Observations of Dyeing;
Or, A Practical and Theoretical Overview of Lichens in Singapore

Fig. 1: Dirinaria picta (right) and a lichen from Graphidaceae (centre) touching each other on a tree. Image credit: Alvin Chang Jia Jun.

It is often said that knowledge is a curse. But never did I understand the true weight of this statement, as we so often fail to when it comes to such trite aphorisms, until I had experienced it for myself. For I had acquired a vision that oppresses eyesight and a resolve that constantly waylaid me. It is such a burden that I hope that you will bear along with me through this article.

You may wonder: for what did I strain my eyes and unavoidably prolong all outdoor journeys? It was for that which encrusts the earth, the stones, and the trees from pole to pole. It was for lichens. They are one of those things that make our city awash with colour, speckling tree bark and concrete alike with remarkable dissimilitude. Yet it is this incredible patchwork of colours that humbles us into simply accepting the innumerable shades as part of the unknown complexity of all things.

Lichens are essentially fungi that have adopted photosynthetic algae. The fungal component of most lichens hails from the phylum Ascomycota that includes most kinds of mould. The algae live within the fungi in a mutualistic relationship. Since the algae make food through light energy, the fungi no longer need to consume rotting organic matter, which is known as saprophagy, to obtain nutrition. In return, the fungi protect the algae from excessive sunlight and provide them with water and minerals.

The relationship is not, however, merely a situation of scratching each others’ backs, because it involves dramatic changes in both the parties involved. The thallus which makes up the body of the lichen is very different in appearance from that of the free-living fungus, mostly due to the colour of the algae. Moreover, the filaments that make up the thallus, known as hyphae, are different from the hyphae of free-living fungi, even on a microscopic level. For example, lichen hyphae have a higher gelatin content in their cell walls. The algae are also changed by chemical stimulation from the fungus. They uncontrollably leak sugars, mostly glucose, to feed the fungus, a practice that would almost certainly kill them if they were not in the fungus. This continues for up to 24 hours, even after they are taken out of the fungus. Even so, the lichen is usually named for the fungal component, because that makes up the majority of the lichen’s biomass. This intimate symbiosis is so life-altering for both partners that the free-living fungus is often mistakenly identified as a completely different species from the fungal component of the lichen. But in a sense, it still is a different being.

The fungi can partner with either green algae or blue-green algae. Some lichens that have partnered with a green alga can suddenly acquire a blue-green alga as a third partner, keeping it in little dark lumps on the surface called cephalodia. Other types of tripartite symbiosis  have also been recorded, such as a species of yeast that was found to support the structure of beard-like lichens.

Lichens are often classified into three categories: crustose, foliose, and fruticose. Crustose lichens look like a flat crust, foliose lichens have flat and leaf-like lobes, and fruticose lichens grow into complex 3-dimensional forms that are only attached to the substrate by a small base. Generally, crustose and foliose lichens are distinguished from each other by the fact that foliose lichens have both upper and lower cortices, which are protective layers. Crustose lichens only have an upper layer, so they are totally attached to their substrate.

Spotting

Despite the general inconspicuousness of lichens, there are some which are more recognisable. The second most common species in Singapore, Dirinaria picta, is perhaps an archetypal lichen – when you see it, you can probably tell that it is a lichen. This is because of its pale-green colour, which stands out starkly on tree bark, and its distinct thickness, because its lobes are very tightly compacted together. Although technically classified as a foliose (leaf-like) lichen, it can appear somewhat crust-like. The lobes are distinctly visible only upon close examination. The words crustose, foliose, and fruticose, therefore, are more like a spectrum of descriptive terms, rather than a rigid system of classification.

Dirinaria picta is most often seen on tree bark, but it is not dependent upon the tree for anything other than the water that runs off of it, because it produces nutrients itself. It can be found on bare concrete as well. In fact, it is so undiscerning in its choice of substrates that it has even been found on the shells of Galapagos turtles.

There is, in fact, a good reason why this lichen has such a huge geographical distribution, being found all the way from Singapore to the Galapagos Islands, which are halfway around the globe. It has to do with its mode of reproduction. If you look closely at one of these lichens, provided it is mature enough, you may see some raised, powdery bumps on the thallus. If you then touch one of them, a powder will stick to your fingers. The bumps are called soralia, and their role is to carry soredia, which are small balls that make up the powder. Soredia are made of a few algal cells surrounded by just a few strands of fungal hyphae. They can float on the wind or stick to the legs of insects and travel great distances. Since they contain both algae and the fungus, they act as a sort of just-add-water starter set for lichens. These would then lead to the formation of clones of the original lichen.

Although Dirinaria picta does have the ability to produce cup-shaped structures called apothecia, which allow sexually produced spores to be dispersed, it is very reliant on asexual reproduction through soredia. Spores from the apothecia cannot go as far as soredia; when an apothecium is moistened, it forcefully ejects spores a few centimetres before they fall towards the ground and usually land back onto the same surface as its parent. This spore will not grow into a lichen, but just a fungus, because the chance event of it interacting with algae in a specific manner needs to happen for lichenisation (what a wonderful word) to occur. A general rule of thumb is that crustose lichens tend to use sexual reproduction more regularly to produce spores, while foliose and fruticose lichens are usually so reliant on different forms of asexual reproduction, such as producing soredia, that they hardly ever bother to produce apothecia.

