Algen

Algen (algae) are a diverse group of photosynthetic organisms that are not classified as land plants. They range from microscopic unicellular forms to large multicellular seaweeds and occur in freshwater, marine, and moist terrestrial environments. Algae perform photosynthesis, generating a substantial portion of the world’s oxygen and forming the base of many aquatic food webs. They also contribute to carbon cycling by fixing carbon dioxide.

Taxonomically, algae span several lineages. Green algae (Chlorophyta and relatives), red algae (Rhodophyta), and brown algae (Phaeophyceae) are traditional groups of photosynthetic eukaryotes. Other important algal groups include diatoms and dinoflagellates, which are protists, and cyanobacteria, commonly known as blue-green algae, which are bacteria. Because of their diversity, the term “algae” covers a wide range of organisms with similar ecological roles rather than a single coherent group.

Morphology and physiology vary widely. Algae range from single cells to colonies, filaments, and complex thalli. They contain pigments such as chlorophyll a and accessory pigments that give green, red, or brown hues. Some have silica shells (diatoms) or cellulose walls. Reproduction is diverse, including asexual division, fragmentation, and sexual life cycles, with patterns that differ among groups.

Ecology and human uses: as primary producers, algae support aquatic ecosystems and influence nutrient and carbon cycles. Algal blooms can occur in nutrient-rich waters and may produce toxins. Humans harvest algae for food (seaweeds) and supplements (spirulina, chlorella), and for industrial products such as agar, carrageenan, and diatomaceous earth. Research explores algae for biofuels, bioproducts, and wastewater treatment.