What is a Lichen?

Lichens are a unique life form. It's strange that scientists classify them when you consider that they are two different organisms living together in some kind of relationship. (There is debate about whether or not the relationship is a mutualism, i.e. a relationship that is beneficial for both parties, or a form of controlled parasitism.) In short, a lichen is an alga (pl. algae) or cyanobacterium (formerly known as a blue-green alga) and a fungus that live together. But in a lichen, the two partners form a completely unique organism that doesn't look like either partner does alone. The fungus forms the body of the lichen (aka the thallus) as well as the attachments for anchoring it to what it grows on (the substrate). The alga or cyanobacterium performs photosynthesis, providing the "food" for the relationship. Because the fungus doesn't really do much work and takes food from the alga or cyanobacterium, some consider this a controlled parasitism. There is a cute drawing of this scenario in William Purvis's book Lichens, where a blob of a fungus has put some green algal cells behind bars and is holding the key and smiling like a glutton.

I want this blog to help lay people, including children, understand lichens, their significance, and why I think they are the coolest organisms on the planet.