Surely when we have gone to bathe well in a lake or the sea we have wondered what kind of animals and plants can live there. Some, although elusive, we can see with the naked eye, such as fish or crabs. However, if we could look at the water with a microscope we could see a whole universe of beings floating in it, the plankton.
What is plankton?
Victor Hensen was the first scientist to use the term plankton in 1887 to refer to the set of organisms that floated at the mercy of the movements of the sea. So he chose a word that described them so aptly, since plankton means “wanderer” or “wanderer.”
This group of organisms is very numerous and diverse and inhabits both fresh and marine waters. It is most representative in the oceans reaching quantities of trillions and can increase in the coldest seas. However, in freshwater ecosystems they are usually found in lentic systems such as lakes, ponds or reservoirs since in areas with currents they would be washed away.
Types of plankton
Plankton can classify in several ways. According to their diet , these types of plankton are distinguished :
- Phytoplankton : It is a plankton of a plant nature and, like plants, they obtain energy and organic matter by carrying out photosynthesis. It lives in the photic layer, that is, the area that receives sunlight, and can reach up to 200 m in the ocean. It is made up of cyanobacteria, diatoms, and dinoflagellates. We recommend you consult this other post by Green Ecology to learn more about what is phytoplankton .
- Zooplankton : It is a plankton of an animal nature. It feeds on phytoplankton and other zooplankton organisms. It is made up of crustaceans, jellyfish, fish larvae, and other organisms. Zooplankton organisms can be differentiated according to the time of their life that they belong to plankton. Holoplanktonic organisms are part of plankton throughout their lives, whereas meroplantonic organisms do so only during one stage, which is normally the larval stage.
- Bacterioplankton : Formed by bacterial communities. They are responsible for the decomposition of detritus and play a key role in the biogeochemical cycles of some elements (C, N, O, P), climate and food chains.
- Virioplankton : Formed by aquatic viruses. Composed mainly of bacteriophage viruses and eukaryotic algae. They participate in the remineralization of nutrients, in biogeochemical cycles and are part of the trophic networks of plankton.
Most plankton organisms are microscopic in size and therefore the unit of measurement used is the micron (one thousandth of a millimeter). The average size ranges between 60 microns and a millimeter. In this sense, the different types of plankton that exist are:
- Ultraplankton : 5 microns. Bacteria and small flagellates are included.
- Nanoplankton : From 5 to 60 microns. Formed by unicellular microalgae such as coccolithophores and small diatoms.
- Microplankton : From 60 microns to 1 millimeter. Some unicellular microalgae (diatoms, dinoflagellates), mollusk larvae and copepods (small crustaceans).
- Mesoplankton : 1 to 5 millimeters. Fish larvae.
- Macroplankton : Between 5 mm and 10 cm. Sargasso, salps and jellyfish.
- Megaloplankton : More than 10 cm. Jellyfish
In addition, plantonic organisms have different body shapes that respond to the needs of the environment in which they live, such as buoyancy or the viscosity of water. Among the strategies or adaptations that have been promoted to float in the water is to increase the body surface area, incorporate fat droplets in the cytoplasm and shed armor, molts and other structures. However, there are some organisms that have a small swimming capacity.thanks to flagella and other locomotor appendages such as copepods. The viscosity of water changes with temperature, being higher in warm areas and this affects the buoyancy of individuals. Some diatoms have developed cyclomorphosis, that is, the ability to develop different body shapes in summer (long, wide shell with pointed ends) and in winter (short, obtuse shell).
Why is plankton important?
