The Atlantic Ocean supports a diverse array of plant and plant-like organisms. These organisms are primary producers, forming the base of the marine food web by converting sunlight into energy through photosynthesis, underpinning nearly all other ocean life. The term “plants” in this marine context extends beyond the familiar terrestrial definition, encompassing true flowering plants, large multicellular algae, and microscopic single-celled organisms.
Seagrasses
Seagrasses are unique as the only true vascular flowering plants that live entirely submerged in marine environments. They possess roots, stems, leaves, and produce flowers and seeds. These plants evolved from terrestrial plants, recolonizing the ocean approximately 70 to 100 million years ago. Seagrasses typically thrive in shallow, sheltered coastal waters with clear conditions, where they can anchor their roots and rhizomes in soft sediment like sand or mud.
In the Atlantic, prominent examples include eelgrass (Zostera marina) and turtlegrass (Thalassia testudinum). Eelgrass is common in the North Atlantic, found as far south as North Carolina and extending north to Greenland. Turtlegrass is more prevalent in warmer, tropical Atlantic regions. Seagrasses reproduce both asexually, by spreading through an underground network of rhizomes, and sexually, through underwater pollination. These dense underwater meadows are productive ecosystems, providing habitats, nursery grounds, and food for marine animals, while stabilizing sediments and improving water quality.
Macroalgae
Macroalgae, commonly known as seaweeds, are large, multicellular algae distinct from true plants. Unlike vascular plants, they lack true roots, stems, and leaves. Instead, they absorb nutrients directly from the surrounding water through their entire surface. Their structure consists of a holdfast (anchoring to substrates like rocks), a stem-like stipe, and leaf-like blades where photosynthesis occurs.
The Atlantic Ocean hosts macroalgae, categorized by their primary pigments. Green algae, such as sea lettuce (Ulva lactuca), are found in intertidal zones and shallow waters. Brown algae include species like kelp and rockweed (Fucus vesiculosus), which can form extensive underwater forests or dense mats along rocky shores. Red algae, such as Irish moss (Chondrus crispus), thrive in deeper subtidal zones and along rocky coasts. Irish moss is abundant along Atlantic coasts of Europe and North America and is valued for its carrageenan content. These macroalgae contribute to coastal ecosystems by providing food, shelter, and primary production.
Microalgae
Microalgae are microscopic, single-celled or colonial organisms fundamental to marine life in the Atlantic. Collectively known as phytoplankton, these tiny photosynthetic organisms form the base of nearly all oceanic food webs, converting sunlight, carbon dioxide, and nutrients into organic matter as primary producers. Microalgae are responsible for producing a substantial portion of the Earth’s oxygen, estimated to be at least half, significantly more than all terrestrial forests combined.
Types of microalgae found throughout the Atlantic include diatoms, dinoflagellates, and coccolithophores. Diatoms have intricate, glass-like cell walls made of silica and are abundant in nutrient-rich waters, often dominating spring blooms. They are a major component of the marine food web, consumed by zooplankton and small fish. Dinoflagellates are a diverse group, with many species possessing two flagella for movement. They can be autotrophic (photosynthetic), heterotrophic, or mixotrophic, and some species form extensive blooms. Coccolithophores produce microscopic calcium carbonate plates called coccoliths, which can reflect light and give the water a bright blue appearance during blooms. These calcifying organisms are abundant in the Atlantic, particularly in temperate and sub-polar environments, and play an important role in the global carbon cycle.