Phytoplankton

We explain what phytoplankton is and how it feeds. Also, why it is so important and what differentiates it from zooplankton.

What is phytoplankton?

Phytoplankton is the group of microscopic and photosynthetic organisms that inhabit the planet's fresh and salt waters. They are free-living, which means that they do not remain attached to any substrate, but rather float in the water.

These autotrophic microorganisms proliferate in the superficial regions of bodies of water, where they have access to sunlight and the mineral salts they need. Most live between 10 and 45 meters deep although this may be reduced in cloudier waters that allow less sunlight to pass through (as occurs in some streams, reservoirs and lagoons).

Phytoplankton is composed of photoautotrophic bacteria (also called cyanobacteria or blue-green algae) and microscopic unicellular or colonial algae. Diatoms, dinoflagellates and coccolithophores are the most abundant phytoplankton algae.

All of these microscopic organisms generate much of the oxygen on the planet as a result of photosynthesis. In fact, cyanobacteria were responsible for the increase in oxygen in the atmosphere during the geological period called The Great Oxidation, approximately 2.4 billion years ago.

Phytoplankton is one of the two components of plankton. Plankton is the enormously diverse set of microscopic (or small) organisms that live suspended in water. The other component of plankton is zooplankton, made up of small heterotrophic organisms: protozoa, tiny crustaceans, fish larvae, aquatic worms, etc.

In recent times, phytoplankton levels in rivers, lakes and oceans have declined alarmingly, primarily due to increased levels of ultraviolet radiation filtering through the atmosphere. In regions less protected by the ozone layer, plankton productivity has declined by between 6 and 12%.

See also: Aquatic ecosystem

What do phytoplankton feed on?

The organisms that form phytoplankton are autotrophs, that is, make or synthesize their own food from inorganic substances (carbon dioxide and water), just like plants do.

To carry out photosynthesis they need sunlight as a source of energy. Therefore, the penetration of light into the water limits the depth at which phytoplankton can live.

During photosynthesis, phytoplankton organisms release oxygen. Some remains as oxygen dissolved in the water, while another part ends up in the air as molecular oxygen (O₂). In fact, marine phytoplankton is responsible for producing 50% of the oxygen in the Earth's atmosphere.

Cyanobacteria and microscopic algae also need phosphorus and nitrogen to synthesize proteins, fats and other substances. These elements are found as salts dissolved in water, which are produced by decomposing bacteria.

Examples of phytoplanktonic organisms

Phytoplankton comprises two well-differentiated groups of photosynthetic microorganisms. On the one hand there are cyanobacteria, which are prokaryotic unicellular beings. On the other hand, there are microscopic algae (microalgae), which are unicellular eukaryotic beings. Both microorganisms can form colonies or filaments.

Some phytoplanktonic organisms are:

Importance of phytoplankton

Phytoplankton is the basis of all food chains aquatic s (especially marine ones), just as plants are the basis of terrestrial food chains. They occupy the first link of the food pyramids.

These producer microorganisms constitute the food for zooplankton and many other larger aquatic animals (cnidarians, mollusks, crustaceans, fish, etc.).

Humans benefit from the productivity of aquatic food chains through fishing, which would not be possible without phytoplankton. On the other hand, people have used different phytoplankton species to make food supplements and antioxidants. Other species are also used to produce biofuels and to bioremediate environments.

On a global scale, phytoplankton is responsible for the largest share of oxygenation on the planet since it produces as much or more molecular oxygen than plants. This means that without phytoplankton, oxygen levels in water and air would decrease, and the world would become a less suitable place for life.

The increase in surface temperature of seas and oceans has caused the reduction of phytoplankton during the last century. In contrast, the dumping of pollutants (both in the sea and in freshwater bodies) generates the proliferation of some cyanobacteria and algae that can be toxic to marine life and humans. Therefore, phytoplankton species constitute important environmental indicators.

Phytoplankton and zooplankton

Plankton comprises all microscopic organisms that live freely in fresh or salt water. Some of these organisms are autotrophs and are grouped under the name “phytoplankton”, such as cyanobacteria and microalgae. Others are heterotrophs and are collectively called “zooplankton.”

Zooplankton are made up of protozoa (organisms unicellular heterotrophs) and tiny animals These include rotifers, some crustaceans (such as copepods, cladocerans and krill) and the larvae of some fish and invertebrate animals (poriferans, cnidarians, echinoderms and mollusks). All of them feed on bacteria or phytoplankton.

Within zooplankton, we can differentiate between holoplankton (organisms that are part of plankton throughout their lives) and meroplankton (organisms that only make up plankton during the larval stage). This distinction is not made for phytoplankton since their organisms are always part of the plankton.

Both phytoplankton and zooplankton live in the area of ​​the water body that receives sunlight. Although zooplankton can live in slightly deeper regions, the illuminated region is where they get their food.

Continue with: Biotic factors

References

• Curtis, H., Barnes, N.S., Schnek, A., & Massarini, A. (2008). Biology. (7th ed.). Panamericana Medical Editorial.
• Nabors, M. W. (2006). Introduction to botany. Pearson Education.
• Rushforth, SR, Robbins, RR, Crawley, JL, & Van de Graaff, KM (2008). A photographic atlas for the botany laboratory. (5th ed.) Morton Publishing Company.
• Oliva Martínez, MG, Godínez Ortega, JL and Zuñiga Ramos, CA (2014). Biodiversity of phytoplankton of continental waters in Mexico. Mexican Biodiversity Magazine85, 54-61. Doi.org
• López, R. (January 7, 2019). Phytoplankton, basic for life on Earth. UNAM Gazette. Gazette
• UNAM Foundation. (February 11, 2019). The importance of phytoplankton. Unam Foundation
• Aquae Foundation. (September 16, 2022). The plankton that feeds the world. FoundationAquae
• National Ocean Service. (January 20, 2023). What are phytoplankton? National Oceanic and Atmospheric Administration. OceanService
• Britannica, T. Editors of Encyclopaedia (March 6, 2023). Encyclopedia Britannica. Britannica
• Institute of Marine Sciences. (sf). The sea in depth. The plankton. Teaching guide. Higher Council for Scientific Research. Elmarafons
• Cifuentes Lemus, JL, Torres García, P and Frías Mondragón, M. (1997). The environmental picture of phytoplankton. In The ocean and its resources V.: Plankton. Economic Culture Fund. Digital Library
• Gough, E. (May 8, 2020). New data show how phytoplankton pumps carbon out of the atmosphere at an enormous scale. universe today