More than just tiny pebbles - the sand on beaches can also consist to a large extent of the limestone shells of microscopic marine life. As a study has now revealed, this "biological sand" can apparently contain nanoplastics: The researchers have shown how the so-called foraminifera incorporate the particles into their sediment-forming shells. The scientists say that the significance of this for the important suppliers of building materials on many coasts of the world should now be clarified in more detail.
The ugly signature of civilization cannot be overlooked in many places: Garbage is lying around or bobbing on the banks of water. Plastic is particularly problematic because it does not biodegrade. Instead, the parts only break down into smaller and smaller pieces until they are referred to as microplastics and finally as nanoplastics from sizes below one micron. This does not make the problem any smaller – on the contrary: Studies show that the environment is teeming with these tiny particles and they are ingested by many different living beings. In the body of the organisms, they can then possibly cause problematic effects. However, the exact meaning of the growing burden of micro- and nanoplastics remains unclear. When researching this question, information on how the particles could affect living beings that are particularly important from an ecological point of view seems particularly important.
Important tiny things in sight
The researchers led by Marlena Joppien from the Leibniz Center for Tropical Marine Research in Bremen are now focusing on the so-called foraminifera. These are unicellular organisms, some of which form calcareous shells up to a few millimeters in size with chambers reminiscent of tiny mussels or snails. They are found worldwide, but mainly in warm, flat coastal areas. There, with their shells, they make a significant contribution to the formation of sand that is deposited on beaches and the seabed. As a result, they are of great importance for the stability of coasts. Joppien and her colleagues have now looked more closely at previous indications that foraminifera ingested plastic particles.
“These organisms feed, among other things, on microalgae or organic material particles that they find on the seabed. Micro and nano plastic particles are similar in size and could easily be mistaken for potential fodder,” explains Joppien. In order to investigate corresponding processes, the scientists carried out experiments with foraminifera from the group of Amphistegina, which are up to about one millimeter in size. In seawater tanks, they exposed the tiny creatures to polystyrene nanoparticles in a concentration that can occur under today's environmental conditions. They then used fluorescence microscopy to investigate the extent to which they absorbed the particles and incorporated them into their calcareous shell.
Eaten and integrated into the calcareous skeleton
As the researchers report, they found a large number of the yellow fluorescent nanoplastic particles in most foraminifera. "If plastic particles are small enough, the foraminifera take them up as food," says senior author Marleen Stuhr from the Leibniz Center for Tropical Marine Research. Although some of the protozoa rejected the plastic after the feeding attempts, around half of the foraminifera retained the plastic load. After eight weeks, an examination using scanning electron microscopy revealed that many of the protozoa had already encrusted the plastic particles with a layer of lime. In other words: the particles have been integrated into the limescale housing. The incorporation also seemed to be associated with the formation of new chambers in the structures, the researchers write.
But what do the results mean now? “The installation of the particles could have advantages and disadvantages for the environment. For example, the trillions of foraminifera on the sea floor could represent a sink for nanoplastics - a system that extracts plastic from the ocean," says Stuhr. But on the other hand, the particles could also harm the tiny creatures. To what extent this is the case, however, remains unclear. However, if the protozoa confuse plastic particles with their natural food and build them into their calcareous shell, their fitness, the formation of the shells and their stability could be disturbed - with consequences for their population as a whole, say the scientists.
This, in turn, could have long-term effects on coasts and islands that are already suffering from the burden of sea-level rise and increasing erosion processes. The contribution of foraminifera to coastal material can be significant: on beaches and in shallow sea areas, the shells are often deposited in densities of more than one kilogram per cubic meter. "The current results make it clear that the effects of plastic pollution on calcifying organisms need to be further investigated," the scientists write in conclusion.
Source: Leibniz Center for Tropical Marine Research, specialist article: Scientific Reports, doi: 10.1038/s41598-022-18547-4