Another microplastics post for the blog. It seems endless, but they are now a leading subject in ocean pollution research. Microplastics are just about everywhere in the marine environment, found across trophic levels spanning from baleen whales to zooplankton. There are many laboratory studies to date that have confirmed the ability of planktonic species to ingest and egest microplastics. The typical results have shown that spherical microplastics either; pass through unaffected and excreted or are sufficiently small for translocation to occur. Recently, Australian researchers have completed a landmark study and identified a new pathway, discovering that krill can digest certain types of microplastics. Their study, recently published in Nature Communication, found Antarctic krill, Euphausia superba, can break down 31.5 µm polyethylene (PE) balls into fragments off less than one µm in diameter.
Dr Amanda Dawson, who completed the study as part of a PhD with Griffith University, exposed the crustaceans to PE microbeads (27–32 µm diameter) together with an algal food source to determine the fate of microplastics ingested by a planktonic crustacean of high dietary flexibility and ecological importance. Krill filter feed by forming a feeding basket through which water is passed. The particles are retained on the basket and then transported to the mandibles for mastication. The mandible has a cutting and grinding surface. Next, the food is directed through the oesophagus into the stomach and gastric mill where it is mixed with digestive enzymes for further mastication. The digested fragments were on average 78% the size of the original fragments, with even up to 94% smaller.
Co-Author Dr So Kawaguchi, a krill biologist from the Australian Antarctic division, said the krill had effectively turned microplastics into nanoplastics. The plastics they do consume are broken up by the animal’s gastric mill, which Dawson said functioned like a mortar and pestle. Within five days after ingesting the PE beads, all the plastics had left the krills’ digestive system, meaning that bioaccumulation in predators further up the food chain was unlikely.
The study also used brand new plastics. Dawson said that microplastics in the ocean would be even easier to digest because they have already been degraded by UV radiation. Now before we get all excited and think that Krill are the plastic munching answer we’ve been waiting for - “It’s not necessarily helping plastic pollution, it’s just changing it to make it easier for small animals to eat it,” she said. “It could be a new source of plastics for the deep ocean.”
Some additional interesting questions still remain. Which other groups of organisms have these abilities as well? Also, what about fibrous microplastics as they make up a great proportion of microplastics?
For the free full text of this interesting work please see:
https://www.nature.com/articles/s41467-018-03465-9
Dawson AL, Kawaguchi S, King CK, Townsend KA, King R, Huston WM and Bengston Nash SM (2018) Turning microplastics into nanoplastics through digestive fragmentation by Antarctic krill. Nature Communications. 9. Article number: 1001 doi:10.1038/s41467-018-03465-9
