• Kelp grow in huge forests in the ocean.Paul Souders/Photodisc/Getty Images
  
  Oceans are an active community of fish, microorganisms, marine wildlife and plants. Plants in the ocean, such as seagrass, kelp and phytoplankton, serve a vital purpose. They provide food and shelter for animals and produce oxygen. However, they can also be dangerous to the marine life that feeds off of them.
  
  

Seagrasses

• Seagrasses grow underwater along coastlines. The plants have thick roots, long, green stems and thin leaves that branch from the tops of the plants, similar to grasses found on land. There are 52 species of marine seagrasses, according to the Florida Department of Environmental Protection. Seagrass needs a source of light to grow and produce oxygen. Seagrass provides a habitat and protection for many types of ocean life, including shellfish and young ocean animals. The grasses also provide food for animal life in the ocean, such as manatees. Other animals feed off the algae and small organisms that grow along the blades of the seagrasses. According to the Florida Department of Environmental Protection, seagrasses also gives ocean water clarity by trapping sediments inside their leaves.
  
  

Kelp

• Kelp are thick groupings of brown algae that grow near the surface of the ocean, with the tops of the plants often floating on the ocean surface itself. The plant has thick roots that act like anchors attaching it to rocks in the ocean. The wide, brown leaves branch off at the top of the stems. Kelp grow predominantly in rocky terrains with cool water, such as the coast of Alaska, Canada and California, according to National Marine Sanctuaries. Dozens of invertebrate animals live inside kelp forests and several large animals, such as sea lions and whales, feed off of the leaves of the kelp.
  
  

Phytoplankton

• Phytoplanktons are single-cell plants that use light to produce energy. Like plants on land, phytoplanktons absorb carbon dioxide and produce oxygen. Phytoplankton grows in large blooms near the surface of the ocean. However, according to the National Aeronautics and Space Administration, individual phytoplankton only live for a few days. Phytoplanktons provide a source of food for many fish, whales and other marine wildlife. However, they can also cause widespread death and disease in the ocean. The plant produces a toxin that results in an algae bloom. When fish eat the toxic phytoplankton, they can die. The phytoplankton can also sink to the bottom of the ocean, causing a rapid depletion in oxygen that suffocates fish and marine life.