Lichens of this species have a beautiful texture made up of overlapping lobes, but their colour is usually less inspiring, as it is pale almost to the point of sickliness. Even so, a remarkable change occurs after it rains. Its colour deepens to a vivid green, resembling opaque jade. It reaches its highest possible rates of respiration and photosynthesis, making the world seem more saturated with colour. Since lichens that live on stones or tree bark do not have access to an absorbent source of water that can stay moist for relatively long periods of time, unlike those that grow in soil, they show low levels of metabolic activity during dry periods. Lichens’ way of life, therefore, consists of cycles of wetting and drying.

Dehydrated lichens hardly photosynthesise at all, which is why they grow so slowly. But just 20 minutes after being wetted, Dirinaria picta reaches its peak photosynthetic rate. The algal partner of this lichen belongs to the genus Trebouxia, and it is very resistant to drying. The way that the isolated alga photosynthesises is markedly different from the way it photosynthesises as part of a lichen. When it is free-living, it has to go through a process called non-photochemical quenching at high light intensities, dissipating excess light energy as heat to protect its photosynthetic machinery from damage. This happens at a much lower level when it lives as part of a lichen. The fungal component thus protects the alga from the harms of excess light in Dirinaria picta.

Sickening

Lichens are hardy, as seen by their colonisation of bare rock throughout the globe, but they have a specific weakness that leaves them vulnerable to one ever-present aspect of the modern world: air pollution. Most plants and animals react badly to heavy metals, but many lichens are unable to do anything about it, so they keep accumulating them until lethal levels are reached. Quite distinctly from this passive uptake of metals, there is one soil lichen from Belgium (Diploschistes scruposus) which takes in so much zinc that the metal makes up over 9% of its dry weight, and has a zinc content nearly 10 times that of the soil. No one knows why it does this.

Dirinaria picta, being a conspicuous and widespread lichen, is often used as an indicator of air quality levels. Like all lichens, it has a very high surface-area-to-volume ratio, and no other means of acquiring nutrients other than water running off of the substrate it grows on, so it is very receptive to anything that seeps into it. Its thallus can be analysed for the presence of pollutants. Since it has no connection to the soil, all the metals it absorbs are from the air – and learning about what it absorbs from the very same air we breathe is quite disconcerting. Some researchers found that in the industrial areas of Jurong and Sembawang, there are high levels of zinc and copper generated from industrial activities such as manufacturing and petroleum refining. Near Changi Airport, the samples of lichen contained high levels of lead, accumulated from the use of leaded petrol in aeroplanes and cars, even though adding lead to petrol was phased out years before the research was carried out. The slow growth of lichens is thus a living record of our city. This particular lichen is rather resistant to air pollution when compared to others.

Breeding

But the pale green of Dirinaria picta is by no means emblematic of lichens, and white patches on trees are far more common. Upon looking closer at them, something amazing is revealed. Far from being blank, waxy spots, they are often peppered with black spots, or intricately etched with labyrinthine lines that twist and branch all over the surface. Though some are slightly harder to see clearly with the naked eye, zooming in with a phone camera should allow you to notice the patterns. These blackened areas are the reproductive structures, or apothecia, of the lichen. That is where spores are produced, within specialised cells called asci (meaning “sacs” in Latin).

Spores are the reproductive units of the lichen. Before that, one lichen produces conidia (“little dust” in Greek) and microconidia (“tiny little dust”) in flask-shaped structures called pycnidia. Conidia can grow on their own and allow the fungus to reproduce asexually, while the microconidia act as gametes, analogous to sperm cells in humans. Another lichen of the opposite sex will then develop a curved, projecting structure called a trichogyne (“female hair” in Greek) which changes upon coming into contact with a microconidium, becoming an apothecium.

The lichens that produce the convoluted branching patterns generally belong to the family Graphidaceae, named for the resemblance that their reproductive structures bear to writing. The lines are actually like little trenches with ridges at the edges. The blackened edges are considered to be “carbonised” and contain dark-coloured pigments in high concentrations – mostly melanin, which is the same pigment that darkens human skin. This likely protects the spores in the reproductive structures from UV damage from the sun. Some of these lichens produce new reproductive structures on top of the old ones, leading to very tiny folds which appear as stripes on the “lips” (known as exciples) of the apothecia. Theoretically, these could be read like tree rings to approximate the age of the lichen. It turns out that something is, in fact, written on these lichens.

