Marine Plants: What Grows in the Ocean?

Marine "plants" include true vascular plants (seagrasses, mangroves) and diverse photosynthetic protists (algae, phytoplankton). Here are the major groups:

1. Seagrasses (Angiosperms)

True flowering plants with roots, stems, leaves. Form underwater meadows in shallow, clear water.

• Eelgrass (Zostera) – temperate zones.
• Turtle grass (Thalassia) – tropical Caribbean.
• Widgeon grass (Ruppia) – brackish to saline.

Critical habitat for fish, turtles, dugongs; carbon sequestration; sediment stabilization.

2. Kelp & Large Brown Algae (Phaeophyceae)

Not true plants (lack vascular tissue), but form towering underwater forests.

• Giant kelp (Macrocystis pyrifera) – up to 45 m/day growth; California, Tasmania.
• Bull kelp (Nereocystis) – Pacific Northwest.
• Sugar kelp (Saccharina) – North Atlantic/Pacific.

3. Sargassum

Free-floating brown algae; forms vast mats in the Sargasso Sea. Nursery for turtles, fish, invertebrates.

4. Corals (Animal–Algal Symbioses)

Reef-building corals host Symbiodinium (zooxanthellae) dinoflagellates—photosynthetic partners. The calcium carbonate skeleton builds reefs.

5. Phytoplankton (Microalgae)

Microscopic drifters: diatoms (silica shells), dinoflagellates, coccolithophores (calcium carbonate plates), cyanobacteria. Base of marine food web; produce ~50% of Earth's oxygen.

6. Red & Green Algae (Rhodophyta, Chlorophyta)

• Coralline algae – cement reefs.
• Nori (Porphyra) – edible seaweed.
• Sea lettuce (Ulva) – green sheets.

7. Mangroves

Woody trees/shrubs in tropical intertidal zones. Prop roots, pneumatophores, salt-excreting leaves. Nursery habitat; coastal protection; massive carbon storage.

Together, these groups drive marine productivity, oxygenate the planet, and support human fisheries and climate regulation.