Plant life is relatively rare undersea. Most of the niche occupied by plants on land is actually occupied by macroscopic algae in the ocean, such as Sargassum and kelp which are commonly known as seaweeds.
The non algae plants that do survive in the sea are often found in shallow waters, such as the seagrasses (examples of which are eelgrass, Zostera, and turtlegrass, Thalassia).
These plants have adapted to the high salinity of the ocean environment. The intertidal zone is also a good place to find plant life in the sea, where mangroves or cordgrass or beach grass might grow.
Prokaryotic algae
Cyanobacteria have been included among the algae, referred to as the cyanophytes or Blue-green algae, (the term "algae" refers to any aquatic organisms capable of photosynthesis) though some recent treatises on algae specifically exclude them. Cyanobacteria are some of the oldest organisms to appear in the fossil record dating back to the Precambrian, possibly as far as about 3.5 billion years. Ancient cyanobacteria likely produced much of the oxygen in the Earth's atmosphere.
Cyanobacteria can be unicellular, colonial, or filamentous. They have a prokaryotic cell structure typical of bacteria and conduct photosynthesis on specialized cytoplasmic membranes called thylakoid membranes, rather than in organelles. Some filamentous blue-green algae have specialized cells, termed heterocysts, in which nitrogen fixation occurs.
Eukaryotic algae
All other algae are eukaryotes and conduct photosynthesis within membrane-bound structures (organelles) called chloroplasts. Chloroplasts contain DNA and are similar in structure to cyanobacteria, presumably representing reduced cyanobacterial endosymbionts. The exact nature of the chloroplasts is different among the different lines of algae, reflecting different endosymbiotic events. The table below lists the three major groups of eukaryotic algae and their lineage relationship is shown in the figure on the left. Note many of these groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost them entirely.
Reefs
Reefs comprise some of the densest and most diverse habitats in the world. The best-known types of reefs are tropical coral reefs which exist in most tropical waters; however, reefs can also exist in cold water. Reefs are built up by corals and other calcium-depositing animals, usually on top of a rocky outcrop on the ocean floor. Reefs can also grow on other surfaces, which has made it possible to create artificial reefs. Coral reefs also support a huge community of life, including the corals themselves, their symbiotic zooxanthellae, tropical fish and many other organisms.
Corals.Much attention in marine biology is focused on coral reefs and the El Niño weather phenomenon. In 1998, coral reefs experienced a "once in a thousand years" bleaching event, in which vast expanses of reefs across the Earth died because sea surface temperatures rose well above normal. Some reefs are recovering, but scientists say that 58% of the world's coral reefs are now endangered and predict that global warming could exacerbate this trend.
