After more than 40 years collecting, classifying and studying the life forms known as lichens, Dr Gintaras Kantvilas, from the Tasmanian Herbarium, has only scratched the surface of Australia’s biodiversity for this unusual group of organisms. “Each year, I discover at least 10 more lichen species that are new to science – and that’s just in Tasmania!” says Gintaras, one of Australia’s leading lichenologists.
So far, science has documented more than 3200 species of lichen in Australia. It’s an often-overlooked group found in almost every terrestrial environment, from tropical rainforests and arid deserts to rocky coastlines and city sidewalks.
From a global perspective many are cosmopolitan, with distributions that span continents. Venture outdoors and you’re almost guaranteed to see lichen growing on a tree trunk or concrete footpath. “From the seashore to the highest mountains, lichen can grow on many different surfaces, including rocks, trees, leaves, soil, glass and plastic,” Gintaras explains.
For such a common group, it’s surprising how little most people know of lichens. Many think, mistakenly, that they’re like mosses. But mosses are plants. And lichens certainly aren’t plants. In fact, when it comes to classifying them scientifically, lichens are so distinct that they are identified, like plants and animals, within a separate overarching kingdom: fungi.
Perhaps the most fundamental feature about lichens is that they are composite – not single – organisms. Each is formed through a symbiotic relationship between a fungus and an alga. The fungus enters this partnership because it needs to tap into the capability of its algal partner to produce food through photosynthesis – the process that converts light energy into carbohydrates. So, the algal partner delivers sugars and other carbohydrates. In return, the fungus provides most of the lichen’s physical structure in the form of a vegetative structure known as a thallus, as well as minerals and water. The fungus is an ‘obligate symbiont’, meaning it’s completely dependent on its photosynthetic partner and wouldn’t survive without it. The alga, however, could survive independently.
How do lichens reproduce? For many species, the answer is asexually, often producing fungus–alga cell bundles that break away from the thallus or lie sprinkled across its surface, to be dispersed by wind or water. Asexual reproduction can also occur when a section of the thallus breaks off and continues to grow.
But the fungal partner can, in addition, reproduce sexually by creating spores, which can be dispersed by wind, water or animals. These spores can then form into lichens if they encounter the right type of photosynthetic partner.
Lichens come in a variety of colours and forms. Foliose lichens are probably what comes to mind for most people when they hear the word ‘lichen’. These typically have a leaf-like thallus that’s not firmly attached to the surface it’s growing on. Many commonly encountered foliose lichens are classified in the genus Xanthoparmelia – one of the world’s largest lichen genera, occurring on every continent except Antarctica. Xanthoparmelia species are typically found growing on rocks.
Then there are fruticose lichens, which have a branching, pendulous structure, much like corals. Notable examples include cup lichens (Cladonia spp.) and beard lichens (Usnea spp.).
At the opposite extreme are crustose lichens, which have a flat, two-dimensional thallus like a thin layer of crust that’s tightly bound to the surface it’s growing on, so you can’t see its underside. Common crustose lichens in Australia include rim lichens (Lecanora spp.) and the many paint-like, black-speckled lichens seen on rocks.
“In Tasmania’s temperate rainforests and wet eucalypt forests, there are white lichens, including species of the genera Pertusaria and Phlyctis, that adorn tree bark. Most people think the trees grow white bark, but it’s actually the lichen,” Gintaras says.
Some lichens have earthy colours while others are vivid and eye-catching, including gold dust lichens (Chrysothrix spp.) and orange lichens (Xanthoria spp.). There are also firedot lichens (Caloplaca spp.), a cosmopolitan genus that spans every continent, including Antarctica.
Firedot lichens are widespread across Australia, but they’re arguably best-known for cloaking the large granite boulders in the Bay of Fires and surrounds in north-east Tasmania, giving the rocks their distinctive fiery orange hue. Experts are concerned the firedot lichens in this much-photographed bay might one day be impacted by rising sea levels and storm surge. “Caloplaca lives in a very specific part of the tidal area,” Gintaras says. “It certainly cops a lot of sea spray, but it cannot live submerged underwater, and it may not have the opportunity to relocate to higher ground.”
Lichens are often dubbed ‘pioneer species’ because they’re usually among the first life to colonise a barren surface such as a paved road or newly formed volcanic rocks. But many are also slow-growing. That’s a problem, considering climate change is prompting other organisms such as animals and plants to migrate into new areas more suitable for their survival. It’s not yet known if lichens will be able to keep pace with these range shifts. “Lichens have long survived slow changes to their environment, but we’re experiencing change much faster now,” Gintaras says.
