Phytoplanktonare microscopic plants that grow in the upper regions of the ocean where sunlight is plentiful. These small plants, which are composed of algae, are the bottom of the food chain for the entire planet. Phytoplankton require light for photosynthesis, so they usually are found near the surface of the water. A wide variety of adaptations help them stay afloat. Some beat their flagellas and tread water. Others have fins and spines that act as water wings. Others store extra food as oil, which buoys them up near the surface.
When phytoplankton become full of oil, they die and sink to the bottom. They become buried under mud and sand. Over millions of years, heat and pressure within the earth transform the oil from the algae into crude-oil deposits that can later be used by humans.
Climate change has a profound effect on ocean circulation and mixing patterns, and these in turn control nutrient availability to the ocean's phytoplankton and their access to the solar radiation that is required for photosynthesis. During normal years, the Earth's oceans go through a process known as upwelling in which the easterly winds blow across the equator and drag the warm surface water with them. Then the denser, colder water from the depths of the ocean rises, restoring oxygen and allowing mineral nutrients to return to the surface where phytoplankton can use them once again.