Lichens are symbiotic, composite organisms that are generally categorized in the Fungi kingdom. However, certain members are also found in the kingdom Protista (algae) as well as kingdom Monera (cyanobacteria or blue green algae). According to Lichen.com, lichens typically grow in harsh environments that are too limiting for other organisms.
Lichen grow in a variety of unwelcoming environments including desert sand, cleared soil, deadwood, bare rock, rusty metal and animal bones. Their thrive in those areas because of their ability to shut down their metabolisms for a period of time in order to survive extremes like drought, cold and heat, according to Lichen.com. Furthermore, given enough moisture, light and air, lichens are able to colonize almost anywhere on earth.
Lichen are known for their particularly slow growth rate of less than a millimeter per year. They are also considered among the oldest living organisms, according to Lichen.com. Their slow growth rate is often used to estimate geological time of certain events in the history of the planet, such as determining when glaciers from the Ice Age retreated.
The life cycle of a lichen begins when a mycobiont (fungal part) combines with a phycobiont (algal part). The fungal filaments enclose and grow into the algal cells and provide the lichen with the majority of its physical structure and shape. The apothecium, a fungal reproduction structure, produces spores. These spores disperse and germinate into new fungi but do not by themselves create new lichens, according to the University of California, Berkeley. The lichen will only reproduce when the fungus and the alga disperse together. Furthermore, lichens are known to reproduce in two main ways, via fungal reproduction and via asexual reproduction.
Lichens can reproduce as a result of fungal reproduction. When a spore lands and germinates, the resulting hypha needs to find the correct species of alga in its proximity. After the hypha finds the alga, the hypha will grow through the algal cells and a new lichen will develop, according to BackyardNature.com.
Lichens can also reproduce via asexual reproduction. One type of asexual reproduction includes fragmentation in which a section of a lichen breaks off and then grows into a new lichen. Another type of asexual reproduction includes soredia, according to BackyardNature.com. Soredia are microscopic particles that are made up of several algal cells enclosed in a fungus hyphae. Each soredium can produce a new lichen and both fragments and soredia disperse through the air with the aid of wind.