Mosses and lichens come to the rescue in battle against air pollution

We cannot avoid breathing in the air around us. When we breathe, tiny air pollutants such as toxic trace elements can penetrate and spread throughout our bodies. These pollutants can also be absorbed by our skin. Once inside, trace elements find their way into our lungs and enter our blood system.

We might not be able to see these harmful elements with the naked eye, but we can detect them by using plants as well as nuclear physics and related techniques.

There is a rising need for every country to monitor concentrations of pollutants in the air. But studies have focused mainly on classical ones like carbon monoxide. Heavy metals have not received the attention they deserve despite having been identified as the most harmful active air pollutants by the World Health Organisation.

But new scientific research is making headway into ways of tracking less visible air pollutants. A collaborative research initiative is using a simple method called biomonitoring to assess levels of toxic trace elements in the Western Cape area. We are using mosses and lichens as air filters.

Once the invisible toxic trace elements in air have been revealed, certain air quality standards can be enforced. Industry will be encouraged to invest in technologies that emit fewer pollutants. Moreover, people will be encouraged to find ways of reducing the air pollution emissions like unrestricted waste burning.

Dangers of air pollution

Air pollution can cause chronic diseases, degrade the environment and even destabilise economies. Vehicle emissions and industrial growth are the major causes of air pollution.

At high concentrations, the effects of heavy metals in the air can lead to mortality. The World Health Organisation has linked premature mortality and reduced life expectancy to air pollution exposure. Air pollution also leads to forest decline and loss in agricultural production.

Unfortunately air pollution is not always noticeable. Heavy metals such as lead, cadmium and mercury carry the highest and most dangerous toxicity and can be found in clean-looking air.

Why mosses and lichens

What makes mosses and lichens especially suitable for studying air pollution is their underdeveloped root system. As a result they get their nutrients from the air through atmospheric dry and wet depositions.

Both mosses and lichens are non-vascular plants that act as natural air filters against toxic trace elements. They can be thought of as analogues to air filters. Their underdeveloped root system minimizes their contact with soil and so the amount of pollutants they get from the soil can be considered negligibly small. To survive they accumulate trace elements from atmosphere. In this way they are able to accumulate and retain air pollutants.

Mosses are green land plants with small leaf-like structures. Mosses lack some of the adaptations to dry environments that are found in the vascular plants and so are only able to grow and reproduce in wet environments.

Lichens are a complex life form that have an interdependent partnership with fungus and algea. Lichens do not have any roots, stems or leaves. They usually create disc-shaped structures and often have a grey or pale white appearance.

To identify the toxic trace elements in plants, we bombard them with neutrons, in a nuclear physics technique called neutron activation analysis. Once the accumulated trace elements in the plants absorb the bombarded neutrons, they become unstable. However, the trace elements prefer to exist on a less excited state which has energy higher than the absolute minimum also known as the ground state. Once they are radioactive, they de-excite by emitting high energy photons called gamma rays. These help in identifying different elements in the sample.

Each element will give a gamma-ray of its own unique energy. Gamma-rays indicate the presence of a specific element in the sample under study. Gamma-rays are presented in a spectrum in a form of peaks at particular energies and the intensity of each peak is related to the concentration of that particular element. In that way, scientists can be able to identify an element in the sample as well how much of it is there.

Apart from just identifying the kinds of toxic pollutants available in air, it is also important to know how much of those pollutants are there. This is because the extent to which one is affected by air pollution depends on the length of exposure and the amount of pollution in the air.

That is where mosses and lichens are unique. Their air pollutant fighting capability can be used worldwide.

This is important because, on a larger scale, long term results of air pollution will affect the planet’s ability to sustain life. Fresh air, pure water and unpolluted earth are the basic needs for humanity to continue to exist. Hence all living creatures have the right to an environment that is not harmful to their health and nations have a responsibility to keep the quality of air in a good state.

Zikhona Ndlovu is affiliated with Stellenbosch University and iThemba LABS. She works as a Junior Research Scientist of iThemba LABS in Cape Town. For her PhD studies at Stellenbosch University, she has been receiving funding from Stellenbosch University and National Research Foundation.