Tracking down microplastic

They drift in seawater, are eaten by amphipods and isopods, and are frozen in the sea ice of the Arctic by the million – and yet they survive. These tiny plastic particles are referred to as microplastic. Up until just a few years ago, hardly anyone had heard of them; today, researchers have determined that they’re practically ubiquitous. “That being said, we still don’t know just where the microplastic comes from or how much of it there is in the ocean,” says Dr Gunnar Gerdts, an AWI microbiologist working on the island of Helgoland. That’s troubling news, given the fact that no one can say to what extent the tiny particles harm marine organisms, or whether they could ultimately pose a threat to human health.

Chemical analysis of tiny particles

Above all, Gerdts’ goal is to determine which substances the microplastic actually consists of. For his purposes, looking through the microscope won’t suffice. “You simply overlook too much plastic, because the particles sometimes look like grains of sand,” he says. Instead he relies on cutting-edge analytical equipment that can precisely identify the substances present in the particles. One example is the der Fourier Transform Infrared Spectrometer (FTIR), which essentially bombards the microparticles with infrared light, then uses an advanced mathematical method to analyse the amount of radiation it reflects back. Since different substances absorb and reflect different wavelengths, each can be recognised by its optical fingerprint. Using the FTIR, Gerdts and his colleagues can determine whether a given particle consists of polypropylene, polyethylene or a mix of different plastics. The working group has created and maintains a database containing the chemical fingerprints of several hundred polymers, which allows the researchers to immediately recognise which substance they’re looking at.

Catching particles with plankton nets

The North Sea is Gunnar Gerdts’ “hunting grounds” for marine plastic: not just on the open sea, but also in the mouths of the Elbe, Ems and Weser rivers. He’s also conducted research in the Baltic. To test for microplastic, the AWI biologist uses a neuston net, a fine-meshed net designed for gathering the organisms active near the ocean surface – collectively referred to as neuston – but which also “catches” microplastic.

The net is dragged behind a small catamaran some distance behind the mother ship and subsequently emptied into barrels on board. In order to measure how much microplastic is present, Gerdts and his colleagues have to cleanly and precisely separate it from the biological material and count the particles. “It’s a daunting task, because the mass of material that accumulates in the neuston net isn’t microplastic: instead, it’s mostly algae, crabs and sediment. To get to the microplastic, we basically have to dissolve it all.”

In the past he’s used nitric acid, caustic soda, and enzymes to dissolve the organic material. In some cases, the plastic is also dissolved in the process. “We had plenty of failed attempts before we learned how to cleanly separate the microplastic from the samples,” he says. Further, the FTIR had to be adapted for use with microplastic.

According to Gerdts, many research groups still exclusively rely on optical microscopes to investigate microplastic and estimate the concentration in seawater samples. “But that approach is extremely imprecise. We’ve analysed samples in which 97 per cent of the microplastic was initially classified as sand.” His own analyses indicate that one cubic metre of water from the North Sea contains an average of between three and ten microparticles. He has also examined the arctic sea ice that fellow AWI researchers brought back from their expeditions. The results are alarming. “We’re finding roughly one million particles per cubic metre of ice – and no one knows why there is such an enormous amount there. We have plenty of work ahead of us!”

Microplastic as a transporter for pathogens

As a microbiologist, Gerdts is not only interested in the plastic particles themselves, but also the microorganisms that settle on plastic surfaces. This is a perfectly natural process, since bacteria and single-celled organisms in the sea settle on practically every surface they can find – stones, ships’ hulls, or snail shells.

Gerdts and his colleagues have found various groups of microorganisms on the microplastic. Their most troubling discovery is that they also include pathogens – like the bacteria Vibrio parahaemolyticus, which can cause gastroenteritis, diarrhoea and vomiting. “Who knows,” he says, “In the future, the masses of microplastic accumulating in the ocean might promote the spread of diseases.”

The long-term consequences of the growing amount of microplastic in our ocean also interest Gerdts’ colleague Lars Gutow, a biologist whose work explores e.g. the effects of marine organisms consuming plastic particles when they feed. Gutow has found that different groups of organisms react differently to the particles: when it comes to isopods, the particles pass through their digestive tract and are simply excreted.

Yet the research of his AWI colleague Angela Köhler show that, in bivalves, the particles can spread from the digestive tract to the tissues and cells, where they can cause inflammations. “However, we should bear in mind that the microparticle concentrations used in the laboratory experiments were extremely high, so we can’t yet say for sure if the same inflammations would occur under natural conditions,” Gerdts adds.

Identifying the “super-loser”

Gutow now has an initial explanation for why the different organisms differ in terms of their responses. Isopods’ digestive systems include a filter of sorts, which prevents microparticles from finding their way to sensitive organs: as bottom feeders, it’s only natural that they often swallow small particles, like the tiny shells of diatoms, or grains of sand. They’ve developed an effective filter, unlike bivalves.

In the future, Gutow plans to compare further types of animals with a range of feeding strategies. “Ultimately, I hope to determine which group is the ‘super-loser’: the one most vulnerable to microparticles.” Is it the crustaceans, which ingest microparticles when they feed on algae, or is it the bivalves after all? Are animals that sift through the sand most at risk, or those that graze on rocks? “We might just succeed in finding the highest-risk groups.”

Working together to combat micro-litter

Yet even if they do, we still won’t know what can be done about the huge amount of microplastic already in our environment. “This problem affects a range of systems, since microplastic is produced in various sectors,” explains Gerdts. Massive quantities of plastic film are used in agriculture, and eventually break down into microparticles.

Gerdts and his fellow researchers have also detected considerable amounts of microplastic in wastewater and sewage sludge at treatment plants. Cosmetic products like peelings also contain microparticles, which usually end up going down the drain. “There are so many sources that in the future we’ll only be able to put a real dent in the microplastic by working together. The problem affects everyone – different interest groups, government offices, the industry and political decision-makers alike.”