Climate change is also fuelling more frequent and extreme weather events such as floods and bushfires. Both spell trouble for lichen. “Fire is a terribly destructive thing,” Gintaras says. “When it burns through an area, it can completely wipe out lichen habitat and species that won’t ever recover.”
Climate’s impact
Organisms in a symbiotic relationship are often more susceptible to the impacts of climate change, because more than one species is reacting to climatic pressures. “It’s usually the algae part of the lichen that cannot cope,” explains Dr Simone Louwhoff, a lichenologist from Australia’s Institute of Innovation, Science and Sustainability at Federation University. “But without the algae to photosynthesise and produce food, the symbiotic relationship between the fungus and algae breaks down and the lichen cannot survive.”
Because lichens can’t regulate their own water content, they’re dependent on water levels in the surrounding environment. Excessive rainfall can impact their algal component’s ability to perform photosynthesis, leading to oxidative stress and dieback, which may take a lichen community years to recover from. The good news is that many lichens fare much better in drought conditions because they’re desiccation-tolerant, able to survive periods of extreme dehydration by entering a dormant state.
This involves a sponge-like mechanism, which makes lichens highly sensitive to pollution. Some species are more susceptible to certain pollutants than others. These can be liquid, gaseous or particulate in form, and include sulphur dioxide, heavy metals, nitrogen oxides produced by car exhausts, agrochemicals and ammonia emissions from livestock, coastal oil spills and more.
The impact of sulphur dioxide is arguably the most well-documented. A stinky, colourless gas – produced during the burning of fossil fuels, smelting of sulphur-bearing mineral ores, and volcanic eruptions – sulphur dioxide dissolves when it comes into contact with atmospheric water vapours, creating acid rain. It’s a toxic precipitation that can interfere with photosynthesis in lichens and impact their reproduction and spore germination. It can also affect a lichen’s ability to carry out nitrogen fixation in species containing the blue-green algae known as cyanobacteria.
As well as soaking up harmful liquids, lichens can also absorb air pollutants, including fine particulate matter – everything from soot and heavy metals to dust from construction sites and roads, and even airborne microplastics. These microgranules can become embedded into the thallus, releasing harmful chemicals over time.
Lichens’ sensitivity to pollution makes them important indicators of environmental health. The presence or absence of certain species reflects local pollution levels; if an area has poor air quality, lichens will be less abundant and less diverse than in areas with clean air. Like a canary in a coal mine, changes in local lichen populations can be an early warning an area’s air quality is deteriorating. Scientists can also assess an area’s pollution levels by measuring concentrations of pollutants stored within lichen tissue.
Ecological marvels
Lichens play many ecological roles, from nutrient-cycling to sustaining small animals. “Lichens can provide food, camouflage and shelter for invertebrates,” Simone says. “Some larger species, like the eastern yellow robin, also use lichens to help build and camouflage their nests.” Lichens containing cyanobacteria can fix nitrogen from the atmosphere and convert it into compounds such as ammonia and ammonium, which plants can use. In semi-arid and drier regions, there are lichens that protect sediment by adding to the biological soil crust, capturing moisture and preventing erosion. They contribute to soil formation by trapping wind-blown soil particles and colonising rock surfaces, which they slowly weather through physical and chemical processes, releasing minerals.
These unique organisms have played an important role in human societies for thousands of years as medicines and dyes. “Different cultures have recognised lichens’ antimicrobial, antifungal and anti-inflammatory properties for many years, and now many of these properties are being supported by scientific research,” Simone says.
Image credit: Alison Pouliot
In this respect, it’s particularly valuable that lichens protect themselves using organic compounds produced by the fungal partner. These compounds, which are secondary metabolites, serve a variety of functions, from filtering harmful UV radiation to deterring grazing animals by making lichen taste foul or toxic.
They’ve been extensively studied because they show promise in biomedicine and pharmaceuticals due to anticancer, antimicrobial, antibiotic and antioxidant properties. Usnic acid, produced by genera such as Cladonia and Usnea, is one of the most studied secondary metabolites, and has already been used in commercial products such as topical creams, sunscreen, mouthwash, toothpaste and deodorant.
When it comes to lichen, there’s still much to learn – but Simone says everyday nature lovers can help uncover lichens’ secrets by reporting their sightings through apps such as iNaturalist. “Because they’re so small, they’re often overlooked,” she says. “We don’t know that we need to protect something, or how to protect it, until we know it exists and can learn about its role in the environment.”
