Researchers have found that the impact of microplastics on aquatic plants may not be as clear-cut as expected, depending on the prevailing aquatic conditions.
In recent years, microplastics are acknowledged to be virtually ubiquitous, having been found in the most remote locations on earth, from the summit of Mount Everest to the bottom of the Mariana Trench. Naturally, interest in their effects on environments and living things has been growing in recent years and has attracted the attention of many scientists.
To address the lack of knowledge regarding the effects of microplastics on heavy metal toxicities and fates on aquatic biomes, a team of scientists from the Wuhan Botanical Garden of the Chinese Academy of Sciences and Wuhan University studied the adsorption of copper ions (Cu2+) on different sizes of polyethylene microplastic particles, which included particles of 5 and 150μm. The effects of adsorption of the copper ions on the surface of the microplastic particles were observed in four species of aquatic flora, namely Cabomba caroliniana, Ceratophyllum demersum, Elodea nuttallii and Hydrilla verticillata, using absorption tests and batch experiments.
In their experiments, it was found that Cu2+ was fatally toxic to aquatic plants. From previous studies, metal ions in aquatic habitats were found to significantly inhibit carbonic anhydrase (CAext) activity in aquatic plants, reducing their bicarbonate utilisation efficiency, henceforth affecting their growth, which may explain Cu2+’s fatal toxicity.
It was also found that the polyethylene microplastic particles alone did not negatively affect the growth of plants. However, the addition of microplastics to the Cu2+ contaminated environment helped reduce the Cu2+ concentration, making the environment less toxic to plant growth.
Since different species of plants have different tolerances to Cu2+, the effect of the reduced toxicity of the Cu2+ ions due to adsorption by the polyethylene microplastics varies, but the trend of reduced toxicity of Cu2+ ions in the presence of the microplastics for all species was observed. The researchers hypothesised that the microplastics keep the ions from moving freely in the aquatic environment by adsorption, reducing their bioavailability and thus their overall toxicity to the aquatic plants.
Besides the polyethylene microplastic particles’ ability to reduce the toxicity of Cu2+ ions, they may also act as a carrier for these ions. If these microplastic particles travel to an aquatic environment with a low concentration of Cu2+ as a result of external environmental factors such as water currents or wind, the desorption of Cu2+ ions might also occur, releasing the Cu2+ ions into the new environment and increasing their toxicity. This is due to the adsorption equilibrium of copper ions on the surface of the microplastic particles, which will be recalibrated when exposed to a change in concentration of Cu2+ in the environment.
With this study, more has been understood about the effects of microplastic particles in aquatic environments, where they might serve as a “remedy” to “neutralise” the toxins, or “poison” in desorbing and releasing the Cu2+ ions into environments with lower Cu2+ concentrations.
Source: Zhou et al. (2022). Antidote or Trojan horse for submerged macrophytes: Role of microplastics in copper toxicity in aquatic environments. Water Research, 216, 118354.