Microplastics have long been ubiquitous in the ocean. But at least mussels seem to be coping well with this water pollution so far, as long-term tests show. The animals showed no loss in growth rate, filtration or the formation of their adhesive hair due to microplastic contamination. However, the activity of certain enzymes was comparatively lower in them.
Whether on the surface of the water, in the deep sea or in the polar ice – plastic parts or microplastics can now be found everywhere. With partially fatal consequences for the animal world: some whale species, sharks and rays perish if they swallow too much larger plastic parts or if they get tangled. In addition, it is estimated that 90 percent of all seabirds have had plastic in their stomach and some of them die from a single piece of plastic swallowed. And among the fish in the middle depths of the Atlantic today, around three quarters of the animals probably live with microplastics in their stomachs.
How do microplastics affect mussels?
How much small marine organisms are at risk from microplastic contamination, however, has so far been investigated less often. That is why Thea Hamm and her colleagues from the GEOMAR Helmholtz Center for Ocean Research Kiel have now conducted a laboratory experiment to study mussels for the effects of microplastics. They are very well suited as model organisms for such a study because they are widespread in many coastal ecosystems and absorb and filter large amounts of seawater contaminated with microplastics for ingestion.
For their long-term experiment, the scientists exposed around 300 young mussels to various concentrations of microplastics over a period of 42 weeks. “The special thing about this study is not only the long period of time, but also that the pollution in the test tanks corresponded to values that we actually measure in the environment,” says Hamm. “Many earlier studies only ran over significantly shorter periods of time, but used unrealistically high plastic concentrations. That can of course falsify the picture, ”adds co-author Mark Lenz.
The team also tested the reaction of the mussels to different types and sizes of microplastic. “We used uniformly round particles, such as those used in cosmetics, for example, but also irregularly shaped ones, such as those that arise when larger plastic parts break down,” explained Hamm. During and after the experiment, the researchers measured, among other things, the growth rate of the young mussels, the production of the adhesive threads with which they hold on to the ground, and the rate at which they filtered food algae out of the water. They compared the data with control animals that were kept in clean water at the same time.
Hardly any negative reactions
“Contrary to popular fears, our study shows that the mussels are hardly affected by microplastics in the water, even over a longer period of time,” reports Hamm. She and her team found that the growth of the mussels did not differ from that of the control mussels until the end of the test, despite the plastic particles. “The growth rates of the test mussels were positive throughout the experiment and ranged between 0.005 and 1.850 millimeters per week, with no significant difference in the growth rates between the test groups being observed,” said the researchers. The volume of water filtered by the mussels per unit of time also hardly differed. Only those animals that were exposed to the highest concentration of microplastic particles with tens of thousands of particles per week had a slightly lower filtration rate.
In addition, the researchers were able to observe that the production of the adhesive threads was comparable in all test groups. Only the mussels that were exposed to the smallest polystyrene particles, such as those found in insulation materials, produced around 40 percent fewer adhesive threads at the end of the study. It was not until late in the experiment that the microplastic had another weak negative effect on the performance of the mussels: the activity of certain enzymes in the gills of the mussels, the superoxide dismutases, was significantly lower in comparison to the control group in clean water, according to the research team .
Overall, the laboratory study nevertheless indicates that microplastics – at least in the concentrations in which they are currently found in the sea – only pose a comparatively low threat to blue mussel populations. “Their high adaptability to the presence of inedible particles in the water presumably makes them robust against this new form of marine pollution,” the scientists conclude. “This is reassuring news at first,” said Hamm. “But it does not mean the all-clear regarding pollution with microparticles in general. Other species may react differently. We just need more long-term experiments under realistic conditions. “
Source: GEOMAR Helmholtz Center for Ocean Research Kiel, Article: Science of the Total Evironment, doi: 10.1016 / j.scitotenv.2021.146088