Dictionary > Microalgae


Microalgae are microscopic algae. They are single-celled algae that may exist independently or in colonies. They are comprised of the unicellular algal species, e.g. green algae, diatoms, and dinoflagellates.

Microalgae definition

Microalgae (singular: microalga) is defined as any of the microscopic algal species.


The term microalgae came from micro-, meaning “small” and‎ alga(e). Synonyms: microphytes; phytoplankton.

Microalgae vs. phytoplankton

Phytoplankton refers to a group of planktons that are photosynthetic. Most of them are the microscopic algal species or the microalgae. Thus, microalgae are members of the larger group of phytoplankton. However, there are references wherein microalgae and phytoplankton are considered synonymous. They are important in the ecosystem as they serve as the primary producers of an aquatic food chain. When microalgae grow profusely, algal bloom results. When this happens, they become perceptible especially due to the pigments present in every cell. An algal bloom caused by phytoplankton is referred to as phytoplankton bloom.

Microalgae vs. macroalgae

Microalgae and macroalgae are the two major types of algae based on cellularity. Microalgae are unicellular algal species that may either live singly or in colonies. Macroalgae are multicellular algal species. They are commonly called seaweeds because they can grow profusely at any time. Microalgae include the dinoflagellates, the diatoms, and other single-celled algal species. Macroalgae include macroscopic red algae, brown algae, and green algae. Both microalgae and macroalgae are important contributors of atmospheric oxygen through photosynthesis. They serve as a food source for many aquatic habitats. They do not have true stems, leaves, and roots. A true stem, leaves and roots would have a vascular system as found in higher plants. Macroalgae, though, are similarly multicellular and the cells may function together, forming an organ. The macroalgal thallus is comprised of the following major parts: (1) lamina, (2) stipe, and (3) holdfast and haptera. The lamina (also called blade) is the leaf-like structure, the stipe is the stem-like structure, and the holdfast is the root-like structure of the macroalgae. A special organ on the blade is called pneumatocyst or air bladder may be present. It helps macroalgae to stay afloat. Another floatation-assisting organ is float. It is located between the lamina and the stipe.


Microalgae are microscopic, single-celled, and mostly photosynthetic. Although they are single-celled, some of them can form colonies, such as filaments or spheres with the same species. Their ability to photosynthesize is due to the presence of photosynthetic pigments. The color of the algae is influenced by the predominant pigments in an algal cell. Thus, they are grouped according to their color: green, red, or brown. Some of them though have diverse modes of nutrition. Those that lack the photosynthetic pigments are heterotrophs and therefore feed on other organisms. Others are a mix; they are at times photosynthetic and at other times, heterotrophic. They are referred to as mixotrophs.


Photosynthetic microalgae contribute a huge percentage of atmospheric oxygen. Some of them are also capable of fixing nitrogen. Others form symbioses with other species, such as fungi in lichens. When the conditions are conducive for algal growth, they could grow at an exponential rate and cause algal blooms, e.g. red tide. With the increased algal population is the increased production of toxin. Algal toxin, when consumed, can lead to fish kills.

See also

References and Further readings

  1. Khan, M. I., Shin, J. H., & Kim, J. D. (2018). The promising future of microalgae: current status, challenges, and optimization of a sustainable and renewable industry for biofuels, feed, and other products. Microbial Cell Factories, 17(1). https://doi.org/10.1186/s12934-018-0879-x
  2. 10.2 What are Algae? EGEE 439: Alternative Fuels from Biomass Sources. (2018). Retrieved December 15, 2019, from Psu.edu website: https://www.e-education.psu.edu/egee439/node/693

© Biology Online. Content provided and moderated by Biology Online Editors