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Red algae owe their color primarily to phycoerythrin, an accessory pigment organized within phycobilisomes that function as light-harvesting antennae. These phycobiliproteins, including phycocyanin and allophycocyanin, mask the green chlorophyll a present in their chloroplasts. Species in deeper waters produce more phycoerythrin, appearing darker red, while shallow-water varieties show less pigment and may appear greenish.

Red algae inhabit terrestrial, freshwater, and marine ecosystems, with most species found in oceans. They exist as both unicellular and multicellular organisms. Multicellular varieties display diverse morphologies, including filamentous forms like Polysiphonia, leafy structures, and coralline types that deposit calcium carbonate and contribute to coral reef formation.

Unicellular red algae like Galdieria thrive in hot, acidic springs by acquiring at least 75 genes through horizontal gene transfer from prokaryotes. These transferred genes encode adaptations for salt stress resistance, metal toxicity protection, and cytoplasmic membrane strengthening. Galdieria can even grow in complete darkness using organic compounds as energy sources, an unusual trait among phototrophic organisms.

Cyanidioschyzon merolae is an exceptionally small unicellular red alga, measuring only 1 to 2 micrometers in diameter. It possesses one of the smallest known eukaryotic genomes at approximately 16.5 megabase pairs. Despite its tiny size and minimal genetic material, this organism thrives in extreme acidic and hot environments ranging from pH 0.5 to 4.0 and temperatures of 30 to 60 degrees Celsius.

Red algae produce valuable commercial compounds including agar, a solidifying agent essential for microbiological culture media, and carrageenans, which serve as thickening and stabilizing agents in food production. Additionally, Porphyra species are harvested, dried, and processed into edible wraps used in sushi preparation, making red algae economically important across multiple industries.

Polysiphonia, a filamentous red alga with nearly 200 recognized species, follows a complex reproductive cycle involving alternation of generations. Haploid male and female gametes from diploid organisms mature into haploid multicellular individuals; male algae release sperm-like cells that fuse with female reproductive structures, forming diploid zygotes that complete the cycle through meiosis. These organisms commonly attach to rocks and artificial structures near shorelines worldwide.

Red algae are phototrophic organisms that use chlorophyll a for photosynthesis but lack chlorophyll b, distinguishing them from green plants. Instead, they rely on phycobiliproteins as major light-harvesting pigments, similar to those found in bacterial phylum cyanobacteria. Most red algae are multicellular and lack flagella, and their chloroplasts contain unique pigment organization that enables efficient light capture in various aquatic depths.