A symbiotic way of life: the lichen

A closer look at the miniature ecosystem within the ubiquitous lichen

By Lara Williams

Lichen are often mistaken for plants, or glanced at without a second thought, yet the way they thrive is fascinating. Only about 200 years ago did botanists themselves lump lichens together with mosses under the Latin name ‘Muscus,’ meaning ‘moss’. However, despite growing in similar locations, mosses and lichens are completely different kinds of organism.

A symbiotic relationship is where two organisms come together in order to survive, and to be able to thrive. This is done by having somewhat of an ‘exchange’. They take from each other to better themselves…but they are friends. The symbiosis of a lichen is between a fungus and one or more partners, known as photobionts. The fungal component of lichen is called the mycobiont, whilst the photobionts are either an algal or cyanobacterial species.

The structure of the lichen comprises an upper cortex composed of the fungus, with the algae/cyanobacteria growing right below it. Below the algal layer is the medulla; the widest layer of heteroamorous thallus, composed of loosely arranged interlaced hyphae. From the structure, it is evident that the fungus provides the protection, building the structure of the lichen thallus, and the algae/cyanobacteria supply the source of nourishment.

Their symbiotic nature affords them the remarkable ability to tolerate complete desiccation for extensive periods of time, as well as being able to survive in extreme heat and cold. Although no growth will occur at these times, they persist in a form of suspended animation. They are also able to withstand UV radiation and damage caused by grazing animals. This is achieved through the presence of rich secondary fungal chemicals within lichens, which cannot be found anywhere else in nature!

This relationship arose due to the fungus being unable to produce food for itself, i.e. the fungus is heterotrophic, meaning it requires an external food source. The algal species, however – like most plants – can manufacture their own food simply from carbon dioxide and water, via photosynthesis (i.e. they are autotrophic). For this reason, the fungus is slightly more dependent on the algae or cyanobacteria than the other way around; the algae are the producers of food for both themselves and the fungus. The relationship is still a mutual one, however, as the fungus allows for the algae/cyanobacteria to grow in places that it could not possibly grow alone. This allows lichen to grow most commonly on bark, leaves and mosses, which the fungus enables by providing the photobionts with optimal living conditions. This allows for the photobionts to have a larger population size within the symbiotic relationship than they would alone. Photobionts also gain access to mineral nutrients from fungal digestion that occurs outside of their cells (extracellular digestion).

The symbiosis within lichens provides a living environment that would not be possible alone. Together, the fungi and photobionts achieve balance by combining their qualities to create a new little world; a harmonious environment where they can be mutually better adapted to life.

From Issue 17