Cellular Respiration
This is a process that takes place in the mitochondria and results in making ATP, the energy currency for cells used to perform functions.
After photosynthesis has occurred in the algae, cellular respiration takes the carbohydrate molecule, breaks it down and harvests the energy from it in the form of ATP. O2 and the carbohydrate molecule produced in photosynthesis are the input for cellular respiration, and CO2, H2O and ATP are the output. This ATP is later used by the detachment of a phosphate in the ATP to release the stored energy to be used to move or create things, or as a catalyst in reactions.