Lichens are extremely resilient organisms
As we near winter and the forest begins to shut down – photosynthesis is over for most deciduous trees and is slowing down for the conifers – leaves drop, animals have either fled south or are finding burrows below the frost line or protected by insulating leaves or dirt to overwinter. You walk through the woods and the leaves rustle along with your steps and the bare branches shiver, the wind blows and those last remaining dried-out leaves that still cling to the trees rattle.
But, on the tree trunks and bare rocks, lichens plod along, doing what they always do – photosynthesis when it is warm or wet enough, going dormant when it gets too cold or dry. The U.S. Forest Service waxes poetic (I think) about this. When lichens are wet, they "turn on" and start photosynthesizing and growing. When lichens are dry, they "turn off," become brittle and go dormant. This process is known as "poikilohydry," and other organisms such as mosses and liverworts operate in the same way. So, the simplest way to tell if lichen is dormant or growing is by looking at its color. The darker black or brighter green a lichen is, chances are that it is photosynthesizing.”
When it gets cold, any kind of living tissue is in danger of freezing, frost-bite rupturing its cells. This happens to our fingers and toes, but it also happens to plants. In addition to frostbite, freezing temperatures also mean that it is dry, there is no liquid water, all water is frozen and so life must adjust. Lichens do this by shutting down and going dormant. They can even survive outer space in dormancy. Back in 2005, in an experiment led by Leopoldo Sancho from the University of Madrid, two species of lichens were released from a Russian rocket into the vacuum of space. They endured temperatures ranging from -20°C on the night side of Earth, to 20°C on the sunlit side. They were also exposed to damaging ultraviolet radiation from the Sun (our atmosphere filters out a large amount of this damaging radiation). They survived just fine. Once returned to normal conditions, they went about their daily lives as if nothing had happened. This is pretty remarkable. Those conditions would kill most living organisms.
Among their many adaptations, lichens that can tolerate freezing temperatures do so by having ice nucleation sites in their tissues that ensure that ice crystallizes in between cells and not within cells. Ice crystallizing inside cells is what causes them to rupture.
One of the reasons lichens have been tested in space is because they are a potential candidate for terraforming other planets. On Earth, when land is reduced to bare rock by a glacier or a volcano the first organisms to establish a foothold are lichens and mosses. Lichens can grow on bare rock and as they do they break down the rock, contribute organic material as they die and are the first step involved in building soil. They are the pioneer species that pave the way for eventual forests. After the last ice age, when much of New England was reduced to bare rock, it was the lichens that began the slow process of rebuilding.
There is some debate as to whether a lichen is a single organism or whether it is an ecosystem of closely interacting life forms. Lichens are a symbiotic relationship (a close association) between a fungus and an algae or a cyanobacteria (“a division of microorganisms that are related to the bacteria but are capable of photosynthesis. They are prokaryotic and represent the earliest known form of life on the earth” from the Oxford Dictionary). The fungus, in theory, provides protection for the algae-cyanobacteria and the algae provides the fungus with sugars (produced during photosynthesis) or cyanobacteria provide nitrogen, a necessary element for the production of proteins and genetic material, produced when cyanobacteria fix unusable atmospheric nitrogen into usable forms. This symbiosis benefits the fungal partner the most, so it has been proposed that a lichen is really a fungus that has developed agriculture, that it "cultivates" the algae or cyanobacteria in the same way that we grow crops for food.
A walk in the woods this time of year will always yield some green, perhaps not leafy green, but the ever-present gray-green of the lichen provides some color and a reminder of summer as we head into the darkness of winter.
Susan Pike, a researcher and an environmental sciences and biology teacher at St. Thomas Aquinas High School, welcomes your ideas for future column topics. She may be reached at spike3116@gmail.com. Read more of her Nature News columns online.