Reference: Al-Jaibachi, R.; Cuthbert, R.; Callaghan, A. (2018). Up and away: otogenic transference as a pathway for aerial dispersal of microplastics. Biol. Lett., 14: 20180479. http://dx.doi.org/10.1098/rsbl.2018.0479
Microplastics are now found all over, including in oceans, rivers, lakes, and land, creating a global environmental concern. When ingested by wildlife, microplastics can cause gut blockage, physical injury, and reduced energy levels. Microplastics can also carry harmful chemicals into wildlife, including humans. Microplastics have been detected in seafood and scientists are still working to understand the impacts of ingested microplastics on human health.
A microplastic is a plastic particle smaller than 5 mm in size. Some common types of plastic include polypropylene (found in bottle tops), polyethylene (found in plastic bags and food containers), and polystyrene (styrofoam). Microplastics can come in many shapes (fibers, pellets and cosmetic beads) and sizes. They might begin small (already at the size of a microplastic) or enter enter environment systems at a larger size and degrade to a microplastic .
MOSQUITOES AND MICROPLASTICS
While there has been a lot of research on microplastics in marine systems and organisms, very little research has focused on freshwater environments. Scientists know that microplastics can be ingested by aquatic organisms and transferred through the food chain, but do not know if microplastics can be transferred between life stages that use different habitats (also known as ontogenic transfer). For example, young mosquitoes begin their lives in freshwater, but move to air and land as adults. Specifically, they have four feeding larval stages and a non-feeding pupal stage that occur in freshwater environments, and then emerge as a flying adult and move to a terrestrial environment (Figure 1).
The scientists in this study wanted to determine if mosquitoes could transfer the microplastics they consumed as larva to other creatures as adults on land. Adults mosquitoes are eaten by terrestrial invertebrates (such as spiders and flies), meaning that microplastics may be able to move up the terrestrial food chain. This means that mosquitoes could be responsible for microplastics entering terrestrial ecosystems from freshwater habitats.
STUDYING MICROPLASTICS IN MOSQUITOES
The researchers chose to study the Culex pipiens mosquito (Figure 2) because it is found worldwide. They placed mosquitoes at the third larval stage into beakers containing water, pelleted guinea pig food and one of the following: 2 µm microplastic beads, 15 µm microplastic beads, or a mixture of the two sizes of beads.
The microplastic beads used in the study were fluorescent, meaning they glow under a fluorescent microscope. The fluorescence made detecting and counting the microplastic beads in the mosquitoes easier. The researchers collected one mosquito from each beaker when every mosquito had molted into the fourth larval stage and one mosquito when all the mosquitoes had pupated or emerged as adults (Figure 1). Then, the researchers extracted the microplastics from the mosquito and counted the number of microplastics under a fluorescent microscope.
THE RESULTS ARE IN
The scientists found microplastics in both larvae and adult mosquitoes. As mosquitoes matured through different life stages, the density of microplastics decreased. In other words, the larvae had higher densities of microplastics than the adults. The scientists also found more of the 2 µm size microplastic particles in adult mosquitoes than the 15 µm, suggesting that the size of the microplastics is an important factor in the transfer between larvae and adults. The scientists also determined that the mosquitoes took in more microplastics in the treatment with both sizes of beads versus the treatments when only one size bead was present.
Microplastics were found in the Malpighian tubules of the adult mosquitoes. Unlike the gut, Malpighian tubules are passed from larvae to adult stages without visible reorganization. The question now is how do the microplastics move from the gut to the Malpighian tubules. Scientists are not sure how this occurs in mosquitoes yet, but it has been documented in other organisms such as fish, fiddler crab and marine mussels.
WHAT’S THE BIG DEAL?
This was the first study to show that microplastics can be transferred between life stages of a mosquito. This is important because it means microplastics can be transferred from water to air and land. Further, when mosquitoes are eaten by flies, spiders, dragonflies, birds and bats, microplastics can be transferred up the food chain where they can cause harm to additional wildlife and possibly enter animals consumed by humans. While the researchers in this study focused on mosquitoes, other freshwater insects that live on land or in the air during their adult stages may also transfer microplastics.