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Microalgae definition and example

n., singular: microalga
Definition: Microscopic photosynthetic aquatic organisms

Microalgae Definition

Microalgae (singular: microalga) are microscopic algal species as opposed to other algae that are macroscopic. Thus, similar to other algal species, microalgae are photosynthetic organisms. They play a pivotal role in ecosystems as primary producers. Through the process of photosynthesis, microalgae make use of sunlight and convert this energy into chemical energy that these organisms use to power up a variety of biological processes. Through photosynthesis, they release oxygen as a byproduct, and thus, they occupy a vital contributor of oxygen in the environment.  Microalgae thrive in various aquatic environments, encompassing both freshwater and marine habitats. These diminutive life forms are often unicellular as some microalgae can form colonies. Their adaptability and diverse taxonomy make them a crucial component of aquatic food webs, supporting various organisms and influencing nutrient cycling. Despite their size, microalgae hold immense ecological significance and have garnered attention for their potential applications in biotechnology and environmental sustainability. Examples of unicellular algal species are green algae, diatoms, and dinoflagellates.

Etymology: The term “microalgae” is derived from the Greek words “mikros” meaning small and “alga” meaning seaweed or plant.
Synonym: microscopic algae; phytoplankton
See also: algae

  • Microalgae vs. phytoplankton

Phytoplankton refers to a group of plankton 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 a 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 dinoflagellates, 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.

Understanding micro-algae (by ENICBCMedProgramm):

Characteristics Of Microalgae

Microalgae exhibit several distinctive characteristics that contribute to their ecological and evolutionary success:

  • Typically unicellular, though some species may form colonies or filaments
  • Come in a variety of shapes, e.g., spheres, rods, spirals
  • Possess pigments, such as chlorophyll a, chlorophyll b, and carotenoid, vital to capturing light energy (photon) for photosynthesis
  • The color of the algae is influenced by the pigments predominant in the cell — a characteristic that is used in classifying species based on their color (e.g., green, red, or brown)
  • While many are photosynthetic, some of them may have various or mixed modes of nutrition. Those algae without photosynthetic pigments are heterotrophs, feeding on other organisms. Others are a mix of both modes — sometimes photosynthetic at other times, heterotrophic, depending on ecological factors. This flexible mode of nutrition is what defines the mixotrophs.
  • Rapid growth rate and adaptability to varying environmental conditions, making them capable of surviving dynamic ecological parameters
  • They are found in aquatic environments, e.g., freshwater and marine habitats


Microalgal Groups and Examples

Microalgae belong to various phyla and examples of microalgal groups are as follows:

  • Chlorophyta (Green Algae): For example, Chlorella, a unicellular green alga widely used in research and commercially for its nutritional content and potential applications in biofuel production
  • Rhodophyta (Red Algae)
  • Heterokontophyta (Brown Algae)
  • Diatoms: microscopic algae with intricate silica cell walls; diatoms are diverse and abundant in both marine and freshwater environments, contributing significantly to phytoplankton communities
  • Dinoflagellates: These unicellular organisms are known for their flagella and are often responsible for bioluminescence in the ocean. Some species are symbiotic with coral reefs, contributing to their vibrant colors.

Cyanophyta (blue-green alga) may be regarded as an algal group although it also belongs to Eubacteria wherein the group is referred to as “cyanobacteria”.

Volvox green algae
Volvox sp. (Photo by Frank Fox, CC BY-SA 3.0)


Watch this video about diatoms:

Watch this video about dinoflagellates:

Ecological Roles

Microalgae play various crucial roles in the ecosystem:

    • The primary contributor of atmospheric oxygen through photosynthesis
    • The primary food source for various aquatic organisms, forming the foundation of the trophic pyramid, sustaining higher trophic levels
    • Nutrient cycling: Some species are capable of fixing nitrogen
    • Form symbiosis with fungi, as in lichens
    • Algal blooms, where an increased algal population could lead to increased toxin production


  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


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