Infinitesimal particles inside our cosmetics, drugs, and processed foods are making their way into streams and oceans. There, they become a whole new food group for fish and other aquatic life. Although we treat them as harmless, the nanoparticles added to fish's diets may put them off their lunch altogether.
Manmade nanoparticles--bits of material built to be 300 microns across or smaller--have been booming over the last 10 to 15 years. In pharmaceuticals, they carry tiny doses of drugs into our bodies. In sunscreen, they protect our skin without creating an opaque white coating. Eddie Bauer uses them to make stain-repellent "Nano-Care" khakis.
But once these products have passed through our bodies or been washed off our skin, nanoparticles can journey out into the world, perhaps to be ingested by other organisms. And they don't travel alone. Like staticky socks, nanoparticles collect a coating of hangers-on as they pass through the hallways of an animal's body. Instead of dust and hair, though, they like to gather proteins. For example, several types of nanoparticles are known to trap molecules called apolipoproteins. These proteins are crucial to animals: they help us process the fat that we eat.
Researchers in Sweden set out to discover whether nanoparticles in a fish's environment would affect its metabolism. Would nanoparticles travel up the food chain and into the bodies of predatory fish? And after they'd ingested nanoparticles, would fish have trouble breaking down fats?
Inside the lab, the scientists set up a simple food chain. On the first day of their study, they added plastic nanoparticles to bottles of growing green algae. After 24 hours, they filtered out the algae and fed it to speck-sized crustaceans called daphnia, or water fleas. After another 24 hours, these tiny animals were removed and rinsed off. (The filtering and rinsing steps ensured that only nanoparticles that had actually been consumed would travel up the food chain.)
On the third day, the daphnia were fed to tanks of carp. The researchers observed the fish's feeding behavior, and timed how long it took the fish to gobble up 95% of the hapless water fleas. Then the cycle started over: new nanoparticles were given to new algae, which was fed to new daphnia, which were fed to the same tanks of carp. This went on for 30 days, with the fish fed every 3 days and weighed periodically.
The effects of the fish's new diet didn't show up right away. But after a couple weeks of eating water fleas infused with plastic nanoparticles--and accumulating those nanoparticles inside their own bodies--the carp behaved strangely.
Fish fed a nanoparticle-free diet consistently munched through their food in about five minutes. But the nanoparticle-eating fish slowed way, way down. It took these fish more than twice as long as the others to eat their meal. They moved sluggishly, not actively hunting for the tiny food animals that had been freed in their tank. Bizarrely, the researchers write, "test fish let daphnia swim in and out of their mouth without trying to eat them."
Because they expected nanoparticles to screw up the fish's fat processing, the researchers intentionally gave the fish too little to eat. This made all the fish lose weight as they began to burn up their fat reserves, just like any animal on a diet.
But as time went on, the nanoparticle-fed fish stopped losing weight. By the end of the month, they'd even gained a little.
The authors think that because nanoparticles had sucked up more and more of the carp's apolipoproteins as they ate the contaminated food, the fish couldn't use those proteins to process fat. When starved, they were unable to burn up their stored fat. And somewhere in the complex system of feedback loops that control eating and energy, the fish actually stopped losing weight--even as they lost nearly all interest in their food.
The plastic nanoparticles used in this study are a type that's handy for research, but not especially common outside of the lab. The nanoparticles in sunscreen, for example, are made of zinc oxide or titanium dioxide. Nanoparticles can have various shapes and surface charges, which determine what proteins they'll cling to as they pass through the world. But previous research has shown that the types of metal nanoparticles found in sunscreen can bind to lipoproteins, just as the plastic particles in this study did.
The authors don't address how the concentration of nanoparticles used in their study compares to the concentration that might be found in, say, a contaminated pond--or in your body. But it wouldn't hurt to look into how nanoparticles affect our own metabolisms. And the effect on the fish in this study intensified over time, as nanoparticles seemed to accumulate in their systems.
Every bit of nanoparticle-containing material we add to the waterways, then, might be contributing to some effect on fish. Maybe in the future we'll have to medicate them with nanodrugs.
Cedervall, T., Hansson, L., Lard, M., Frohm, B., & Linse, S. (2012). Food Chain Transport of Nanoparticles Affects Behaviour and Fat Metabolism in Fish PLoS ONE, 7 (2) DOI: 10.1371/journal.pone.0032254
Photo: Benson Kua/Flickr
Thanks to Michael Shuler at Cornell University for talking to me about nanoparticles. He published a paper earlier this month on the effect of dietary nanoparticles on chickens.