macroalgae

Macroalgae, commonly called seaweeds, are large, multicellular, photosynthetic organisms that inhabit marine and some freshwater systems. They are not true plants but are eukaryotic algae that lack true vascular tissue. Macroalgae are traditionally grouped into three major lineages by pigment and biochemical characteristics: brown algae (Phaeophyceae), red algae (Rhodophyta), and green algae (Chlorophyta). Brown algae are mostly marine and include large forms such as kelps; red and green algae occur in a range of marine habitats and, in some cases, freshwater.

Morphology and life cycles: A macroalga typically has a thallus with a holdfast, a stem-like stipe, and leaf-like blades; structures vary by group. Without true vascular tissue, they rely on diffusion for transport. Many macroalgae have complex life cycles with alternation of generations, featuring sporophyte and gametophyte phases that may be morphologically similar (isomorphic) or different (heteromorphic). Reproduction can be sexual or asexual, via spores or fragmentation.

Ecology and distribution: Macroalgae occur mainly in coastal, sunlit waters, from tropical to temperate regions, with brown kelp forests forming major underwater habitats. They serve as primary producers, provide habitat and food for a wide range of marine organisms, and influence nutrient cycling. Some species form dense blooms, while others are sensitive to warming and pollution.

Uses and significance: Humans harvest and cultivate certain macroalgae for food, particularly red and green species (for example, nori and sea lettuce) and for hydrocolloids such as agar, carrageenan, and alginate derived from red and brown algae. They are also studied for biofuel potential, carbon sequestration, and bioremediation. Conservation and sustainable harvesting are important where kelp forests and other macroalgal communities support biodiversity and coastal protection.