Lichens

This article gives a brief description of what lichens are.

Lichens are organisms often found growing on bark, leaves, rocks, and even walls.

A lichen looks like one organism but is, in fact, a combination of two very different types of organisms.

One of the component organisms of a lichen, called the phycobiont, is an autotroph. It can make its own food, which it shares with the other component organism, the mycobiont. The mycobiont, in turn, provides shelter, minerals, and water to the phycobiont.

This type of a relationship between two organisms, where each helps out the other, is called mutualism.

Symbiosis is a general term for a close relationship between two species, say species $1$‍ and species $2$‍. Symbiosis can be of different types:

The third type of symbiosis is called mutualism, which is what exists in lichens.

The component of a lichen is an organism that can perform photosynthesis. Depending on the species of the lichen, the phycobiont can be either an alga or a cyanobacterium, or sometimes both.

Although both algae and cyanobacteria can photosynthesize, they are fundamentally very different types of organisms. Cyanobacteria are prokaryotes and belong to the kingdom Monera, while algae are eukaryotes and belong to the kingdom Protista.

The is always a fungus. The hyphae of the fungus provide shelter and protection to the phycobiont. Also, the fungus obtains water and nutrients from the environment. The phycobiont can then use this water and nutrients for its photosynthesis.

Lichens absorb nutrients directly from the environment. So, if there are pollutants such as sulfur dioxide in the air around them, they absorb the pollutants. The pollutants can be toxic to the lichen bodies, and too much of them can kill the lichens. Hence, whether or not lichens are thriving in an area is a good indicator of the extent of air pollution.