Photosynthesis
During photosynthesis, algae uses chlorophyll (and carbon dioxide and water) to gather light and convert it to energy. Once the conversion is complete, the organism stores that energy as sugar, while releasing oxygen as a byproduct.
Red Algae
Generally any organism that uses photosynthesis will be green because chlorophyll acts as a green pigment. However, red algae, or rhodophyta, contains phycoerythrin, which lends it a red color. Phycoerythrin also absorbs blue light and thus allows red algae to live in deeper water.
Geography
Algae is located in all environments where there is some moisture available. Species are most commonly found in fresh and salt water, but they also appear in arctic and desert environments.