algae

Algae are a diverse group of photosynthetic organisms that live mainly in aquatic environments. The term is used for a wide range of organisms, from microscopic phytoplankton to large seaweeds. In a strict sense, true algae are eukaryotes, while cyanobacteria (blue-green algae) are prokaryotes that also photosynthesize and are not classified as algae in modern taxonomy.

Three major lineages of macroscopic algae are brown (phaeophytes), red (rhodophytes), and green (chlorophytes). Diatoms and dinoflagellates are important microalgae in marine and freshwater ecosystems. Algae display a variety of body forms, from unicellular to multicellular thalli. They generally lack true roots, stems, or leaves; many have holdfasts, stipes, and blades. Pigments such as chlorophyll a, along with accessory pigments, give different colorations and ecological niches. Most algae store energy as starch and have cell walls composed of cellulose or unique materials such as silica (in diatoms).

Ecologically, algae are primary producers, forming the base of aquatic food webs and contributing substantially to global oxygen production. They inhabit oceans, rivers, lakes, damp soils, and even ice or hot springs. Algal populations can form blooms, including harmful algal blooms that produce toxins and deplete oxygen. Reproductive strategies are diverse, including asexual division, fragmentation, and sexual reproduction, with many groups exhibiting alternation of generations.

Humans use algae in food and industry: some red and green algae are consumed as sea vegetables; alginates, agar, and carrageenan are extracted from various species for gelling and thickening; diatoms contribute to filtration and as bioindicators. The fossil record includes early eukaryotic algae dating to the Proterozoic, with stromatolites providing evidence of ancient photosynthetic life. Algae continue to be studied for biofuel potential, wastewater treatment, and biotechnological applications.