Existing from the Antarctic to the Arctic, desert to ocean, mountain high to coast, city to wilderness, on rock, cement, plants, detritus, soil, in water, and wherever else it might be, it is a pioneering symbiosis that can establish itself in lands and waters where nothing visible to the eye is living. It can take on many shapes, sizes, colors, patterns, etc., but at the foundation of it all is a fungus and algae and/or bacteria living together. Alone, the algae/bacteria and fungi, can't survive as many environmental extremes as they can when they work together. Different types of lichen have been frozen to near absolute zero, left dried out for decades, exposed to temperatures over 200 degrees for hours, and then revived undamaged. There is evidence that some lichens buried and frozen by glaciers during the last ice age might have thawed in modern times with the melting ice and carried on like thousands of years passed in suspended animation was of little consequence. In the wild they can live where there is no organic food to consume, on rocks in deserts, buried in ice, etc. Some scientists involved with the space program felt that if there was life to be found on Mars or on other planets with harsh climates that the lichen would be it.
How and why the lichen symbiosis initiates has proved difficult to understand, but under lab conditions it has been shown that when the environment lacks the nutrients essential for the algae's/bacteria's and fungus' survival individually, more often than not, a lichen will form as the symbiosis can endure harsher climes than the individual partners. When the time is right and the select algae or bacteria are near (depending on the lichen), the fungus wraps root-like hyphae around them and together they become something that neither on its own can be.
Often times the bacteria/algae and fungi that make up the lichen symbiosis in a given area can be found living separate lives as well as living together. When they're living apart the fungus looks like a fungus and the bacteria/algae looks like a bacteria/algae, but when the fungus and bacteria/algae get together they transform into a lichen. The lichen they transform into has different color, form, physiology, chemistry and biology than either of the partners do on their own. When a lichen symbiosis breaks down the fungus will go back to being a fungus and the algae/bacteria will go back to their solitary form. In laboratory experiments, the lichen symbiosis is broken by providing a nutrient-rich medium wherein the algae/bacteria and the fungus, separately, can find the nutrition they need.
In summary, the lichen symbiosis might be generally stated as: when times are hard, let's work together. When times are good, go your own way. Of course the fact that all lichen symbioses don't fall apart during good times indicates that there are advantages to working together beyond just obtaining nourishment (or maybe it's foresight, realizing that just because the environment is abundant today it may become scarce by tomorrow).
The lichen relationship is complex and varies from type-to-type. Sometimes the fungus seems to farm algae, harvesting them whenever it needs nutrition. In other circumstances the fungus seems to act like a parasite putting a straw-like appendage into the algae or bacteria and sucking out their juices. In yet other lichens the fungus and algae seem to work together toward the mutual goal of survival and even thriving. The fungus partner is often credited for protecting the algae/bacteria, wrapping itself around it, preventing some desiccation, shielding the algae from overexposure to sunlight, and also with providing the algae with water and minerals. The algae/bacteria provides the fungus with carbohydrates, lipids and various vitamins. Some fungi are particular about which algae/bacteria they will cohabitate with. Some fungi collect a myriad of algae and/or bacteria to transform into a lichen with.
Philosophically I think there can be some interesting questions, observations and insights derived from lichens that might have broader implications. For instance, the fact that lichens can survive in more extreme environments than either bacteria/algae or fungi partner can alone indicates that one need not possess all abilities if they can find another who possesses those missing abilities. Are relationships a means by which the whole can become greater than the sum of the parts? Can cooperation between individuals lead to greater results than the accomplishments of all the individuals acting alone added together?
And what can lichens teach us about the capacity to transform? Neither the algae/bacteria nor the fungus alone can become a lichen. It requires both partners to gain the rather unique abilities of the lichen. That such a dramatic transformation can occur through the simple act of bacteria/algae and fungus changing living status from single to partnership is truly remarkable. Can relationships change us so dramatically. I think the transition from hunting/gathering, for Homo sapiens, to agriculture/pastoralism as well as the domestication of other animals like horses, dogs, cats, etc., might be such an example. Literally, civilization can be partially attributed to the transformation that occurred to humans when we changed our relationship with the plants, animals and microorganisms of this Earth, becoming more intimately connected to the welfare of crops, herds and companions of the non-human kind.
Who knows what future associations born might change the world fundamentally, but we can be sure, because of lichens, that the potential transformative power of relationships has proven a successful adaptation for life.