Earthworms are kind of a big deal for environmental scientists. As it turns out, they’re not just food for those early birds – earthworms are detritivores, meaning they feed on dead organic matter and help decompose it, releasing nutrients that were sealed up back into the environment for plants and microbes to use. They’re a key player in the circle of life. However, sediment and soils have a tendency to accumulate a lot of pollutants, making it tough living for soil dwelling animals. Luckily, earthworms are masters at detoxifying pollutants through their unique physiology. Heavy metals, like lead or cadmium, are easily captured and stored away using specialized proteins called metallothioneins. These proteins have a remarkable capacity for binding onto free metals and transporting them away where they can’t cause any harm to sensitive tissues, sequestering them for long periods of time. Metallothioneins are common proteins – even we, humans, have them. What makes the earthworm system special is that the metallothioneins transport the bound metals to the liver (or the earthworm version of the liver, the chloragogen) where it covers the protein-bound metals with layers of amino acids and proteins, most likely to help eventually excrete them later. Earthworms are so good at this that oftentimes entire populations of worms that live in highly contaminated areas become almost completely resistant to the metals. This makes earthworms ideal candidates for cleaning up major chemical spills, remediating and cleaning the environment.
What’s even more interesting, however, is that this same metal detoxification pathway makes them an efficient (and more importantly, cheap) semiconductor factory. Researchers have found that the choragogen provides just the right conditions that allow metals like cadmium and tellurium to react and create tiny (high quality) nanoparticles called quantum dots. These miniscule particles, ranging from 2 to 10 microns across (that’s about 10 to 50 atoms!), are incredibly useful in the tech industry. When quantum dots are hit with a beam of light or have an electric current passed through them, they emit colored light, which happens to be sharper, brighter, and more vibrant than traditional LED lights. If you’ve recently bought a high definition TV then it’s very likely that the display you use to watch your favorite TV shows uses quantum dot display technology. But that’s not all, quantum dots can potentially revolutionize much of the tech industry, changing the way we approach anything from solar panels to lights, inks, and even biomedical technology.
What makes earthworm quantum dots so intriguing (aside from the fact that they come from worms) are the potential biological uses. Remember that layer of amino acids and proteins that the worms use to cover the metallothionein-metal complex? That is what chemists call a passivating layer, which means it helps protect the interior complex but also helps them dissolve and distribute in water. Nanomaterials are notoriously hard to dissolve in water as they tend to clump (much like when you mix oil and water), and as they say, the human body is mostly just water (roughly 60% or so). So any biomedical use needs to find a way to make quantum dots behave in watery bodies. So far all solutions people have come up with either make the quantum dots toxic to living organisms (which kind of defeats the purpose) or alters the quantum dots so much that we see a loss in performance. The humble earthworm seems to have found a way around all of that. Laboratory experiments show that quantum dots made by earthworms are easily dissolved and taken up by mammalian cells in petri dishes, with no signs of any toxic effects.
To be sure, there’s still a lot of quirks that need to be worked out and much to be learned still about the system before we’ll be seeing earthworm biotechnology farms cropping up (though wouldn’t that be something fun to imagine…). So for now, just bask in the mysterious glow of natural selection and ponder the series of serendipitous events that led to the evolution of a tiny nanotechnology factory within the humble earthworm. That’s certainly enough to keep me busy for a while.
Read more about earthworms and quantum dots doi:10.1038/nnano.2012.232