alga

Alga is a term used for a large and diverse group of photosynthetic organisms that live in water or damp environments. Most are non-vascular and range from unicellular microalgae to large, multicellular seaweeds. Algae are not a single taxonomic group; they span several lineages within the eukaryotes and include the green algae (Chlorophyta), red algae (Rhodophyta), and brown algae (Phaeophyceae), as well as diatoms and dinoflagellates. Cyanobacteria are commonly called blue-green algae in everyday language, but they are bacteria, not true algae.

They are primarily photosynthetic, using pigments such as chlorophyll a; green algae also contain chlorophyll b, brown algae contain fucoxanthin, and red algae contain phycoerythrin and phycocyanin, giving them distinctive colors. Morphologically, many algae lack true tissues and organs; macroscopic forms have holdfasts, stipes, and blades that resemble roots and stems but are not true plant organs.

Algae occupy a wide range of habitats, from oceans and rivers to damp soil and rocks. They are foundational to aquatic ecosystems as primary producers, supplying organic matter and oxygen. Some species can form blooms that alter water quality or harm other organisms, particularly when nutrient levels are high.

Humans rely on algae for food, such as seaweed used in various cuisines, and for hydrocolloids like agar, carrageenan, and alginates used as thickeners and gelling agents. Algae are also studied for biofuels, nutritional supplements, and biotechnological applications. Their diversity and ecological importance make them a central component of many aquatic environments.