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Algae are eukaryotic photoautotrophs that lack roots, stems, leaves, and vascular tissues like xylem and phloem found in plants. Though historically classified as plants, algae lack embryos and the structural complexity of true plants. Instead, their bodies consist of simple structures: unicellular forms, filamentous chains, or thalli with holdfasts, stipes, and blades in larger species.

Unicellular algae reproduce asexually through mitosis followed by cytokinesis, producing two genetically identical cells. Multicellular algae reproduce asexually through fragmentation, where each fragment develops into a new organism. Some species exhibit alternation of generations, cycling between asexual and sexual reproduction across different generations.

Algae rely on water for physical support, reproduction, and nutrient diffusion. Without vascular tissues, algae absorb nutrients directly from water rather than through roots. The surrounding water supports the thallus structure, and some algae use gas-filled structures called pneumatocysts for buoyancy in aquatic habitats.

Red and green algae, part of the kingdom Archaeplastida, originated from primary endosymbiotic events, unlike other protists with chloroplasts from secondary endosymbiosis. Both groups contain chlorophyll and perform oxygenic photosynthesis. Their unique evolutionary origin and photosynthetic capabilities distinguish them within the diverse protist kingdom.

Though predominantly aquatic, algae thrive in diverse terrestrial and unusual habitats including soil, tree surfaces, polar bear fur, and the fur of South American sloths, provided sufficient moisture is present. Algae distribution is influenced by nutrient availability, light wavelengths, and suitable growth surfaces across these varied environments.

Algae are classified into multiple phyla based on rRNA sequences, structures, pigments, and other characteristics using modern molecular taxonomy. Major groups include brown algae, diatoms, dinoflagellates, water molds, red algae, and green algae. Most are photosynthetic with chlorophyll a and accessory pigments, though some groups like oomycotes are chemoheterotrophs.

Photosynthetic algae predominantly inhabit the photic zone of aquatic environments where sufficient light is available for photosynthesis. Light wavelengths influence algal distribution patterns, determining which species thrive in specific aquatic locations. This light dependency shapes algal communities from coastal waters to deep ocean zones.