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A small light in the darkness of plastic pollution

Thursday 12 May 22

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Torkel Gissel Nielsen
Professor
DTU Aqua
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A new PhD lights a candle in the darkness of plastic waste. Contrary to the concerns so far, some of the smallest in the marine ecosystem do not ingest microplastics

 

In a world where plastic is no longer fantastic because it poses massive pollution and is a potential threat to a wide range of marine animals and birds, research is now turning on a small light in the dark.

"These organisms are at the base of marine food webs. If they ingested all the encountered microplastics, the presence of these particles in other organisms would be much higher."
Rocío Rodríguez Torres

New research on microplastics and copepods refutes for the first time the widespread fear that copepods confuse microplastics with food, eat it and thereby carry microplastics further up through the food chain.

Read the PhD thesis Ingestion and effects of microplastics on marine planktonic food webs 

But this is not the case according to new research results in the PhD thesis Ingestion and effects of microplastics on marine planktonic food webs. It turns out that copepods are able to sort their food and that they reject 80 percent of the microplastic they swim around in and that they capture:

Therefore, based on our findings, the role of copepods as vectors for entry of MPs in marine food webs can be considered negligible,” concludes author and Ph.D. Rocío Rodríguez Torres in her thesis.

Why is this new discovery important?

”This finding is relevant to better evaluate the impact of the microplastics in marine ecosystems. These organisms are at the base of marine food webs. If they ingested all the encountered microplastics, the presence of these particles in other organisms would be much higher,” says Rocío Rodríguez Torres and emphasizes:

Copepod Screendump
Click the picture and see copepod rejecting a microplastic particle. Video by Jiayi Xu

“However, observing the opposite, demostrates the need for more investigation in the fate and entrance of microplastics in marine food webs.”’ Microplastic ’ is plastic particles of less than 5 mm, ie. max the size of a grain of rice. Most microplastics occur as a result of the degradation process that breaks down large pieces of plastic into smaller ones.

Pioneering research with highspeed camera and hair
Previous laboratory studies led researchers to consider copepods as the link  for microplastics into marine food chains. There has therefore been great concern about how it could affect ecosystems – not least because both copepods and microplastic pollution in marine environments are found all over the globe.

Compared to previous studies, Rocío Rodríguez Torres' research is groundbreaking in several ways.

First, she has made use of the most advanced method available to take seawater samples. It is not an easy task to quantify small microplastic particles in the marine environment, as there is a lack of standardized methods, she writes in the PhD.

However, using a special plastic-free pumping system in the seawater surface, it was possible to measure the content of microplastic per cubic meters in the Arctic.

In addition, for the first time in microplastic research, Rocío Rodríguez Torres has used a trait based approach to study the copepods behavior with microplastic particles.

In the laboratory, based on her measurements of the amount of microplastics in the Arctic ecosystems, she has set up an experiment with a high-speed camera. She has used a suspension of microplastics and algae, besides a human hair, where she has mounted a copepod.

In this way, she has been able to keep the camera firmly focused on the activity and film the copepods' behavior in relation to different microplastic or algae particles. And here you can see how the small copepods reject microplastic particles.

How did you come up with that method?

“This method has was used in previous studies to observe the response of copepods when exposed to toxic algae. We got inspired by those investigations and we saw it was a good method to observe how copepods handle microplastics,” says Rocío Rodríguez Torres.

When Rocío Rodríguez Torres focuses on the Arctic, she also focuses on a particularly vulnerable ecosystem and, together with Antarctica and the Indian Ocean, a very little studied area in relation to the occurrence of microplastics.

Targeted at energy food

Rocío Rodríguez Torres’ discovery of how copepods do not just blindly confuse microplastics with food is related to their feeding behavior. Ie. that different copepods have different behaviors in terms of how they catch their food. It allows them to target the prey that gives them a high energy intake.

"Using this small scale video observations, we could directly observe and document how feeding-current feeders can capture the MPs (microplastics), taste it and finally decide to reject or ingest the particle,”writes Rocio Rodríguez Torres' PhD.

Can you explain how the copepods reject microplastics?

”All studied copepods behaviors on this thesis have shown low ingestion of microplastics. Different copepod species have different feeding strategies. One strategy called “ambush feeding” is a very efficient mode to feed on motile prey. So, since MPs are non-motile, we thought those copepods would ingest less microplastics.”

“However, the ingestion is as low as for other copepods with other feeding strategies. Other behavior, for example, spend some time handling and tasting the particle and they detect is not eatable. There are several reasons behind the low ingestion of microplastics by copepods,” says Rocío Rodríguez Torres.

Next step – nanoplastics among many others

Although Rocío Rodríguez Torres’ discovery of the copepods' lucky ability to avoid microplastics is good news, the extent of plastic pollution in the sea is still a environmental problem.

Marine waste such as old nylon fishing nets have been shown to kill many marine mamals, fish and birds. For example, it is believed that a large proportion of the world's sea turtles lose their lives because they are suffocated in the sea's plastic waste.

According to Rocío Rodríguez Torres, there is still a long way to go to get a complete overview of the concentration and distribution of microplastics in the oceans. At the end of her Ph.D. she emphasizes the importance of continuing to research plastic pollution, e.g. in nanoplastics, which are particles smaller than microplastics.

According to Rocío Rodríguez Torres, not much is known about nanoplastics, which are particles smaller than 1μm. However, they are known to have the potential to cause critical ecological damage because, due to their small size, they have the ability to migrate into the body's tissues.

In other words, a new darkness in the sea of plastic, which future research can hopefully shed a clarifying light on.

Furthermore, the increasing plastic production and consequently increasing plastic pollution could worsen and change the panorama in the future.

For the piture is my name: Rocio Rodriguez Torres

TOP PHOTO:

PhD candidate Rocio Rodrigeuz Torres in the algae room checking the status of the algae culture used to feed the copepods in the experiments, photo private.

 

 

 

 

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