What two types of organism make up a lichen?
snakes and bacteria
insects and bacteria
animals and viruses
fungus and bacteria
Have you ever seen an organism called a lichen? Lichens are crusty, hard growths that you might find on trees, logs, walls, and rocks ( Figure below ). Although lichens may not be the prettiest organisms in nature, they are unique. A lichen is really two organisms, sometimes referred to as a composite organism, that live very closely together: a fungus and a bacterium or an alga. The cells from the alga or bacterium live inside the fungus. Besides providing a home, the fungus also provides nutrients. In turn, the bacterium or the alga provides energy to the fungus by performing photosynthesis, obtaining energy directly from the sun. A lichen is also an example of a mutualistic relationship. Because lichens can grow on rocks, these organisms are some of the earliest life forms in new ecosystems.