Microplastics make sense. Plastic doesn’t degrade. Well it does, but just very slowly, piece by piece. And it is everywhere.
In a related review by the Royal Society I came across another statistic which caught my eye. Mostly because it relates to where I live.
There are 84, 030 microplastic particles per square metre in the upper River Mersey. This feels a bit like ‘4000 holes in Blackburn, Lancashire, and although the holes were rather small, they had to count them all’*. The authors did not ‘count’ them all but that sounds like a lot.
Microplastic particles are bits and pieces of plastic less than 5mm – roughly the size of an ant. They come in all shapes and sizes; films, fibres, beads, regular, irregular, spherical. You name it.
They also persist in the environment. As mentioned, plastic waste does not degrade into the ether. It degrades into smaller and smaller, micro- and nanoparticles (<0.1microm). The study in the catchment in the River Mersey is the largest volume of microplastics recorded to date in a freshwater ecosystem. There will be higher concentrations elsewhere.
What has this got to do with mosquitoes?
Microplastics enter oceans and river systems in a variety of ways including sewage, erosion, and even via offshore winds. Once in the environment, organisms such as fish, mussels and insects ingest the particles. Here, they get stuck in the food chain.
A few months ago, we ran a journal club on a paper led by the University of Reading, testing something called ontogenic transference. In other words, the transfer between two different life stages of an organism that live in different habitats.
Mosquito larvae fed fluorescent plastic beads in the lab light up under a fluorescent microscope. This isn’t too surprising in itself but perhaps more interestingly, a proportion of the fed microplastics remain in the tissues of the mosquito as it develops from pupae and, finally, to adult.
Could microplastics have an impact on mosquito behaviour or their capacity to transmit pathogens? This is debatable given that only a small percentage of microplastics seem to remain in the adult (about 1% at most looking at the paper), however, a clue might lie in where microplastics concentrate. The Malpighian tubules are five canal like structures that form part of the excretion system and are vital, in the case of female mosquitoes, for dealing with all the toxins and foreign compounds from ingested blood meals.
The presence of microplastics lodged in these structures could disrupt the finely-tuned process of excreting the waste from a blood meal equivalent to their body mass.
What is sure is that mosquitoes, like other aquatic organisms, can act as a vector of microplastics in the environment.
So can mosquitoes vector microplastics through the food chain, from aquatic habitats to terrestrial animals such as birds and bats which rely on mosquitoes for food? Quite possibly, although whether the volume of plastics are sufficient to have any impact on other forms of life is unclear.
What I do know is that there are a lot of microplastics in the Upper River Mersey. So do your bit and recycle.
*A Day in the Life, The Beatles (but you knew that, right?)