Phytoplankton are microscopic marine organisms that photosynthesize, forming the basis of aquatic food webs. They are primarily found in the upper layers of oceans and freshwater bodies and play a crucial role in global carbon cycling by absorbing carbon dioxide and producing oxygen. Phytoplankton not only contribute to a large portion of the Earth's oxygen supply but are also vital for the survival of various marine species, making them essential for ecological balance and health.