Symbiotic Relationships: Cnidarians, Fungi, and Protozoa

Symbiosis means different species living closely together. The partners do not always gain equally. This section sorts the types before the exam examples.

1.1 What does the word symbiosis mean?

De Bary defined symbiosis in 1879 as the living together of unlike organisms.

Researchers today say symbiotic associations form a gradient. The gradient includes mutualistic, commensal and even parasitic relationships. These associations can also shift when conditions change.

So the word only says that the partners live together.

1.2 How do mutualism, commensalism and parasitism differ?

The Class 12 biology book separates the three types by who gains.

• Mutualism. Both species benefit.
• Commensalism. One species benefits. The other is neither harmed nor benefited.
• Parasitism. One species benefits. The other, the host, is harmed.

The same book gives these examples.

• Mutualism: lichens, and mycorrhizae.
• Commensalism: an orchid growing as an epiphyte on a mango branch, and barnacles on the back of a whale. The cattle egret with grazing cattle is also called a classic example.
• Parasitism: lice on humans, ticks on dogs, and Cuscuta. Cuscuta is a parasitic plant that has lost its chlorophyll and leaves.

The egret example shows how commensalism works. Egrets forage close to grazing cattle. The cattle stir up and flush out insects that are hard for the egrets to find.

Symbiosis covers all three types. Mutualism is only one of them.

The exam asks whether cnidarians, fungi and protozoa all have symbiotic species. The answer is yes for all three.

2.1 How do cnidarians take part?

Corals and sea anemones are cnidarians. Most reef-building corals contain photosynthetic cells called zooxanthellae. These cells live in the coral’s tissues.

Sea anemones also partner with algae of the family Symbiodiniaceae. The algae sit inside the animal’s tissue. They make sugars by photosynthesis. The sugars support the host’s growth and reproduction.

2.2 How do fungi take part?

Fungi form two well-known partnerships.

• Lichens. A fungus lives with photosynthesising algae or cyanobacteria.
• Mycorrhizae. Fungi live with the roots of higher plants.

2.3 How do protozoa take part?

Some protozoa live in the gut of termites. Lemuel Cleveland reported in the 1920s that termites need them to live on wood. Section 4 tells that story.

Reef-building corals have algae called zooxanthellae living in their tissues. NOAA calls this a mutualistic relationship.

3.1 What does each partner give?

NOAA calls the coral and zooxanthellae partnership mutualistic. Each side gives something.

• The coral gives the algae a protected environment.
• The coral’s polyp cells make carbon dioxide and water. The algae need both for photosynthesis.
• The algae supply the coral with the building blocks of sugars and proteins.
• The coral uses these products to make proteins, fats and carbohydrates. It also uses them to produce calcium carbonate.

3.2 How much of the algae’s food reaches the coral?

As much as 90 percent of the organic material that the algae make by photosynthesis is transferred to the coral tissue. NOAA calls this the driving force behind the growth and productivity of coral reefs.

The algae also give many stony corals their colours.

3.3 What is coral bleaching?

Sometimes conditions change. NOAA lists changes in temperature, light or nutrients as stress factors. Stressed corals expel the algae living in their tissues. The coral then turns completely white. This is coral bleaching.

NOAA says a coral that bleaches is not dead. Corals can survive a bleaching event, but they are under more stress. If the algae loss is prolonged and the stress continues, the coral eventually dies.

Why can a long bleaching be fatal? Bleaching cuts the sugars that reach the host. If the cnidarian stays bleached for a prolonged period, starvation can result in death.

3.4 How big was the fourth global bleaching event?

NOAA confirmed on 15 April 2024 that the world was in its fourth global coral bleaching event.

• From 1 January 2023 to 30 September 2025, bleaching-level heat stress affected about 84.4% of the world’s coral reef area.
• The previous record was 68.2%, in the third event of 2014 to 2017.
• Mass coral bleaching was documented in at least 83 countries and territories.

The figure counts reef area under bleaching-level heat stress. NOAA says the event likely ended in 2025.

How do scientists know termites truly depend on their gut protozoa? Lemuel Cleveland tested the idea in the 1920s. His story also shows how science checks old results.

4.1 What did Cleveland find?

Cleveland did this work as a fellow at the Johns Hopkins University School of Hygiene. His papers came out between 1923 and 1928. He worked on termites and the protozoa in their guts.

• He removed the protozoa. His methods included incubation at 35 C, exposure to pure oxygen, and starvation.
• The termites still had their gut bacteria and spirochetes.
• On a diet of only wood or cellulose, the termites without protozoa died of starvation.
• Termites that were reinfected with the protozoa were saved.

The National Academy of Sciences memoir calls this a first. It was the first instance in which a mutualistic relationship between internal microorganisms and their animal host was clearly proved.

4.2 Did later work change the picture?

Partly, yes. Later researchers checked Cleveland’s experiment again.

• Two groups said it was not enough to prove the partnership. They noted that the treatments could have damaged the termites.
• Another study found that termites still broke down labelled cellulose after the protozoa were removed. Its authors suggested that termites have their own cellulose-digesting activity.

A 2003 study describes lower termites this way. Cellulose seems to be broken down by flagellates, bacteria and yeasts together. The termite’s own cellulases also help.

So the protozoa are important partners. They are not the only digesters.

Lichens were long taught as a two-way deal. One fungus joined one photosynthetic partner. A 2016 study changed that picture.

5.1 Who are the two classic partners?

The Swiss botanist Simon Schwendener studied lichens. He was the first scientist to propose that some organisms are combinations of unrelated species.

