Utricularia vulgaris, common bladderwort, is often called a carnivorous plant thanks to the suction traps it has beneath the water. Recent research has suggested that it might be better to call the plants omnivorous, as they can eat other plants too. They’ll eat anything they find in the water. That can be a problem when the water contains microplastics, inhibiting their growth. Hongwei Yu and colleagues have looked at Utricularia vulgaris to see how microplastics affect them. They grew the plants in different concentrations of microplastics, with varying levels of nutrients.
They found that microplastics significantly altered the trap-associated microbial community structure and diversity. Plants had a lower relative growth rate, shoot length and chlorophyll content. At the same time, superoxide dismutase and peroxidase enzyme activities increased to cope with the stress. They found that the plants weren’t simply adsorping microplastics; they were also accumulating them in their bladders.
The authors conclude: “High nutrient contents could be a factor in alleviating depression from microplastic-exposure… There is a possibility that U. vulgaris could be used in phytoremediation.”
Usually, when botanists talk about phytoremediation, they’re talking about hyperaccumulators. These are plants that can pick up heavy metals from the soil to clean it. Samantha Lott at Lab Roots has found that a few people are now looking at aquatic plants as a tool for phytoremediation for microplastics. Another study she highlights is on Eelgrass, Zostera marina, and the bacteria that live on it.
Lingchao Zhao and colleagues examined how microplastics interacted with seagrass meadows. They found that the sediment in seagrass meadows collected microplastics, so they tried to find out how that happened. It turns out that the process starts with the plastics getting caught on the leaves of seagrass and forming a biofilm growing over grass, similar to how plaque grows on teeth. This biofilm then develops into a floc, which is like a free-floating film that can trap microplastics, eventually causing them to sink into the sediment.
The key to the film formation appears to be two bacteria that grow on the plant, Vibrio and Exiguobacterium. The team found that isolating these bacteria could cause the concentration of suspended microplastics to drop by 95% in forty-eight hours. Therefore, planting seagrass meadows and periodically collecting the sediment could also help remove microplastics from water.
Searching for plants and microplastics is a painful task. You’ll get every result on waste treatment plants that see microplastics as a problem and very few references to green plants. This confusion is why phytoremediation is such a helpful term. Industrial plants, as well as green plants, can filter out microplastics – but an industrial plant isn’t phytoremediating while it does it.
A search reveals two other recent studies on phytoremediation of microplastics. Auta and colleagues examine how mangrove environments can help. Like the seagrass study, they find it’s not actually the plant that tackles the microplastics, but the plant provides an environment for the microbes that can get to work. As with the seagrass study, bacteria formed a biofilm with the plastics to degrade them. The authors believe that a number of factors in the soil, including heat, moisture and salinity, contribute with the mangroves to provide a home for the plastic-digesting microbes.
In contrast to the other recent studies, Kat Austen and colleagues look at the potential of phytoremediation in a terrestrial context. They examined microplastic inclusion in birch tree roots. The team grew silver birch, Betula pendula, in pots containing microplastic beads between five and fifty micrometres in size. After five months, they examined the saplings’ roots with fluorescence and confocal laser scanning microscopy.
The botanists found particles between five and ten micrometres inside the plants’ lateral roots. It’s not clear how the particles entered and moved through the root, but the lack of larger particles indicates a threshold for mechanism. Austen and colleagues conclude that their study adds to previous work using silver birch for phytoremediation of chemical contaminants.
A common feature of all the research is that there has to be a balance between accumulation and toxicity. Simply throwing more nutrients at a plant to overcome toxicity will likely swap one environmental problem for another. However, breeding for traits to compensate for toxicity remains possible. If that happens in the future, it will be building on these early steps.
Austen, K. et al. (2022) “Microplastic inclusion in birch tree roots,” The Science of the total environment, https://doi.org/10.1016/j.scitotenv.2021.152085
Auta, H. S. et al. (2022) “Enhanced microbial degradation of PET and PS microplastics under natural conditions in mangrove environment,” Journal of environmental management, https://doi.org/10.1016/j.jenvman.2021.114273
Yu, H. et al. (2022) “Impact of microplastics on the foraging, photosynthesis and digestive systems of submerged carnivorous macrophytes under low and high nutrient concentrations,” Environmental pollution, https://doi.org/10.1016/j.envpol.2021.118220
Zhao, L. et al. (2022) “Eelgrass (Zostera marina) and its epiphytic bacteria facilitate the sinking of microplastics in the seawater,” Environmental pollution, https://doi.org/10.1016/j.envpol.2021.118337