Said to be the origin of primitive plant types, algae are photosynthetic plants. They thrive on light energy and the absorption of minerals for photosynthesis, whereby they convert inorganic sunlight and minerals into organic sugars and oxygen. Algae also absorb mineral nutrition from the water that they grow in. They may appear green or brown and are non-parasitic, but differ from most vascular plants in that they do not have stable structures like roots or leaves.
Fungi are decomposers that digest organic material, grow on land and only in the dark. They act as fertilizers, consuming carbon-based organic matter and, by digesting it, convert it to inorganic matter necessary for plant growth. In cases like athlete's foot, the fungus is categorized as parasitic in that the carbon it is consuming is from a living organism. Fungi take many forms, like mushrooms, molds and yeasts.
Despite their differences, algae and fungi coexist very successfully as lichens. Lichens are a sturdy, symbiotic collection of algal and fungal components that grow together on rocky and porous surfaces just about anywhere. These multifaceted organisms may appear crusty, leafy or shrubby.
The fungi form the base or crust, sheltering the algae from sunlight and loss of water. Many animals in high mountains or arctic regions eat the lichens that grow on the rocks in these inhospitable conditions. The algae in the lichen produce sugar by photosynthesis, which is then consumed by the fungi. The fungal component also absorbs water and nutrients from the environment that benefit the algae.
Since they absorb rainwater from their immediate environment, lichens are extremely sensitive to pollution and will not grow well or at all near heavy industry.
Some algae cling to surfaces near or underwater, while others drift within or on the surface. Algae occur in films, unicellular cloudy colonies and some leafy or blade forms. Seaweed is a leafy, edible algae often accompanying seafood or served as a salad.
Fungi are far more versatile in the kitchen than algae. Edible fungi occur in many forms. Mushrooms can be edible or poisonous. Other types of fungi are also edible, such as Penicillium Roqueforti, the mold in blue cheese. Fungi also occur as single-celled yeast, used for bread making and beer brewing.