Phytoplankton, shown on the left in the image below, are microscopic organisms that can be found in freshwater and salt water environments. They perform photosynthesis to get energy and act as an important food source for larger organisms, such as the whale that is shown on the right. Phytoplankton A large whale. Which characteristic is found in the whale but not in its food source, the phytoplankton? heterotrophic cell walls unicellularity prokaryote cells