Outstanding among the many ecological functions of plankton are that of being at the base of the trophic chain , being part of biogeochemical cycles and regulating the climate . In addition, it is very useful for humans due to its economic and environmental importance. Here is how important plankton is to other living things and to the planet in general:
Plankton is a community of organisms where food webs are established between producers, consumers and decomposers. Phytoplankton, by photosynthesis, transform solar energy into energy available to consumers, mainly zooplankton. Within zooplankton there are herbivores that feed on phytoplankton, carnivores that feed on other species of zooplankton, and omnivores that have a mixed diet. Some of which establish mutualistic relationships and there are others that are parasites. In turn, plankton is consumed by fish, mammals, crustaceans, birds and other animalsTherefore, plankton, as a whole, is located at the base of the trophic pyramid of marine, freshwater and even terrestrial ecosystems, as they are the food of many waders.
On the other hand, both the remains of the dying organisms and the excrement of zooplankton and the rest of the animals need to be decomposed. This task is what bacterioplankton performs and scientists have called this process the microbial loop. Bacteria decompose dissolved organic matter in the environment and also remineralize inorganic compounds, providing the environment with nutrients that are used by phytoplankton to grow and photosynthesize. New research has included virioplankton in the trophic chains, as they attack phytoplankton, zooplankton and bacterioplankton, releasing more organic and inorganic matter to the environment so that it can be reused and therefore feeding back this circle .
Bioindicators of waters
A bioindicator is a living organism that is used to know and determine, in this case, the quality of the water and its level of contamination. Plankton organisms are very sensitive to any change in the environment and therefore can provide us with information on its state of conservation . Some cladocerous crustaceans of the genus Daphnia, popularly known as water fleas, are used in toxicology studies to study the effect that certain contaminants can have on organisms and the environment. Some pollutants can bioaccumulate and in other cases cause the loss of the species by destructuring the trophic chains as they are the basis of food for other organisms.
Bacterioplankton plays a fundamental role in most of the biogeochemical cycles of the aquatic environment; especially in the carbon, nitrogen and sulfur cycles. Without these transformations, life would cease to exist on Earth as all those elements would not be available to plants and animals. This ability to regulate biogeochemical cycles is especially important in oceans where there is a greater volume of water and bacteria. It is estimated that there are around 10 29 bacteria in the oceans and in areas where light shines, about 500,000 bacteria can be found per cubic centimeter of water.
Microorganisms have internal machinery that enables them to transform organic matter into inorganic and vice versa using different sources of energy. Bacteria can use atmospheric nitrogen for their growth, and they also obtain energy from gases such as carbon monoxide (CO), hydrogen (H2) or hydrogen sulfide (H2S) and heavy metals such as iron or manganese.
In addition, photosynthetic bacteria and cyanobacteria (unicellular algae) fix CO2 and contribute almost 50% of oxygen to the atmosphere, reducing the greenhouse effect.
On the other hand, when plankton dies it falls to the bottom of the ocean and creates a layer of sediment that once fossilized after thousands of years gives rise to oil.
Plankton also has the ability to regulate the climate locally on coasts and seas. It is produced during one of the phases of the sulfur cycle, that of DMS (the acronym for dimethyl sulfide). The DMS is responsible for the well-known “smell of the sea”. DMS appears when DMSP, one of the simple organic compounds that we can find in the most abundance in the ocean, breaks down. Phytoplankton synthesize and accumulate DMSP in their cells to counteract the effect of the salt in seawater, preventing dehydration. Thus, the algae release it into the sea when they die and break apart or when they are ingested by zooplankton. Bacteria also use it for carbon and energy and then release DMS, which escapes into the atmosphere.
In the atmosphere, DMS is oxidized by ultraviolet radiation, forming sulfate aerosols that condense moisture into clouds . Since clouds limit the amount of radiation that reaches the earth’s surface, they cause a decrease in temperature , therefore, DMS reduces the greenhouse effect.
It is a delicate process since by increasing the density of the clouds, the amount of ultraviolet radiation that reaches the sea surface where the phytoplankton is found is reduced and it stops producing DMSP.
In conclusion, we should not underestimate the importance of the organisms that inhabit the Earth, since these tiny beings have shown that they have great power over the processes that govern this planet, which makes it necessary to deepen their knowledge and avoid their loss.