The marine biome, Earth’s largest living system, encompasses vast oceans, intricate coral reefs, and nutrient-rich estuaries. Photosynthetic organisms thrive here, forming the base of marine food webs. From rooted plants to microscopic single-celled life forms, they convert sunlight into energy, sustaining marine species.
Vascular Plants of the Sea
Vascular plants adapt to marine and coastal life with features for saline waters. Seagrasses, underwater flowering plants, form extensive meadows in shallow coastal waters. Their roots anchor them in sediments, providing stability, reducing erosion, and serving as foraging grounds and habitats for marine animals.
Mangroves are important vascular plants in tropical and subtropical coastal areas, thriving in brackish and saline conditions. They adapt with salt-filtering roots or glands excreting excess salt. Their aerial root systems, like prop roots and pneumatophores, obtain oxygen in waterlogged soils and provide nursery habitats for fish and invertebrates.
Along temperate coastlines, salt marsh plants dominate the intertidal zones, enduring daily tides. Cordgrasses (Spartina species) and glassworts (Salicornia species) tolerate high salt and saturated soils. These marsh communities filter pollutants, stabilize shorelines, and support coastal ecosystem health.
The World of Seaweeds
Seaweeds (macroalgae) are multicellular photosynthetic organisms inhabiting marine environments. They are not true plants, lacking roots, stems, and leaves. They attach to surfaces with a holdfast and absorb nutrients directly from water. They are categorized by primary photosynthetic pigments, determining their color.
Green algae (Chlorophyta) are found in shallow, intertidal zones, sharing characteristics with land plants, including chlorophyll a and b. Sea lettuce (Ulva species), for example, forms thin, bright green sheets in nutrient-rich coastal waters.
Brown algae (Phaeophyceae) are common in cooler waters, including some of the largest and most complex algal forms. Kelp species, like giant kelp (Macrocystis pyrifera), grow into underwater forests, providing habitats for diverse marine life. Rockweed (Fucus species) is prominent in rocky intertidal zones, enduring wave action.
Red algae (Rhodophyta) absorb blue light, thriving in deeper waters where other light wavelengths don’t penetrate. This diverse group ranges from delicate, branching forms to calcified coralline algae that contribute to reef building. Nori, a common food, comes from certain red algae species.
Microscopic Photosynthesizers
The marine biome hosts microscopic photosynthetic organisms, forming the base of oceanic food webs. Phytoplankton are single-celled or colonial algae drifting in the water column, primarily in the sunlit photic zone. These organisms produce approximately half of Earth’s oxygen annually.
Phytoplankton diversity includes diatoms, with intricate silica cell walls, and dinoflagellates, many possessing flagella for movement. Coccolithophores, encased in calcium carbonate plates, contribute to ocean sediments. Their abundance makes them primary producers in open ocean ecosystems, converting sunlight into organic matter that fuels zooplankton and larger marine animals.
Cyanobacteria, ancient photosynthetic bacteria, contribute to marine primary production. Species like Prochlorococcus and Synechecoccus are among Earth’s most abundant photosynthetic organisms in nutrient-poor open ocean waters. Some marine cyanobacteria also perform nitrogen fixation, converting atmospheric nitrogen into a usable form for other organisms, enriching the marine nitrogen cycle. They are distributed throughout the photic zone, their abundance influenced by light, temperature, and nutrient concentrations.