He saw that a lichen is a fungus plus a photosynthetic partner. The partner is an alga or a cyanobacterium.

• The alga or cyanobacterium makes food. It turns sunlight and carbon dioxide into sugars.
• The fungus forms the main structure of the lichen.
• The fungus also protects its partner from the environment.

5.2 What did the 2016 study find?

A study led by the University of Montana appeared in Science on 21 July 2016. It found a third partner embedded in the lichen cortex, or “skin”. The partner is a yeast.

• The yeasts are basidiomycetes. They are single-celled fungi.
• They were found in lichens across six continents.
• Related yeast lineages turned up in 52 genera of lichens worldwide.

The yeasts likely make chemicals that help lichens ward off predators and repel microbes.

5.3 Why did the yeast matter?

The work began with a puzzle. Two lichens seemed to share the same fungus and alga.

• Bryoria tortuosa is yellow and makes a toxic substance called vulpinic acid.
• B. fremontii is dark brown and makes none.

The yeast could explain such differences. It could also explain why scientists have been unable to build lichens in the laboratory. They failed even when they combined species that partner successfully in nature.

Mycorrhizae are the second fungal partnership in the exam list.

6.1 What does each side gain?

The fungus helps the plant absorb essential nutrients from the soil. The plant gives the fungus energy-yielding carbohydrates.

Members of the genus Glomus show this well. The fungal partner absorbs phosphorus from the soil and passes it to the plant. Such plants also show other benefits.

• Resistance to root-borne pathogens.
• Tolerance to salinity and drought.
• An overall increase in plant growth and development.

6.2 How common is this partnership?

A 2018 review in New Phytologist found that the majority of vascular plants are mycorrhizal. About 72% form arbuscular mycorrhizas. Just 8% are completely non-mycorrhizal.

Cnidarians, fungi and protozoa are not the only groups. Plants run a similar trade with bacteria.

7.1 What happens inside a root nodule?

Clover, alfalfa and other legumes provide a home for bacteria called rhizobia. The bacteria live in nodules. They induce the plant to build these nodules on its roots.

Inside the nodules, the bacteria convert nitrogen from the air into ammonia. The plant can use it as a nutrient. The plant feeds the bacteria with carbon compounds called dicarboxylates.

7.2 Is it the same trade as coral and algae?

The sources describe three exchanges.

• Coral and algae. The coral gives a protected home and carbon dioxide. The algae give sugars and building blocks of proteins.
• Fungus and plant. The plant gives carbohydrates. The fungus helps absorb nutrients such as phosphorus.
• Legume and rhizobia. The plant gives a home and carbon compounds. The bacteria give ammonia made from air nitrogen.

Margulis proposed that some parts of complex cells began as separate cells.

8.1 What did Margulis propose?

In March 1967 Lynn Sagan, later known as Lynn Margulis, published “On the Origin of Mitosing Cells”. It appeared in the Journal of Theoretical Biology.

She proposed that mitochondria, plastids and basal bodies were once free-living cells. They took up residence inside primitive eukaryotic cells. She called this process endosymbiosis.

8.2 Which features fit the idea?

The Class 11 biology book describes these features of the organelles.

• Mitochondria are bound by a double membrane.
• The mitochondrial matrix holds a single circular DNA molecule, a few RNA molecules and 70S ribosomes.
• Mitochondria divide by fission.
• Chloroplasts are also double membrane bound. They contain small, double-stranded circular DNA molecules and ribosomes.

A 2020 review notes that bacteria and mitochondria both contain circular DNA and divide through fission. It adds that the double membrane may act in favour of the endosymbiotic theory.

The same trade shows up in very different bodies. One of these examples is classed differently by different sources.

9.1 Are anemonefish and sea anemones mutualists?

NCERT calls this pair commensalism. The fish gets protection from predators, which stay away from the stinging tentacles. The anemone does not appear to derive any benefit.

A 2020 paper notes that anemonefish give their hosts numerous benefits. These include ammonia from the fish’s waste. So a 2020 paper points to a two-way benefit. For a textbook question, follow the book and answer commensalism.

9.2 How does the bobtail squid use its bacteria?

The Hawaiian bobtail squid hosts the bacterium Vibrio fischeri in a light organ. The bacteria produce light.

• The squid uses the light as camouflage.
• This behaviour is called counterillumination. It disrupts the squid’s silhouette.
• Only V. fischeri colonises the light organ.

The same review says the bacteria switch their metabolism. They use glycerol phosphate respiration in the day and chitin fermentation at night.

9.3 What do acacia trees give their ants?

Some Mesoamerican acacias are “obligate ant plants”. They provide nesting space in hollow thorns. They also give food rewards. The rewards are extrafloral nectar and food bodies. The ants are defending Pseudomyrmex ants.

A study found that acacias giving higher rewards were more effectively defended against herbivores.

Two of the partnerships above show what happens when a partner is lost.

10.1 What happens when a partner is lost?

• Termites. Without gut protozoa, Cleveland’s termites starved on wood.
• Corals. Without their algae, corals bleach. A long bleaching can end in starvation.

UPSC asked this topic as a list question.

11.1 What did UPSC ask in 2021?

UPSC Civil Services Prelims 2021, Q30, asked which of these have species that can establish symbiotic relationships with other organisms: cnidarians, fungi and protozoa. The answer is all three.

11.2 Which traps should you avoid?

• Symbiosis is wider than mutualism.
• A lichen is a fungus with an alga or a cyanobacterium.
• A mycorrhiza is a fungus with the roots of higher plants.
• A bleached coral has lost its algae. It is not dead.
• The textbook calls anemonefish and anemones commensal.