Looking

Just by identifying lichens with the naked eye, they can be used in research in a variety of ways, even without looking at them under a microscope or analysing the chemical compounds they produce. Many studies gather data by checking for the presence of certain lichens on trees or rocks and marking it out on a map. Other studies count the number of species present in different areas. By measuring the sizes of lichens, they can be used to date geographical events such as the shrinking of lakes, and this study (lichenometry) is especially useful in mountainous areas and high latitudes, where lichens can live for centuries. Yet the light of day is not always sufficient to see all of their colours. When they are illuminated by pure UV light, they often reveal glowing colours that can be used to identify them. The ability to absorb and emit UV light allows them to tolerate the sun.

J. R. R. Tolkien, in his magical creation entitled The Hobbit, writes this:

“There is nothing like looking, if you want to find something. You certainly usually find something, if you look, but it is not always the something you were after.”

In the wide, wonderful world, strange and complex patterns should never fade into uniformity in our eyes. Anything that is not uniform can, in fact, be interpreted in some way. And the world is far richer for it.

References:
General information (in depth):

1. Ing., B. (1985). The biology of Lichens, 3rd edition, Mason E Hale. Edward Arnold (1983), 190, ISBN 0 7131 2867 4. Paper, £8.95.

2. General information:

   1. Fungus - Form and function of lichens. (n.d.). Encyclopedia Britannica. https://www.britannica.com/science/fungus/Form-and-function-of-lichens

3. List of lichens in Singapore:

   1. Sipman, H. (2010). A Conspectus of the Lichens (Lichenized Fungi) of Singapore Gardens. Bulletin Singapore, 61(2), 437–481. https://www.nparks.gov.sg/sbg/research/publications/gardens-bulletin-singapore/-/media/sbg/gardens-bulletin/gbs_61_02_y2010/61_02_14_y2010_v61p2_gbs_pg437.pdf

4. Common lichens in Singapore:

   1. Sipman, H. J. M. (2009). Tropical urban lichens: observations from Singapore. Blumea - Biodiversity, Evolution and Biogeography of Plants, 54(1), 297–299. https://doi.org/10.3767/000651909x476328

5. Photosynthesis in Dirinaria picta:

   1. Ritchie, R. J., & Editor: Linda E. Graham. (2014). Photosynthesis in an Encrusting Lichen (Dirinaria picta (Sw.) Schaer.ex Clem., Physiaceae) and its Symbiont, Trebouxia sp., Using Pulse Amplitude Modulation Fluorometry. International Journal of Plant Sciences, 175(4), 450–466. https://doi.org/10.1086/675575

6. Non-photochemical quenching:

   1. Ruban, A. V., & Wilson, S. (2020). The Mechanism of Non-Photochemical Quenching in Plants: Localization and Driving Forces. Plant and Cell Physiology, 62(7). https://doi.org/10.1093/pcp/pcaa155

7. Absorption of a variety of metals by Dirinaria picta:

   1. Xulei Huang, Lei Wang, Anna Karen Carrasco Laserna, Sam Fong Yau Li, Correlations in the elemental and metabolic profiles of the lichen Dirinaria picta after road traffic exposure, Metallomics, Volume 9, Issue 11, November 2017, Pages 1610–1621, https://doi.org/10.1039/c7mt00207f

8. Lichens to detect heavy metals:

   1. O.-H. Ng; B. C. Tan; J. P. Obbard. (2006). Lichens as Bioindicators of Atmospheric Heavy Metal Pollution in Singapore. , 123(1-3), 63–74.doi:10.1007/s10661-005-9120-6

9. Yeasts supporting the structure of lichens:

   1. Spribille, T., Tuovinen, V., Resl, P., Vanderpool, D., Wolinski, H., Aime, M. C., Schneider, K., Stabentheiner, E., Toome-Heller, M., Thor, G., Mayrhofer, H., Johannesson, H., & McCutcheon, J. P. (2016). Basidiomycete yeasts in the cortex of ascomycete macrolichens. Science, 353(6298), 488–492. https://doi.org/10.1126/science.aaf8287

10. Lichen reproduction:

    1. Lichen life cycle. (2024). The British Lichen Society. https://britishlichensociety.org.uk/learning/lichen-life-cycle

11. Graphids:

    1. Wirth, M., & Hale, M. (n.d.). Morden-Smithsonian Expedition to Dominica: The Lichens (Graphidaceae). Retrieved May 13, 2026, from https://www.govinfo.gov/content/pkg/GOVPUB-SI-PURL-gpo113824/pdf/GOVPUB-SI-PURL-gpo113824.pdf

12. Definition of carbonisation:

    1. Smith, C.W.; Aptroot, A.; Coppins, B.J.; Fletcher, A.; Gilbert, O.L.; James, P.W.; Wolseley, P.A., eds. (2009). "Glossary". The Lichens of Great Britain and Ireland. London: British Lichen Society. pp. 21–39.

13. Lichenometry:

    1. Winchester, V. (2023). Lichenometric Dating and Its Limitations and Problems: A Guide for Practitioners. Land, 12(3), 611. https://doi.org/10.3390/land12030611

14. UV light colours of lichens:

    1. https://www.fungimag.com/summer-2014-articles/LR1%20V7I2%2028-31%20Jungle.pd