SOURCE: Dawson, A. L., Kawaguchi, S., King, C. K., Townsend, K. A., King, R., Huston, W. M., & Nash, S. M. B. (2018). Turning microplastics into nanoplastics through digestive fragmentation by Antarctic krill. Nature communications, 9 (1), 1001. doi :10.1038/s41467-018-03465-9
Microplastics are tiny plastic pieces that enter the ocean directly or are produced when larger plastics breakdown. They are found in many health and beauty products such as toothpaste and exfoliating cleansers. Microplastics are prevalent in the ocean and as a result have been detected in many marine organisms. However, planktonic (floating species) that are filter feeders (strain water for their food) may be the most susceptible to ingesting microplastics as they indiscriminately feed on anything in the water. Two common plastic types, polyethylene and polystyrene, are less dense than seawater causing them to float in water. This makes it possible for planktonic species to ingest the plastic along with their food. Previous studies have shown that planktonic species are capable of ingesting microplastics, which can lead to detrimental health effects from obstruction of digestive system to reduced nutritional condition. The researchers in this study wanted to find out what happens to microplastics after they are ingested by Antarctic krill (Figure 1). The Antarctic krill is an abundant planktonic filter feeder in the Southern Ocean.
The goal of this study was to determine the fate of microplastics after they are ingested by krill. To do this, the researchers fed the krill two diet types that contained different amounts of polyethylene microplastic beads: a low plastic diet that contained 20% plastic and 80% algae and a high plastic diet that contained 80% plastic and 20% algae. The low plastic diet contained microplastic concentrations similar to those observed in the North Pacific, which are some of the highest concentrations reported globally. The krill were exposed to their diets in a lab for 4 days. At the end, the scientists collected the microplastics in the krill and their fecal material. They compared the size of the microplastics to that of the original microplastics the krill were fed.
Just as previous researchers had found in other organisms, the scientists in this study determined that the krill ingested the microplastic beads present in their diet. While some of the beads present in the krill and their fecal material matched the size of the original beads, the majority were smaller (Figure 2). On average the plastic beads inside the krill were 78% smaller than the original beads. This indicated that the krill were physical fragmenting the beads after ingestion. The researchers hypothesized that the microplastic beads were fragmented in the stomach and gastric mill, which have a cutting and grinding surface to break down food (Figure 3). However, digestive enzymes may have also contributed to the fragmentation of the microplastic beads.
This is the first time the ability to fragment microplastic has been documented in planktonic crustaceans. This may be because the researchers in this study used polyethylene microplastic beads whereas most other studies used polystyrene plastic. Polystyrene is more rigid and has a higher capacity to withstand stress than polyethylene. Polyethylene is one of the most common type of plastic in the marine environment. Therefore, even if the phenomenon is limited to polyethylene it could have an important impact on the marine ecosystems. What is that impact? It is up to future scientists to determine the fate of the altered plastics after they are released as fecal matter or following the death of krill that ingested them.