All plants require sunlight to live. This limits marine plant life to the topmost layers of the ocean, known as the epipelagic and mesopelagic zones; although more than 250,000 land-based species of plants exist, oceanographers have counted only about 4,000 species in the ocean. But in spite of their limits, aquatic plants do reach the edge of permanent darkness underwater.
Single-cell algae called phytoplankton comprise most plant-like life in the sea. Since they rely on sunlight, as do land-based plants, these free-flowing organisms live near the surface, but settle to the ocean floor when they die. Their remains provide food for fish who live in the sunless zones.
While most kelp forests live within 98 to 131 feet (30 to 40 meters) below the ocean surface, a study posted in the Proceedings of the National Academy of Sciences (PNAS) website by Michael H. Graham, Brian P. Kinlan, Louis D. Druehl, Lauren E. Garske and Stuart Banks predicted tropical kelp forests ranging from 98-656 feet (30-200 meters) Their survey in 2007 discovered kelp as deep as 196 feet (60 meters) underwater.
In 1999 the U.S. Geological Survey found coralline algae in a coral reef in a submerged island southwest of Florida. About 250 feet below the surface, algae living in this reef are the deepest photosynthesizing plants known today.