A carnivorous plant suffers poisoning from microplastics

Botanists identify microplastics entering traps as reducing the growth of a carnivorous plant.

We’ve mentioned before that microplastics could pose a threat to plants as well as animals. Jingzhe Zhou and colleagues have more evidence that we’ll be hearing more of this. They have studied how polyvinyl chloride (PVC) and polyethylene particles interfered with bladderwort plants. They found that it was the bladderwort’s focus on microscopic prey that made it particularly susceptible to poisoning from microplastics.

The bladderwort, Utricularia spp., are the fastest carnivorous plants in the world. They feed through small bladders attached to long stems that run underwater. The bladders are emptied and primed so that when an animal triggers the trap, a door opens. The trap sucks in water, and anything floating in the water, with amazing speed. The acceleration is over 600g, and over faster than you can even think of blinking an eye.

Botanists have been investigating what you need to do to trigger these traps. They’ve found that some seem to fire at random. In recent years it’s been discovered that bladderworts not only eat animals, they eat microscopic plants too. It’s thought that they might even ‘farm’ algae as a food source.

The problem for bladderworts is that the traps grab whatever is outside when they fire. That means if pollution increases they may have a problem. In particular, botanists are now looking at the pollution on the microscopic scale, meaning that microplastics and nanoplastics are now a serious matter for research.

To see how microplastics were a problem, Zhou and colleagues looked at how Utricularia aurea Lour, the golden bladderwort, handled them. The scientists though that it would be the smaller plastics that would pose the more potent threat to the plant, thanks to the size of the traps.

Utricularia aurea in a lake/
Utricularia aurea. Image: Canva.

To test the plants, the scientists got spheres of polyethylene and PVC to put into tanks containing the bladderworts. They mixed the plastic in with water in the same concentration, 50mg per litre. They then set up the bladderwort plants in a series of tanks with different conditions to see what effect the plastics would have.

The first result showed there was a definite difference in the effect of the plastics, with PVC having a worse effect on plant growth than polyethylene. One key difference between the two plastics was that the PVC particles were usually below 100μm in size, whereas the polyethylene particles were often larger. This difference mattered because the traps are able to take prey of up to 100μm in size. The polyethylene particles were simply too large to be caught by the traps as often. The plants catching PVC had reduced elongation suggesting that plastic in the traps was having a toxic effect.

To check that it was the plastic in the traps that caused the problem, Zhou and colleagues had another experiment. Bladderworts sat in the same solution of PVC and water – but they had their bladders stripped. Comparing the growth of this plants with stripped plants in plastic-free water showed that the plants in the plastic solution did no worse. So it was the combination of plastic particles, and the traps to ingest them, that caused the toxic effect. In addition they found that when plants caught the plastic, it adhered to the inside of the traps, possibly making them less useful to the plant when reset.

The results are worrying when you consider that microplastics and nanoplastics are getting everywhere. We don’t rely on bladderworts, but they are a canary in a coal mine. Other plants won’t take up plastics in the same way, but may be at risk from picking them up via their roots. Understanding the dangers will be a first step to mitigating them.