Lichens- Symbiosis of Distant Organisms
I began to really notice them in one winter two years ago. Our family had a winter break visiting Shennandoah River in West Virginia near the Harper’s Ferry. We had a long walk admiring the open vistas of the light-bouncing winter river through mature trees of oak, hickory, ash, maple, hackberry, and cherry. We had frequent stops examining and identifying tree species, but we also stopped to admire numerous boulders on the banks of the river. Those rocks, as well as trees, are elaborately decorated with lichens. I’ve seen lichens before, a good deal of them indeed, in Smokey Mountains and other old-age forests in Tennessee. It was Spring then and the fantastic array of spring ephemerals overwhelmed my senses. Then I forgot I saw lichens.
Here in a balmy winter afternoon at Shennandoah, however, lichens beckoned me. They revealed themselves in all sorts of different shapes and colors. Once I got home, I realized that lichens are pretty much everywhere, hidden in full view. Those Common Greenshield lichens (Flavoparmelia caperata), for instance, are growing almost on every street tree in our neighborhood and even on our fences. Then I learned that about 7 to 10 percent of the Earth’s surface is covered by lichens. Lichens are among the earliest land colonizers. How on earth I managed to ignore their presence? I still know very little about lichens and my ability to identify their species is miserably weak. Yet their life story fascinates me and want to share a bit of their story.
Pictured above: Foliose lichen called Greenshield (Flavoparmelia caperata)
What are lichens? They seem to grow everywhere: on fences, rocks, tree barks (living or dead), on soil, on mosses and practically on all types of objects that are not moving. Yet, lichenology is a relatively young branch of science and a lot more studies needed to fully understand their evolutionary processes and their roles in ecosystem. It’s perhaps easier to say what they are not. Lichens are not plants. They are not fungus. They are not mosses. They are not algae. No, they are not parasites, as they are self-feeding. They do not hurt the substrates on which they grow.
Lichens are the end-result of a union between several different organisms. Individually and alone, fungus or algae cannot survive too long in dry, extreme temperatures, or nutrient-deficient conditions. By collaborating, they have gained a perfect environment in which they can provide food for themselves and to reproduce. Lichens have evolved and diversified from the deep past in dinosaurs time to the present modern ecosystems.
The two main partners are fungus and green algae. Scientists discovered at least two more partners in lichens: photosynthetic cynobacteria that can “fix” nitrogen and another single-celled fungus, yeasts. Yeasts in lichens apparently produce chemicals that help lichens to ward off predators and other harmful microbes. Recently, scientists also found that the presence of yeast in lichens can influence the appearances of lichens. There are just too many different types of fungus and algae in the world. What types of fungi and algae can be found in lichens? They almost always involve a phyllum of fungus called Ascomycota (Sac or Cup fungus) and photosynthetic green algae belong to the genus of Trebuxia and Trentepohlia.
To think about it, it involves three life kingdoms to create lichens! A kingdom of fungus, a kingdom of plant (algae is plant), a kingdom of bacteria!
Now, let’s look at the morphology of lichens. Lichens are categorized into three different growth forms: crustose, foliose, and fruiticose.
Pictured here: Crustose lichen growing on bark.
Crustose lichens are micro-lichens as they are intimately encrusted on barks of trees or on the surface of rocks. Sometimes you cannot tell apart if they are part of the tree barks or rocks. Foliose lichens, on the other hand, are macro lichens, producing leafy lobes with lots of variations on their growth. Their lobes can be rather flat and yet convoluted like little lettuce. They can grow quite large, several feet wide and long. In foliose lichens, you can see clearly the upper and lower surfaces (cortex). Fruticose lichens are three-dimensional, shrubby-looking, and have branches rather than lobes. They do not have differentiated upper and lower surfaces (cortex), although there are some exceptions.
Pictured here: Fruiticose lichen called Cladonia on the bed of mosses
Let’s look at closely into this leafy portion of the lichen, called thallus. It is composed of four layers. The upper and lower layers form the cortex and are composed of dense network of fungal hyphae (mycobiont). The middle layer is composed of algal or cynobacteria cells. The upper cortex allows the sunlight pass through so that the algal or cynobacteria cells (photobiont) receive lights to photosynthesize. The upper cortex also has color pigments which play the important role in lichen identification. The layer below the algae is termed the medulla, which is composed of loose to dense network (mycelium) of fungal hyphae. Below this layer of medulla, there are special structures called “rhizines.” Rhizines are not roots and therefore cannot uptake water or nutrients. This root-like structures function only to anchor the lichens to their substrates. In crustose lichens, they only have three layers as they lack rhizines.
Fungus in lichens provide structures of lichens and bring nitrogen and other minerals. Algae photosynthesize and efficiently produce simple carbohydrates. They pass carbohydrates onto fungal partner. In turn, fungus does their usual magical work to transform simple carbohydrates into complex carbohydrates. Lichenized, they can keep themselves hydrated effectively and reduce the dessication period considerably. Lichens are protected from extreme temperatures and UV radiations. Above all, they can reproduce asexual propagules to colonize.
Not too many fungi could live in dry and cold habitats with no or little organic matter. But lichenized fungi (mycobiont) found a niche and thrive in open air. Algae is also non-vascular aquatic organism. It cannot survive outside of water. Once lichenized, however, algae (photobiont) are protected from drying out by being enveloped by fungal layers. Lichens also have this remarkable ability to stay dessicated (state of extreme dryness) for a prolonged period and then resurrect. Scientists are still debating when the first lichens appeared on land. Some argues that not as early as 500 to 400 million years ago as previously believed, but much later at about 250 million years ago, when there already existed land plants like ferns. No matter, lichens have survived the harsh conditions of earlier landscape and have produced oxygen into the atmosphere, thereby paving the way for other more complex plants and animals to live on land.
All these lichenizing players are often playfully referred to as “singers” while the resulting body of lichens (thallus) are called “songs.” In lichens, singers are not that important. What matters is the song. Lichens seem to experiment the combination of singers in order to produce desirable song (structural and functional form of lichens). They change partners with slightly different fungal or algal lineages. What is amazing about lichens is that they have achieved metabolic integration. In other words, the partners in lichens got together and transformed into a new kind of organism.
Lichens reproduce both sexually and asexually. Lichens can reproduce sexually via spores. The spore-producing partner in lichens is always fungus. Once dispersed into the air or in water, the spores must land in ideal locations to meet their ideal algal or bacterial partners. Obviously, that is not easy. So, understandably, lichens’ primary reproduction is through asexual propagules. The lichen asexual propagules are ready-made, in the form of microscopic bundles of algal, fungus, and bacteria woven together. They are dispersed by wind, water, or animals.
There are still a lot of unanswered questions in lichens evolutionary processes. What are the underlying principles, advantages, and fitness of lichens symbiosis for the partnering symbionts. And how do they drive their evolution from there to onward and forward? There is, however, no doubt that lichens – though everywhere but largely left undetected – have developed complex life systems through symbiotic associations. The dynamics of symbiotic associations are the fundamental forces that drive evolution and our environment. In the midst of horrendous climate changes, the success of lichens and their roles in ecosystem suggest it’s time we must pay attention to organisms we have ignored so steadfastly.
- Lisa Bright
Director Emeritus & Co-Founder