By Mitchell Land, Updated Aug 30, 2022
Protozoa and algae are two major divisions of protists—organisms that form a significant portion of plankton. While protozoa exhibit animal‑like behaviors, algae resemble plants. Both possess a true nucleus and require moisture, yet they are evolutionarily distinct.
Algae belong to the Kingdom Protista and range from unicellular to multicellular and colonial forms. They are eukaryotic, meaning each cell contains a membrane‑bound nucleus that stores genetic information. Most algae possess chloroplasts—organelles that carry out photosynthesis via the Calvin cycle, converting light energy into chemical energy. They thrive in moist environments, including freshwater, saltwater, soil, and on rock surfaces, but they lack true roots, leaves, or stems.
According to AlgaeBase, 127,203 species have been documented as of March 2011. The scientific community classifies algae into seven groups based on chemical processes, pigmentation, anatomy, and behavior. These include yellow‑green (colonial freshwater species), brown algae (commonly known as seaweed), green and red algae (ranging from microscopic to macroscopic), and unicellular types such as euglenoids, golden‑brown algae, and fire algae.
The term “protozoa” was widely used in the 20th century but is now considered obsolete because the organisms grouped under this label are not closely related. Protozoa are eukaryotic and unicellular, lack chloroplasts, and therefore show no coloration. They can reproduce both sexually and asexually, and they acquire nutrients by ingesting or absorbing substances from their surroundings.
Protozoa are traditionally divided into four phyla: Sarcodina, Mastigophora, Ciliophora, and Sporozoa. Sarcodina includes amoebae and related species that move by extending pseudopods to engulf food. Ciliophora are propelled by hair‑like cilia, while Mastigophora use flagella for locomotion. Sporozoa are predominantly parasitic and can infect hosts through contact with contaminated soil or water; some species reach up to 16 mm in length and can be lethal to humans.
Protozoa and algae play essential roles in aquatic ecosystems and provide insight into early eukaryotic evolution.
