Phytocleaning our planet

Life on Earth has multiple forms. Humans make up only 0.01% of Earth’s total biomass, while plants total a whopping 82.4%. Yet the value of plants is often neglected. Humans have thrived on this planet because of the other life that Earth protects. Advances in human civilisation, such as industrialisation, have polluted the planet, thereby affecting all living organisms. Efforts to clean up our planet are now at large, and humans have discovered that plants can be put to good use to clean up contaminated environments.

Anthropogenic activities result in organic and metal contaminants, such as persistent organic pollutants (POPs or ‘forever chemicals’), heavy metals, pesticides, petroleum hydrocarbons, and gaseous pollutants. These pollutants from various industry effluents and agricultural practices remain in the soil and leach into waters, thereby posing a major health risk by moving up the food chain. While removing these pollutants using traditional methods (with other chemicals or physical methods) is costly and of low efficiency, using biological organisms is cheaper, reliable, and more efficient.

Plants working to combat pollution

Phytoremediation is a general term describing a variety of processes using specific plants and microbes to diminish the toxic effects of contaminants in the environment. Here, we take a closer look at some of these.

Phytodegradation is the process by which certain plant roots secrete enzymes that degrade organic pollutants in the soil. Plants such as canna are efficient in rendering pesticides and explosives into non-toxic compounds, which are then expelled by transpiration. In association with bacteria residing in the plant, poplar trees can treat petroleum hydrocarbon residues in groundwater and thrive, because they transform the pollutant into less toxic compounds.

Toxic soil organic contaminants can also be broken down by plant secretions that increase the activity of soil microbes (bacteria and fungi), thereby hastening detoxification, via a process called phytostimulation. POPs in the soil have been shown to be degraded via root-associated bacteria.

Heavy lifting of heavy metals

Heavy metals such as nickel, arsenic, and lead are pollutants that can be removed via phytoextraction, whereby certain plants hyperaccumulate metals in their biomass. Some plants accumulate high amounts of nickel in their sap, turning it blue. Application of soil microbes or metal chelators enhance the ability of the plant to hyperaccumulate metal contaminants. The plants are then repeatedly harvested, and required metals isolated via ashing, a process known as phytomining. This procedure has been successfully used to clean up low-producing agricultural lands, non-mined land rich in minerals, or land used to previously mine heavy metals. Following clean up via phytomining, the mined land could be used to grow crops. With a global demand for nickel, sustainable processes such as phytomining should be seriously considered, rather than traditional mining which poses a threat to humans and wildlife, and is responsible for at least 10% of greenhouse gas emissions.

Phytostabilisation retains the pollutant in the soil by making it less toxic and contained. Plants secrete compounds that allow the pollutant to stick to the soil, or soil fungi sequester the compound and retain it in the soil. Certain grasses can stabilise toxic radioactive compounds and keep them contained in an area.

Cleaning the hydrosphere and atmosphere

Rhizofiltration involves growing plants on contaminated waters, whereby the plant roots secrete chemicals, and then get saturated with the absorbed pollutants. This is a method which the cut flower industry can exploit to clean up polluted waters.

Air pollution is now a global issue, and plants are at the forefront of efforts to enable clean air environments. Absorption of particulate matter and volatile compounds by plants and associated microbes enables this clean up.

Future research will make phytoremediation more efficient, for example by engineering plants to enhance the uptake of contaminants, using nanomaterials that reduce the toxicity to plants, and using plants as sources of biofuel. Sustainable, cheap clean up of the polluted planet is, therefore, an exciting and viable option.

Dr Radhika Desikan