Bismuth, a metal regarded by industry as a potential green alternative to lead, can accumulate in certain aquatic organisms, with the risk of being transferred up the food chain. This is one of the main findings of a study published in the international journal Science of The Total Environment and carried out by ENEA, CNR-IRET (coordinated by researcher Massimo Zacchini), Sapienza University of Rome and the University of Tuscia, as part of the PNRR National Biodiversity Future Centre (NBFC) project.
The study assessed the effects of bismuth on two key species used as bioindicators of freshwater ecosystems: Lemna minor, a tiny floating plant known as duckweed, which forms characteristic green mats on the surface of ponds and still waters; Echinogammarus veneris, a small amphipod crustacean that plays an essential role in the recycling of organic matter and serves as a food source for fish, amphibians and birds.
The results show that both organisms accumulate very high levels of bismuth in their tissues, highlighting a potential risk of transfer along the food chain. But there is one key difference. Laboratory tests showed that the Lemna minor plant was more tolerant: even at high concentrations of bismuth, no damage to growth or alterations to the photosynthesis process were observed. “This is the first study on the effects of bismuth in an aquatic plant species and provides the first evidence of a plant’s ability to withstand high levels of this metal. In the absence of toxic effects, the results suggest that duckweed is capable of activating a mechanism to control metal uptake,” explains study co-author Valentina Iannilli, an ENEA researcher at the Biodiversity and Ecosystems Laboratory of the Sustainability Department. “On the contrary,” she adds, “a previous study of ours[1] on terrestrial plants had shown that bismuth can cause toxic effects during the early stages of development. For example, in an edible land plant such as cress (Lepidium sativum), where we observed inhibition of root growth and clear signs of DNA damage even at low concentrations, along with a high capacity for bioaccumulation of the metal.”
The small crustacean Echinogammarus veneris showed DNA damage even at the lowest concentrations tested. “Living in direct contact with sediments, where heavy metals tend to accumulate, these organisms are particularly exposed to bismuth (and other metals): they can ingest particles containing it and absorb it through their bodies, thereby increasing their exposure to and accumulation of the metal,” continues Iannilli.
Although bismuth is considered a low-toxicity metal, new data suggest that it may have sublethal effects on the most sensitive aquatic species, with potential repercussions for the overall balance of ecosystems. And the high capacity of the crustaceans analysed to bioaccumulate it, combined with their particular sensitivity to genotoxic effects, makes them, to all intents and purposes, useful sentinels for assessing the metal’s impact in the wild.
The natural concentration of bismuth in the earth is very low (around 0.025 parts per million), which is why it is classified as a minor metal. Among the ‘anthropogenic’ sources identified as potential routes by which bismuth enters the environment are dust from industrial and transport activities, residues from firearms, and particulate matter from fireworks. Given its increasing use in cosmetics[2] and medicine[3], it is reasonable to expect an increase in its presence in the environment: it has already been detected in Alpine ice, sludge and the wastewater of Swedish cities, likely due to the growing use of cosmetics and plastic products containing it. “This latter case warrants particular attention, especially given the use of sludge in the production of biofertilisers for agriculture. Overall, bismuth therefore deserves greater attention as a potential emerging contaminant, both due to its wide and growing range of uses and the ecotoxicological data that is emerging,” concludes Iannilli.
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[2] In cosmetics, bismuth is mainly used to give powders, foundations and eyeshadows a luminous, pearlescent finish.
[3] IBismuth has long been known for its medicinal properties and is widely used to treat gastric ulcers, burns and malignant tumours. Furthermore, its use is rapidly increasing in diagnostic medicine as a theranostic agent (i.e. one that combines therapeutic and diagnostic functions), for example to enhance image contrast. In recent years, certain bismuth compounds have also attracted attention in research into the development of broad-spectrum antiviral drugs: experimental studies have indeed shown that bismuth is capable of inhibiting key enzymes of the SARS-CoV coronavirus.
