Hunting down air pollution: Meet the lichens

What makes lichens worth studying and admiring?

In minimis natura praestat / Nature excels in the smallest things. - Pliny -

You might already know three facts about lichens: that they are a famous example of a symbiosis; they are pioneer species; and they are bio-indicators of air pollution. But what many people do not realise is that lichens are actually a community of fungi and algae or cyanobacteria. The fungi provide protection from drying out and shield against too much sunshine, while algae or cyanobacteria use photosynthesis to produce sugars for both partners. And since lichens look like just one organism, until the development of microscopes, people were convinced that lichens were just that, and when Simon Schwendener claimed, in 1867, that lichens are in fact two organisms – fungi and algae - living supportively side by side, many lichenologists initially refused to believe it.

Nevertheless, the word “symbiosis” was first used in biology in 1877 to describe a lichen as a close, mutually-beneficial interaction between organisms. After Schwendener’s discovery, it was shown that, in some lichen species, cyanobacteria can replace the algal role as a partner (these are called cyanolichens) or that there can be both algae and cyanobacteria in association with a fungus (these are called "tripartite lichens"). Also, recent discoveries show that bacteria found on lichens can have an important role in the symbiosis. But the latest surprise in the lichen symbiosis was a discovery led by Toby Spribille in 2016. He found that there is one more indispensable partner composing a lichen: a form of unicellular fungus which is present in almost all lichens, but whose role is still unknown. The complexity of a seemingly simple organism is still unveiling, thus bringing intrigue and excitement to many scientists.

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Lichens are amazingly diverse in their form, from the dry crustose “skin” you see on rocks, to the coralline-looking lichens in sand and the beard-like lichens hanging from trees. Symbiotic partnerships like this were thriving even before vascular plants evolved and have originated independently several times, which makes lichens an incredibly diverse group for studying – akin to studying all vertebrates. Such a successful cooperation between lichen partners has enabled them to inhabit and flourish in almost all habitats on Earth. Among the most extreme lichens are those living a cryptoendolithic lifestyle, meaning they live inside rocks. These lichens persist in the driest and coldest places in Antarctica.

Lichens are regarded as pioneers owing to their self-sustainability and low ecological requirements. They need only moisture and sunshine to survive, and this need not even be present throughout the year. Over time, lichens are also able to prepare life conditions necessary to support other organisms. They are effective in biodeterioration of rocks and have the capacity to accumulate different nutrient elements (i.e. nitrogen, sulphur, phosphorus), thereby increasing availability of these elements for other organisms. Furthermore, after they decompose, they leave the soil enriched with these nutrients and other particles trapped from the air. Thus, colonisation of a landscape by lichens is an important first step in the succession of barren locations and areas devastated by disasters like volcanic eruptions. 

How do lichens respond to air pollution?

Lichens are present almost everywhere, from the leaves of tropical trees to windblown rocks in the Antarctic, and from sand dunes in the Sahara to snow-burdened trees in Siberia. But, surprisingly, there are nevertheless some places where even lichens struggle to survive. From the beginning of the industrial age in the 18th century humans have produced numerous sources of air pollution. Air pollution has influenced not only our health, but also the health and stability of all ecosystems and the economy. Although lichens are capable of surviving harsh environmental conditions, many lichen species are highly sensitive to air pollution. In the 19th century, independent observations from England, Munich, and Paris recorded disappearance of lichens from urban areas. Lichens’ sensitivity to air pollution is related to their biology - they take almost all their nutrients and water directly from the atmosphere. They lack stomata and cuticles, so pollutants can be absorbed over their entire surface and they have little biological control over gas exchange. Moreover, they have no deciduous parts to shed after the exposure to a pollutant so excreting toxins isn't an option either. It is important to emphasise though that not all lichens are equally sensitive to air pollutants: different species show different sensitivities to different pollutants.

“In 2012 around 7 million people died - one in eight of total global deaths – as a result of air pollution exposure. This finding more than doubles previous estimates and confirms that air pollution is now the world’s largest single environmental health risk. Reducing air pollution could save millions of lives.” - World Health Organisation (WHO), 2012...

This means that lichens can be used as sensitive bioindicators and biomonitors of air quality and aid in investigating and monitoring air pollution problems. Bioindicators indicate the presence of the pollutants. Biomonitors, on the other hand, are used for experiments measuring the physiological responses to atmospheric pollution over time and providing additional information about the amount and intensity of the exposure. According to the official European standard protocol for biomonitoring with lichens, the benefit of using bioindicators and biomonitors is that the effects on them are the result of time-integrated, complex influences. This approach combines both air quality and local climatic conditions, which reveals biologically-relevant indications of environmental health. Physico-chemical measurements of ambient concentrations and deposition  of pollutants, together with emissions data, are irreplaceable sources of information on air quality. Thus, bioindicators and biomonitors can give us a biologically-relevant response of an organism to a mixture of pollutants from urban and industrial sources.

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There are a few different ways to use lichens like this as air quality detectors. One is to determine the biodiversity of lichens of an area. There are different levels of lichen tolerance to the pollution and lichen biodiversity decreases as air pollution levels rise. Severe pollution results in a "lichen desert" – the complete disappearance of lichens, like in the urban areas of the 19th century. The second way is by determining the changes in lichen physiology. Before they disappear because of pollution, lichens are placed under biochemical stress. This causes damage that can be monitored by measuring the lichen chlorophyll concentration, secondary metabolites, and the permeability of their cell membranes. This type of monitoring can be carried out on native lichens which are already present in the research area, or on lichens that are "transplanted" from the area without air pollution to the polluted area. The third way to use lichens as bioindicators and biomonitors is by measuring the build-up of pollutants in lichens. Lichens are known to bioaccumulate airborne substances, primarily heavy metals, but also non-metals such as sulphur and nitrogen. By measuring the amounts of these substances in lichens, we can deduce the concentrations of these substances in the ambient air over time.

In my PhD research I combine different approaches of air quality research and monitoring on one area, to gain a better theoretical understanding of the problem and to design the optimal system for biomonitoring of air quality using lichens. I chose to study the air quality of the city of Slavonski Brod. This is the city with the biggest air pollution problem in Croatia mainly due to the emissions from an oil refinery. To explain the physiological changes caused by air pollution from the oil refinery I am using two sampling approaches (native lichens and transplants) as well as various physiological parameters of lichens, with respect to other environmental factors including the wind direction and wind speed. My research has already shown concerning air quality problems in the study area, and a “lichen desert” at sites closest to the oil refinery.

Lichens might get overlooked by most people, but they tell an incredible story of cooperation, survival and persistence that all of us could learn from. So, next time you stumble upon lichens on your walk, stop; take a closer look and do not be embarrassed to study the funny colourful flecks. Let yourself be inspired to start cooperating with the most diverse people, appreciate how nothing can stop you from progressing to environments you never dreamed of, and set the ground for others to follow...