Difference Between Algae and Protozoa

Main Difference

The main difference between algae and protozoa is that algae are autotrophic, plant-like organisms whereas protozoa are heterotrophic, animal-like organisms. Algae can produce their food by the process of photosynthesis while protozoa ingest organic molecules by phagocytosis.

Algae vs. Protozoa

In several ways, algae and protozoa are similar. They belong to the same kingdom Protista. Algae and Protozoa are both eukaryotic organisms which mean that they have a membrane-bound nucleus and some other basic cellular organelles. However, their way of obtaining food, as all organisms must, is very different and is the essential difference between these two types of organisms. We can also differentiate them instantly based on their looks or structure. Algae can be found in different colors, shapes, and sizes. Most algae have a green color and slimy appearance. All algae have chlorophyll and other pigments in the chloroplast though they do not have leaves, stems, and roots. On the flip side, protozoans are unicellular, and they are animal-like organisms. We can describe protozoa as a blob without a definite shape because they lack a cell wall. Protozoa are motile and heterotrophic unicellular organisms (such as amoebas, trypanosomes, sporozoans, and paramecia) that are present in almost every kind of habitat and include some pathogenic parasites of humans and other domestic animals

Comparison Chart

Algae are simple, non-flowering, and typically aquatic plant. Algae contain chlorophyll but lack true stems, roots, leaves, and vascular tissue.Protozoa are single-celled microscopic organisms which feed on organic matter and include amoebas, flagellates, and ciliates.
Methods of Obtaining Energy
Algae are autotrophsProtozoa are heterotrophs
Algae are plant-like organisms.Protozoa are animal-like organisms.
Algae contain chlorophyll.Protozoa do not contain chlorophyll.
Cell Wall
Algae have a cell wall which is made up of cellulose.Protozoa do not have a cell wall.
Resting Stage
The resting stage of algae is the spore.The resting stage of protozoa is the cyst.
Algae can produce 70% of the oxygen in the atmosphere by photosynthesis.Protozoa are the foundation of most aquatic food chains.
In Humans
Some Algae may produce toxins, which are toxic to humans.Protozoa can cause diseases in humans; e.g., malaria is caused by Plasmodium.
Seaweeds, green algae, red algae, brown algae, and cyanobacteria are major examples of algae.Amoeba, Plasmodium, Euglena and Paramecium are most familiar examples of protozoa.

What is Algae?

Algae are small, non-flowering, aquatic plants, which contain chlorophyll and other pigments but lack a true stem, root, leaves, and vascular tissues. Algae occur most frequently in water. Also, algae may occur in moist soil or on the surface of moist rocks and wood. Algae live with fungi in lichens. Some algae are single-celled, and they are microscopic. They are called microalgae which include cyanobacteria (blue-green algae) and other green, red, and brown algae. Other algae are multicellular, which are called as macroalgae. Seaweeds are macroalgae such as kelp, which can grow about a hundred feet in length. Most of the microalgae are autotrophic, and they can produce their food through photosynthesis.

Algae can produce 70% of the oxygen in the atmosphere through the process of photosynthesis. Algae can reproduce by both asexual and sexual mode of reproduction. Asexual reproduction occurs by the process of fragmentation of colonial and filamentous algae or by spore formation (as in fungi). Spore formation takes place by mitosis. Binary fission also takes place. In sexual reproduction, algae form differentiated sex cells which fuse to produce a diploid zygote with two sets of chromosomes. The zygote then develops into a sexual spore, which is a resting stage of algae and it germinates when the conditions are favorable to reproduce and reform the haploid organism which has a single set of chromosomes. This process of reproduction is called alternation of generations.

What is Protozoa?

Protozoa refer to single-celled and microscopic animals that belong to the kingdom Protista. Protozoa are located in most moist habitats. Free-living species are found in freshwater and marine environments, and terrestrial species inhabit decaying organic matter. Some species of protozoa are parasites of plants and animals. Protozoa vary considerably in size and shape. Smaller species of protozoa may be the size of fungal cells; however larger species can be seen by the naked eye. Protozoal cells have no cell walls, and so they can assume an unlimited variety of shapes and forms. Some genera have cells which are surrounded by hard shells, while the cells of other genera are enclosed only in a cell membrane. Protozoa are important for their ability to move independently, a characteristic found in the majority of species. They usually lack the capability for photosynthesis. Though the majority of protozoa reproduce by asexual methods, sexual reproduction has been seen in several species.

A large number of protozoal species are aerobic, but some anaerobic species have been found in the human intestine and animal rumen. Most protozoa alternate between a free-living vegetative form which is known as a trophozoite and a resting form called a cyst. The protozoal cyst is to some extent analogous to the bacterial spore because it can resist harsh conditions in the environment. Most of the protozoal parasites are taken into the body in the cyst form. Protozoa are heterotrophic microorganisms, and most species feed on large food particles by phagocytosis. Many protozoan species move independently by one of three types of locomotory organs: flagella, cilia, and pseudopodia.

Key Differences

  1. Algae are plant-like organisms whereas Protozoa are animal-like organisms.
  2. Algae contain chlorophyll while Protozoa do not have chlorophyll.
  3. Algae are autotrophs on the other hand; protozoa are heterotrophs.
  4. Algae have a cell wall which is made up of cellulose conversely protozoa do not have a cell wall.
  5. Algae have a resting stage called spore on the flip side; protozoa have a resting stage called cyst.


Algae are plant-like organisms mainly aquatic and chlorophyll-containing nonvascular organisms. Algae can produce their food through photosynthesis whereas Protozoa are motile and heterotrophic unicellular protists that are present in almost every kind of habitat and contain some pathogenic parasites of domestic animals and humans.