The ocean contains far fewer plant species than land, but the ones that thrive there are extraordinarily productive. Roughly half of Earth’s oxygen comes from photosynthetic organisms in the sea. These range from microscopic single-celled life drifting in open water to towering kelp forests and meadows of underwater grass rooted in the seafloor. Here’s a breakdown of the major groups.
Seagrasses: The Only True Flowering Plants
Seagrasses are the only flowering plants that live entirely submerged in saltwater. About 70 species have been documented worldwide, making them relatively rare compared to land plants. They grow in shallow coastal waters where sunlight reaches the bottom, sending roots into sandy or muddy sediment much like terrestrial grass.
What makes seagrasses remarkable is how they reproduce underwater. Nearly all species rely on water currents to carry pollen from male to female flowers, a process called hydrophily. Male flowers typically open at sunset and release pollen within one to two hours at night. The pollen is either elongated or bundled in strands of sticky mucilage that drifts through the water until it contacts the tentacle-like stigmas of female flowers. Recent research has also found that small invertebrates carry pollen between flowers, meaning at least some seagrasses use a combination of water flow and animal-assisted pollination.
Seagrass meadows are powerful carbon sinks. Coastal vegetated habitats like seagrass beds, mangroves, and salt marshes store three to five times more carbon per acre than tropical rainforests and pull carbon from the atmosphere at roughly ten times the rate. This stored carbon, often called “blue carbon,” accumulates in the sediment beneath the plants and can remain locked away for centuries.
Macroalgae: Green, Brown, and Red Seaweeds
Seaweeds are not technically plants. They’re macroalgae, large enough to see with the naked eye but lacking true roots, stems, or leaves. They anchor to rocks and other hard surfaces with a structure called a holdfast. The ocean’s seaweeds fall into three major groups, each distinguished by its pigments, chemistry, and preferred habitat.
Green algae tend to grow in shallow, sunlit water. They contain the same primary pigment as land plants (chlorophyll), which is why they look familiar. Sea lettuce is one of the most recognizable examples. Green algae produce a unique compound called ulvan, a type of fiber being studied for various health and industrial uses.
Brown algae dominate cooler, deeper waters and include the largest seaweeds on Earth. Kelp is the most well-known member of this group. Bull kelp can grow stipes (stem-like structures) up to 30 meters long, with growth rates reaching 6 centimeters per day under ideal conditions. A single bull kelp individual anchors to the seafloor with a holdfast, grows a long flexible stipe, and supports a floating bulb that holds up to 100 blade-like fronds near the surface. Brown algae are rich in compounds like alginate (used as a thickener in food), fucoidan, and iodine.
Red algae are the most species-rich group and can survive at the greatest depths. A red alga holds the record for the deepest photosynthesizing organism ever found: 268 meters below the surface in the Caribbean. Red algae manage this because their pigments are especially efficient at capturing the dim blue light that penetrates deep water. They’re also the richest of the three groups in dietary fiber, protein, and certain vitamins like C and B6. Agar, the gel-like substance used in cooking and laboratory science, comes from red algae.
Phytoplankton: Microscopic Ocean Producers
Phytoplankton are single-celled organisms that float in the sunlit upper layer of the ocean. They’re invisible individually but produce an enormous share of Earth’s oxygen, collectively responsible for roughly 50% of all oxygen generation on the planet. About the same amount gets consumed by marine life, so the net contribution to the atmosphere is smaller, but their role in the food web is irreplaceable. Nearly every marine food chain starts with phytoplankton.
The two main classes are diatoms and dinoflagellates. Diatoms have rigid shells made of silica (essentially glass) with interlocking parts, and they drift passively on ocean currents. Dinoflagellates are more mobile, propelling themselves through the water with whip-like tails called flagella. Their shells are complex but flexible compared to diatoms. When conditions are right, dinoflagellates can multiply explosively, sometimes producing harmful algal blooms (red tides) that discolor the water and release toxins.
Mangroves: Trees at the Ocean’s Edge
Mangroves are salt-tolerant trees and shrubs that grow along tropical and subtropical coastlines, right where land meets sea. Their tangled root systems sit partially submerged in saltwater, which would kill most trees. Three common species dominate many coastlines: red mangroves along the shoreline, black mangroves in the upper marsh, and white mangroves farther inland.
The key to mangrove survival is an extraordinary salt filtration system in their roots. Some species filter out approximately 90% of sodium ions from seawater before it ever reaches the rest of the plant. The root has a layered pore structure in its outermost skin. The first sublayer does most of the heavy lifting, using an electrical charge on its surface to repel salt ions. A second layer of larger pores provides additional filtration. The result is that the water reaching the tree’s vascular system is nearly fresh.
Mangrove forests also rank among the most effective carbon-storing ecosystems on Earth. Like seagrass beds, they sequester carbon at roughly ten times the rate of tropical forests and store several times more carbon per acre in their waterlogged soils.
Salt Marsh Plants
Salt marshes form in sheltered coastal areas where sediment accumulates and tidal water flows in and out regularly. The plants here are halophytes, meaning they’ve evolved to tolerate high salt concentrations in the soil. Smooth cordgrass dominates the low marsh zone, where flooding is frequent. Higher up, where tides reach less often, you’ll find salt grass, saltwort, glasswort (both annual and perennial varieties), and black rush. In warmer regions, dwarf mangroves sometimes grow scattered among these marsh grasses.
Salt marsh vegetation stabilizes shorelines by trapping sediment in its root systems, buffers coastal communities from storm surges, and filters pollutants from runoff before they reach open water. These plants occupy a narrow band between fully terrestrial and fully marine environments, but that strip of green is disproportionately productive and ecologically important.