plastic particles (ibid).
A study by Farrell and Nelson (2013) investigated the effectiveness of trophic transfer. The authors added 0.5μm fluorescent microspheres to water that contained mussels. Once the mussels had filtered these microspheres from the water, they were fed to crabs. The results showed the approximately 0.28% of microspheres that had been filtered by the mussels were transferred to the crabs (Farrell and Nelson 2013), demonstrating that there is evidence for trophic transfer.
|Figure 1. Potential transport pathways of microplastics and their interaction with the biota (Wright et al. 2013)|
However, some animals manage to avoid the ingestion of microplastics altogether. Interestingly, some bivalves have developed a way to sift through particles before ingesting them, and thus unlike passive feeders actually discriminate between these particles (Wright et al. 2013).
Another threat is the contamination of toxic compounds from their adsorption to the surfaces of microplastics. Because of their small size, microplastics have a large surface area : volume ratio, indicating that they are more likely to adhere pollutants onto their surfaces (Cole et al. 2011). There have been a number of studies published on the types of chemicals found on micorplastics, including trace metals (such as chromium, lead and cadmium) (Holmes et al. 2012), brominated flame retardants (Engler 2012) and Persistent Organic Pollutants (POPs) including organic compounds such as organochlorine pesticides, Polychlorinated Biphenyl (PCBs) and Polycyclic Aromatic Hydrocarbons (PAHs) (Mato et al. 2011). The biggest concern about POPs is that they are known mutagens and/or carginogens and endocrine disruptors (Cole et al. 2011), which suggests that ingestion of these may cause problems, especially if there is significant transfer across the trophic level.
The sources and effects of microplastics are vast and complex, but hopefully I've provided with you with an informative overview over the last few posts. Next up - macroplastics!