Red algae does not eat anything. Instead, like most plant life, red algae employs a process known as photosynthesis to convert sunlight into energy. Photosynthesis requires light energy, water and carbon dioxide to take place; photosynthetic organisms such as plants, algae and some bacteria are known as photoautotrophs.
Photoautotrophs produce energy from carbon dioxide and water, using sunlight as a catalyst for the reaction. Algae and plants have complex cellular structures that allow them to convert light energy into chemical energy and then store it as sugar. The photosynthetic reaction occurs in small organelles within the algae's cells, which are called chloroplasts.