The term “sea plant” is widely used to describe any organism in the ocean that uses sunlight to create energy. In biological science, this group is highly diverse, including true flowering plants and several different groups of algae. The most accurate way to categorize these organisms is based on their cellular structure and whether they possess internal plumbing for moving nutrients.
The Biological Distinction Between Marine Plants and Algae
The fundamental difference between a true marine plant and marine algae lies in the presence of a vascular system. True plants, classified under the Kingdom Plantae, are defined by having specialized tissues—xylem and phloem—that transport water and nutrients throughout the organism. This internal complexity allows them to develop true roots, stems, and leaves, similar to terrestrial flora.
Algae, in contrast, are non-vascular, meaning they lack these specialized transport tissues. They absorb water and nutrients directly from the surrounding seawater through their entire body structure, known as a thallus. Most algae are therefore classified outside of the Plant Kingdom, often belonging to the diverse Kingdom Protista. The structures that superficially resemble roots and leaves on algae are simple anchoring devices called holdfasts and blades, which serve only for attachment and not for nutrient uptake.
True Vascular Marine Plants: Seagrasses
Seagrasses are the only organisms in the marine environment correctly classified as true plants. They are submerged flowering plants, or angiosperms, that evolved from land plants and recolonized the ocean. These organisms possess true roots and underground stems, called rhizomes, which anchor them into the sediment and are responsible for nutrient exchange.
Like their terrestrial relatives, seagrasses have leaves, produce flowers, and undergo pollination to create seeds, completing their entire life cycle fully submerged in saltwater. Approximately 60 to 72 species are known globally, forming dense underwater meadows that stabilize soft coastal sediments.
The Vast Majority: Macroalgae (Seaweeds)
Macroalgae, commonly referred to as seaweeds, constitute the large, visible, and non-vascular forms of marine photosynthetic life. They are generally multicellular and can range in size from small fronds to massive, towering structures. Seaweeds are primarily categorized into three groups based on the dominant photosynthetic pigments they contain, which determines their color and the depth at which they can thrive.
Brown Algae
Brown algae (Phaeophyta) includes the largest forms, such as kelp and wakame, which owe their characteristic color to the pigment fucoxanthin. Giant kelp (Macrocystis pyrifera), for instance, can grow up to 20 meters long, forming underwater forests that serve as complex habitats. These brown seaweeds typically dominate cooler, nutrient-rich coastal waters.
Red Algae
Red algae (Rhodophyta) is the most diverse group, with species like nori (used in sushi) and the rock-hard coralline algae. Their red pigment, phycobilin, allows them to absorb blue light, which penetrates deeper into the water column than other light wavelengths. This adaptation permits red algae to live at greater depths than their green and brown counterparts.
Green Algae
Green algae (Chlorophyta) possesses the same chlorophyll pigments as true land plants, giving them a bright green color. A common example is sea lettuce (Ulva), which often grows in shallow tide pools and intertidal zones.
The Microscopic Photosynthesizers: Phytoplankton
The smallest and most numerous photosynthetic organisms in the sea are phytoplankton, a term derived from Greek meaning “plant drifter.” These single-celled organisms, also known as microalgae, float in the sunlit upper layers of the water column, known as the euphotic zone. Their sheer abundance makes them the largest contributor to marine primary productivity, performing roughly half of the world’s total photosynthesis.
Phytoplankton are the foundation of nearly every marine food web, supplying energy to zooplankton, shellfish, and ultimately, large marine animals. Major groups include diatoms, which are encased in complex shells made of silica, a component of glass.
Another significant group is dinoflagellates, many of which utilize two whip-like tails called flagella for limited vertical movement in the water. Beyond providing food, these microscopic organisms also play a substantial role in the global carbon cycle by absorbing carbon dioxide from the atmosphere.