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Restoring Polluted Soils With Cadmium-Resistant Microbe
The identification of a cadmium-resistant microorganism with the potential to promote plant growth, thereby improving phytoremediation efforts.

The dramatic usage of cadmium in the 20th century has led to an increase in cadmium pollution in our environment. Cadmium is a heavy metal that finds its way into our soils through phosphate fertilisers, sewage sludge amendments, and industrial discharge. A high cadmium concentration in the soil reduces the biological activity of soil microorganisms, which affects crop yield and quality. When taken up by plants, cadmium can enter the food chain and affect animal and human health.

In recent years, with the use of animals, plants, and microorganisms to take up or immobilise hazardous substances from contaminated soils, phytoremediation has been developed as a new way for restoring heavy metal contaminated soil.

To enhance phytoremediation efficiency, researchers from the Wuhan Botanical Garden of the Chinese Academy of Sciences looked into the abundance and composition of the microbial community in heavy metal polluted soil. In their investigation, the team isolated a cadmium-resistant microorganism, Penicillium janthinellum ZZ-2, which could potentially improve plant growth and reduce cadmium toxicity.

Soil samples were first extracted from a heavy metal polluted site in the Qingshuitang industrial district in Zhuzhou City, Hunan Province, Central China. Here, bermudagrass, an important plant for the remediation of contaminated soils, was the dominant plant.

The soil samples revealed a high concentration of zinc and cadmium, establishing a serious ecological risk. Through a series of statistical analyses, the team found that bacteria and fungi richness was significantly decreased in contaminated soils. This could be due to the inability of the organisms to deal with the stressors physiologically, suggesting that many microbial species cannot manage long-term heavy metal exposure.

However, long-term exposure to heavy metals could give rise to resistant microorganisms with significant metal absorption capacity. From the contaminated soil, one fungus strain, Penicillium janthinellum ZZ-2, survived at maximum cadmium concentration.

The data revealed that ZZ-2 has a high tolerance for cadmium, being able to absorb cadmium and survive. Furthermore, the team also observed that ZZ-2 could produce the plant hormone, indole-3-acetic acid (IAA), which helps in promoting plant growth.

To investigate ZZ-2’s potential in improving phytoremediation efficiency, the researchers inoculated bermudagrass with ZZ-2 and compared its results with uninoculated plants. The inoculated bermudagrass recorded a higher plant growth, lowered cadmium-toxicity symptoms, and increased accumulation of cadmium in the shoots and roots under cadmium stress. Furthermore, ZZ-2 potentially solubilised cadmium in the soil and produced the hormone IAA. These results suggest that ZZ-2 could be useful in cadmium-contaminated soil restoration systems as a potential supplement to improve phytoremediation efficiency.

Source: Xie et al. (2021). Identification of Cd-resistant microorganisms from heavy metal-contaminated soil and its potential in promoting the growth and Cd accumulation of bermudagrass. Environmental Research, 200, 111730.

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