Algae are diverse organisms, from microscopic single cells to large seaweeds. They play an important role in ecosystems as photosynthetic producers. Classification clarifies their biological significance.
Understanding Biological Domains
Biological classification organizes life into hierarchical categories, with domains as the broadest groupings. Three universally recognized domains exist: Bacteria, Archaea, and Eukarya.
The domains Bacteria and Archaea comprise prokaryotic organisms, lacking a nucleus and internal compartments. They differ in genetic makeup, cell wall composition, and membrane lipids. In contrast, the Domain Eukarya includes all eukaryotic organisms, characterized by cells with a true nucleus and membrane-bound organelles. This domain includes animals, plants, fungi, and protists.
Algae’s Place in the Tree of Life
Most organisms called “algae” belong to the Domain Eukarya. Eukaryotic algae possess chloroplasts for photosynthesis, a nucleus, and other membrane-bound organelles. However, “algae” is not a precise taxonomic group but a broad, informal term for polyphyletic photosynthetic organisms not classified as plants.
Historically, cyanobacteria were once known as “blue-green algae.” Despite their photosynthetic capabilities, cyanobacteria are distinct from eukaryotic algae and are now classified under the Domain Bacteria. This reclassification highlights cellular structure’s importance in classification, differentiating organisms by fundamental characteristics.
Diverse Forms of Algae
Within Eukarya, algae exhibit great diversity in forms, pigments, and cellular structures.
Green Algae (Chlorophyta)
Green algae (Chlorophyta) are characterized by their bright green color due to chlorophyll a and b, like higher plants. They store food as starch and have cell walls composed of cellulose. They can be unicellular, colonial, or multicellular, found in freshwater and marine environments.
Red Algae (Rhodophyta)
Red algae (Rhodophyta) are mostly marine and owe their red coloration to pigments like phycoerythrin and phycocyanin, which absorb blue light in deeper water. They lack flagella at any stage and store carbohydrates as floridean starch. Many have complex multicellular structures, with cell walls containing cellulose and sulfated polysaccharides (e.g., agar, carrageenan).
Brown Algae (Phaeophyceae)
Brown algae (Phaeophyceae) are mostly marine, from small filamentous forms to giant kelps. Their brown or olive-green color comes from the xanthophyll fucoxanthin, masking other chlorophylls. They store energy as laminarin and have cell walls containing cellulose and alginic acid, which has commercial applications.
Diatoms
Diatoms are unicellular algae known for their intricate cell walls, called frustules, made of silica. These “glass houses” are highly patterned and are significant primary producers. They contain chlorophylls a and c, and fucoxanthin, giving them a yellowish-brown appearance, and store food as chrysolaminarin and oils instead of starch.
Dinoflagellates
Dinoflagellates are unicellular protists; many are photosynthetic and possess two flagella for spinning movement. Their cells are covered by cellulose plates called a theca. Many are photosynthetic, but some are heterotrophic, known for bioluminescence and harmful algal blooms (red tides).