Microorganisms living around the roots of plants may help restore mine tailings contaminated with toxic metals. In a study of an abandoned mine in China’s Guizhou province, researchers found that rhizosphere soils contained more genes associated with phosphorus solubilization and metal tolerance than nearby bare mine residues, suggesting that plant–microbe partnerships could support the recovery of polluted ecosystems.
Why mine tailings are difficult to restore
Mining leaves behind crushed rock and soil known as mine tailings. These materials can contain toxic metals such as lead, zinc and cadmium, which can harm living organisms. After mining operations end, tailings are often left in place, where they can interfere with plant growth and disrupt surrounding ecosystems.
Plants require nutrients such as phosphorus for growth and energy-related processes. However, much of the phosphorus in even healthy soil is bound within minerals and is difficult for plants to absorb. Available phosphorus is phosphorus dissolved in soil water in a form that plants can take up through their roots. Toxic metals in mine tailings can damage roots and prevent plants from absorbing this available phosphorus.
Microbes living around plant roots, in an area known as the rhizosphere, can increase the amount of phosphorus available to plants. Some of these microorganisms carry genes that enable them to produce chemicals that dissolve soil minerals, a process called phosphorus solubilization. The resulting dissolved phosphorus can then be absorbed more readily by plant roots. Researchers, however, have not fully understood how phosphorus-solubilizing genes function in soils contaminated with metals.
Sampling an abandoned mine in China
A team of Chinese scientists investigated this question by collecting rhizosphere soil from three plant species growing in an abandoned mine. They analyzed the DNA of microorganisms in the samples to identify genes involved in phosphorus cycling and metal tolerance. Their broader aim was to determine whether plant–microbe relationships could help remediate metal-contaminated soil.
The researchers selected an abandoned mineral mine in northwestern China’s Guizhou province. Lead concentrations in the mine tailings were approximately 6,400 milligrams per kilogram (mg/kg), more than 30 times the U.S. Environmental Protection Agency’s safe soil limit of 200 mg/kg.
The team collected rhizosphere soil from three local plants growing in the tailings: Artemisia, Buddleja davidii and Carex breviculmis. They also collected soil from areas of bare mine residue as a control. For each plant species, the researchers collected eight rhizosphere samples and eight control samples, producing 32 samples in total.
Metal concentrations and available phosphorus
The researchers dissolved the soil samples in acid and measured their metal concentrations using atomic absorption spectroscopy. In this method, a hot flame vaporizes the sample, and light is used to measure the metal atoms in the vapor.
All of the rhizosphere soils contained approximately 3,000 to 7,000 mg/kg of lead, 4,000 to 5,000 mg/kg of zinc and 30 to 60 mg/kg of cadmium. The control soils had similar metal concentrations.
To measure available phosphorus, the researchers mixed the samples with sodium bicarbonate, or baking soda. Its high pH helps prevent dissolved phosphorus from recombining with soil minerals. Rhizosphere soil associated with Artemisia contained approximately 7 mg/kg of available phosphorus. The concentrations were approximately 9 mg/kg for B. davidii and 7 mg/kg for C. breviculmis. All three values were higher than the approximately 6 mg/kg measured in the control soil.
Microbial genes linked to phosphorus and metal tolerance
The team extracted DNA from the samples and used DNA sequencing to identify the microorganisms and their genes. They found that 65% of the microbial genes involved in phosphorus solubilization were more abundant in rhizosphere soil than in the control samples. Soils with more available phosphorus also tended to contain microbial communities with more phosphorus-related genes.
The researchers focused on two genes: gcd, which is associated with phosphorus solubilization, and zntA, which is associated with resistance to toxic metals. Both genes were most abundant among bacteria in the groups Pseudomonadota and Actinomycetes. The researchers suggested that these microbial groups may contribute both to phosphorus cycling and to metal detoxification in the plant rhizosphere.
Potential for ecological restoration
Overall, the study found that rhizosphere microbes had more genes associated with phosphorus solubilization and metal tolerance than microbes in bare soil. The researchers concluded that these microorganisms may improve soil conditions in contaminated environments and suggested that using them alongside local plants could offer a more environmentally friendly approach to restoring polluted soil.
They recommended that future studies examine gene activity directly in contaminated soils and clarify how rhizosphere microbes regulate genes involved in phosphorus cycling under these conditions.
Source: sciworthy.com