There are two main types of marine plants: seagrasses and algae. Like plants on land, most plants under the sea need sunlight for photosynthesis. This means that they usually live in the top-most layers of the ocean. However, some marine plants, such as kelp and coralline algae, are adapted to live in deeper waters. Unlike terrestrial (land) plants, aquatic plants get their nutrients from the water around them, not through their roots. Instead, their hearts are used to anchor them to the seafloor and stop them from floating away.

Why are marine plants important?

Plants are essential to all aquatic ecosystems, providing fish and other marine life with food and shelter. The carbohydrates produced by plants under the sea form the basis of the entire marine food web, supporting everything from tiny zooplankton to giant whales. In addition, aquatic plants and algae provide over half the oxygen you breathe as they photosynthesize in the ocean.


Seagrasses evolved 100 million years ago and got their name from their long, grass-like leaves. Around 72 species can be found in many parts of the world, living in shallow, salty, brackish waters. Like flowering plants that you can find on land, they have roots, stems, and leaves and even produce flowers and seeds. In addition, they provide food and shelter


The term ‘algae’ refers to many organisms that can produce oxygen through photosynthesis. With about 27,000 different species, ranging from single-celled to multicellular organisms, algae (singular ‘alga’) is the primary source of food for fish and other aquatic life. Because it forms the foundation of the food chain and can be found in both saltwater and freshwater, it is essential for balanced ecosystems.

Seaweed and kelp

Seaweed and kelp are more significant marine algae that are made up of many cells. They all contain chlorophyll for photosynthesis, but some types of seaweed can look brown or red, not green. This is because they have extra pigments that give them a different color. Kelp grows in cold seas worldwide and can form substantial underwater forests that provide a habitat for snails, sea urchins, seals, and sea otters.


Although too small to see with the naked eye, these tiny single-cell organisms clump together to form visible groups floating on the ocean’s top. This microscopic marine algae can be found in all water environments. They can multiply quickly in the right conditions, making the water green or red. Phytoplankton is an essential food source for almost all marine life, so it’s an integral part of the food web.

Are corals animals or plants?

Corals ‘take root’ as plants do but cannot make their food, unlike plants. Instead, they have tiny, tentacle-like arms to catch their nutrition from the water. But unlike a typical animal, they don’t have a face or body parts. So are corals plants or animals?

Well, coral is a sessile animal (an animal that is rooted to the spot). It comprises tiny polyps that secrete a hard, outer limestone skeleton that attaches to a rock or the dead skeletons of other polyps. But here’s where it gets exciting. A type of algae called zooxanthellae resides in many kinds of coral tissue. This alga uses the coral’s waste products for photosynthesis, while the coral benefits from the oxygen and organic products produced due to this photosynthesis. This helps the coral to thrive and grow. In this sense, corals comprise a unique partnership that benefits marine plants and animals.

Facts about plants under the sea

Did you know

  • Seagrasses can form huge underwater meadows, the largest of which can be seen from space.
  • All land plants evolved from algae just under a billion years ago.
  • Giant kelp can reach up to 60 meters from the sea floor to the ocean’s surface. It can be used for human food or as a fertilizer.
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