Algae is an informal term for a diverse group of photosynthetic eukaryotes, including unicellular microalgae like Chlorella and multicellular macroalgae such as giant kelp. Predominantly aquatic, algae lack the complex structures found in land plants, with the largest forms known as seaweeds. Algae do not share a common ancestor, making them polyphyletic. While green algae have primary chloroplasts from cyanobacteria, diatoms and brown algae possess secondary chloroplasts from red algae.
Reproductive strategies in algae range from asexual cell division to complex sexual reproduction via spores. Most algae are autotrophic, although some can be mixotrophic or heterotrophic, relying on external energy sources. Algae contribute to oxygen production through photosynthesis, with fossilized examples dating back 1.6 to 1.7 billion years.
Algae have various industrial and traditional applications, particularly in East Asian cuisine. Modern uses include bioremediation, biofuels, and carbon sequestration, highlighting their potential role in climate change mitigation and economic value.
