By Jean Balchin 09/03/2018

A groundbreaking Griffith University study has found Antarctic krill which ingest microplastics are able to turn them into nanoplastics through digestion.

What are Krill?

Krill is a general term used to refer to around 85 species of free-swimming crustaceans called euphausiids, of which Antarctic krill is one species. Antarctic krill are one of the most abundant and successful animal species on Earth, and it has been estimated that the biomass of this species may be the largest of any multi-cellular animal species on the planet.

As krill mature into adulthood, they aggregate into huge schools that can stretch for kilometres across. Many thousands of krill are wedged in beside each other in each cubic metre of water, which turns red or orange.

Krill are very important to the ecology of the oceans, as they form the staple diet of many animals including seals, whales, fish, squid, penguins and other seabirds. Krill feed on phytoplankton and other zooplankters, filtering their feed by forming a feeder basket through water is passed. This basket retains food particles and the transports it to the mandibles for mastication, where it is cut and ground up. Then, the mushed-up food is directed through the short oesophagus into the stomach and gastric mill where it is mixed with digestive enzymes for further mastication.

“Despite a growing body of research, there are still considerable knowledge gaps regarding spatial patterns and abundance of microplastics in the marine environment,’’ Dr Dawson said.

“The phenomena of digestive fragmentation has never before been reported in other planktonic crustaceans despite the facts that many possess similar gastric mills and mouthparts designed for mechanical disruptions.”

The potential for translocation (movement across biological membranes) to occur after an organism has physically altered the ingested plastics was also studied by the researchers.

“This reveals a previously unidentified dynamic in the plastic pollution threat, with the implication that biological fragmentation of microplastics to nanoplastics is likely widespread within most ecosystems,’’ Associate Professor Bengtson Nash said.

“As such, evaluating the harmful effects of plastic pollution must take into consideration not only the physical effects to the individual arising from macro and microplastic ingestion, but also the potential cellular effects of nanoplastics. Similarly, a biological role in plastic fragmentation will influence life cycle assessment of plastics in the environment.”

Nash and colleagues exposed Antarctic krill to polyethylene (PE) microbeads along with an algal food source to determine the fate of microplastics ingested by a planktonic crustacean of high dietary flexibility and ecological importance.  The krill were exposed to daily feeding either on a ‘high’ diet (80% PE and 20% algae) or ‘low’ (80% algae and 20% PE).

The krill’s faecal material was collected throughout the experiment, and whole krill were enzyme-digested after exposure. It was found that all krill contained a mixture of whole PE microplastic beads and PE fragrments. However, these fragments were on average, 78% smaller than the original beads. Indeed, some fragments reduced by 94% of their original diameter.

Whole microplastic beads were discovered in the stomach, midgut and faecal pellets of the krill. Evidently, exposure concentration played an important role in the ability of krill to fragment the PE beads where lower plastic concentration appeared to facilitate the krill’s capacity triturate (grind to a powder) plastic.

The head of Antarctic krill. Observe the bioluminescent organ at the eyestalk and the nerves visible in the antennae, the gastric mill, the filtering net at the thoracopods and the rakes at the tips of the thoracopods. Wikimedia Commons.

Krill contained significantly more whole beads when exposed to a high plastic diet than a low plastic diet. At the beginning of each daily exposure, krill were efficient at fragmentation but as they ingested more beads the fragmentation efficiency decreased.

“Current contamination levels in the Southern Ocean are theoretically low enough to promote efficient digestive fragmentation by krill species, and in a global context, possibly for other zooplankton with sufficiently developed gastric mills,’’ said Dr Dawson.

The researchers observed microplastics within the oesophagus, stomach, digestive gland and midgut of deceased krill and plastic was also visible in the stomach of live krill.

Their mandibles frequently had plastic fragments enmeshed in the grinding surface. The bulk of plastic maceration took place in the stomach and gastric mill, responsible for mechanically fragmenting food particles under usual feeding conditions.

Antarctic krill survive on a primarily herbivorous diet, and thus have complex digestive systems. The researchers did not examine the examine the effects of digestive enzymes on microplastics. Therefore, the possibility that digestive enzymes contributed to the fragmentation of the microplastics cannot be ruled out. While small food items pass through a filter into the digestive gland, large plastic fragments and full-sized beads were excluded from the digestive gland and directed to the midgut for excretion.

Published in Nature Communications this week, the study formed the PhD research of Dr Amanda Dawson, was published in Nature Communications this week. The work was conducted under Associate Professor Bengtson Nash’s Southern Persistent Organic Pollutants Program (SOPOPP) in collaboration with the Australian Antarctic Division.

This study may be read online at Nature.