UNICELLULAR ALGAE: CHARACTERISTICS AND EXAMPLES OF SPECIES - BIOLOGY - 2025

The unicellular algae or microalgae are microscopic, unicellular eukaryotes and ability to perform photosynthesis. These organisms are practically omnipresent in fresh water bodies, marine, humid terrestrial environments, among others.

They are primary producers, so they are fundamental in the trophic networks of aquatic environments. Their ability to photosynthesize and their wide distribution make unicellular algae one of the most important groups of plants in the production of oxygen on the planet.

Unicellular alga, Chaetoceros affinis. Taken and edited from: Minami Himemiya.

Single-celled algae have been exploited by humans for hundreds of years. Currently they are used as food in the cultivation of many aquatic species of commercial interest, in the pharmaceutical and food industries, as environmental bioremediators and many others.

characteristics

Forms and organization

They are eukaryotic, unicellular organisms, but they can form filaments or colonies. They come in an incredible variety of shapes. Their size varies according to the species, but they can measure from about 5 to 50 micrometers on average, however some can measure several hundred micrometers.

The cell wall when present is complex. There are flagellate forms, usually they have 2 flagella.

Feeding

They are mainly photosynthetic organisms (autotrophs), that is, they use sunlight to transform inorganic compounds into usable organic matter for organisms.

Some species of unicellular algae such as dinoflagellates are heterotrophic, which implies that they depend on other organisms for their food, being predators of other microalgae and microcrustaceans. Parasitic life forms also exist.

Another food condition of some unicellular algae is myxotrophy; This is presented by organisms that are capable of obtaining their food by photosynthesis or heterotrophly.

This characteristic has been observed in some species of dinoflagellates, which, depending on environmental conditions and food availability, use one or another form of food.

Pigments

Most present chloroplasts with chlorophylls a and c, some groups present chlorophylls a and b. Other pigments associated with unicellular algae are beta carotenes, phycobilins, and xanthophylls.

Associations

They can be found associated in symbiosis with fungi (lichens and mycorrhizae), with corals, mollusks, insects and even within salamanders. Some microalgae such as dinoflagellates can become parasites.

Distribution and habitat

Unicellular algae are cosmopolitan organisms, they inhabit fresh, marine, estuarine water bodies, humid terrestrial environments and even underground.

They are in the water column as part of plankton, in sediments, forming a viscous film, or on rocks, algae, aquatic plants, mollusk shells, shells of large crustaceans, turtles and other organisms.

They are found in the photic zone, that is, as far as the sunlight penetrates. Depending on the species, the geographical area, the incidence of light, the transparency of the water and even the availability of nutrients, they can live at different depths and temperatures.

Zoxantella (Symbiodinium sp.), Symbiont dinoflagellate of Cnidarians and other invertebrates. Taken and edited from: Allisonmlewis.

Reproduction

Unicellular algae have two types of reproduction:

Asexual

Where the progeny or descendants inherit the genetic load from the only parent. The main forms of asexual reproduction present in these organisms are binary fission (producing two daughter cells) and multiple fission (producing more than two daughter cells).

Sexual

In this type of reproduction, the progeny obtain the genetic material from the combination of two organisms (progenitors). In this process, meiotic cell divisions generally occur.

During meiosis, the diploid cell divides successively (generally two), producing in most cases 4 haploid cells, although there may be 2. Diploidy recovers with the fusion of two gametes.

Taxonomy

The unicellular algae or microalgae have been considered for many years as a large group that encompasses both prokaryotic organisms (cyanobacteria or blue-green algae) and eukaryotes (true microalgae).

True single-celled algae are eukaryotic organisms of the Chromista and Plantae kingdoms. The Chromista taxon was proposed in 1981 by Thomas Cavalier-Smith, in his work entitled Eukaryotic Kingdoms. Seven or nine? While the taxon Plantae was proposed by Erns Haeckel, in the year 1866.

The Chromista and Plantae taxa are composed not only of unicellular algae, but also of multicellular organisms. The Chromista group is made up of 3 sub-kingdoms, several superphyla and phyla, and more than 30 thousand described species.

On the other hand, Plantae is composed of 2 sub-kingdoms and several phyla, of which the unicellular algae are found in the Viridiplantae sub-kingdom (a taxon also proposed by Thomas Cavalier-Smith).

Ecological importance

Unicellular algae are classified as essential organisms to sustain life on the planet. It is estimated that they produce about 90% of the photosynthesis of the planet and therefore a large part of the oxygen.

They are fast growing and reproducing organisms, being able to have densities of millions of cells per liter. This biomass production capacity makes them very important as primary producers, that is, they are the main producers of organic matter that enters the trophic webs of almost all bodies of water.

Single-celled algae capture some of the excess carbon dioxide and transform it into oxygen. Because of this, microalgae groups that tend to have a large biomass production capacity are essential to counteract the greenhouse effect on the planet.

Some species of algae can exhibit explosive growth phases, known as algal blooms or phytoplankton blooms. When the species involved in this growth phase are capable of producing toxins, then harmful algal blooms or red tides occur.

Examples of species

Chaetoceros gracilis

Chromista unicellular algae species used in aquaculture as food for larvae of species of commercial interest. Most of the species of this genus are used in aquaculture.

Dunadiella salina

A microalgae species from the kingdom Plantae and sub-kingdom Viridiplantae. This species is currently being used to obtain biodiesel, from the extraction of oils in a process called transesterification.

Symbiodinium microadriaticum

It is one of the species of dinoflagellate unicellular algae (Chromista Kingdom), also called zooxanthellae. It lives in association with a wide variety of coral species. Thanks to this association, the coral receives nutrients from the photosynthetic processes carried out by the algae; this for its part receives mainly protection.

Corals form reefs that are home to countless species of microorganisms, invertebrates and vertebrates.

Pyrodinium bahamense

It is a dinoflagellate, whose blooms are harmful both for primary consumers (fish, crustaceans and mollusks), and for the organisms that feed on them (humans and other animals).

Electron microscopy image of the dinoflagellate Pyrodinium bahamense. Taken and edited from: CSIRO.

Gymnodinium catenatum

Another species of dinoflagellate capable of producing harmful algal blooms. It is the only species of dinoflagellate devoid of theca that can produce the toxins that cause paralyzing mollusk poisoning.

References

1. ZC Romero. Systematics of algae. Primitive monadoids. Recovered from personal.us.es.
2. T. Cavalier-Smith (1981). Eukaryote kingdoms: seven or nine ?.
3. WoRMS Editorial Board (2019). World Register of Marine Species. Recovered from marinespecies.org.
4. M. Cerón García (2013). Production of microalgae with nutritional applications for humans and animals. Agri-food study notebooks.
5. A. Medina Jasso, P. Piña Valdez, M. Nieves Soto, JF Arzola González & M. Guerrero Ibarra (2012). The importance of microalgae. Biodiversitas.
6. P. Coutteau. Micro-algae. FAO. Recovered from fao.org.
7. C. Lyre. Gymnodinium catenatum. In lifeder. Recovered from lifeder.com