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How Does Life Work?

Symbiosis and Interliving:
Crossing the Kingdoms
Algae & Protozoa

Protists, algae and protozoans, are incredibly ancient, so many of them have evolved symbiotic associations, mostly endosymbiosis (inside the body of the partner). The great majority of protist symbioses are aquatic, but they are important on land as well.



Chlorella algae are symbionts of a sea anemone

Chlorella algae are symbionts of tiny freshwater protozoan Stentor

ciliate protozoans Trichomytopsis are termite gut symbionts. Their own symbiotes are bacteria that digest the cellulose in wood the termites require to live

the flatworm Convoluta has no mouth and no anus. It lives entirely off the photosynthesis of its algal partner Platymonas

The sea slug Elysia is a symbiont of Chlorella alga. It eats a variety of algae, keeps the chloroplasts alive in its tissues, and becomes red, green, or brown depending on the algae.

This Paramecium contains green plastids that can't live on their own, remnants of an ancient symbiosis with a cyanobacterium


This heliozoan "sun" protozoan
is a symbiont of Chlorella green alga
This freshwater Hydra is a symbiont of the green alga Chlorella. It eats the tiny algae, then keeps it alive in its own tissues to get a free lunch.

Zoanthus sociatus is a polyp colony that gets half its food from zooxanthellae protozoans, much like warm water reef corals.

This Trachelomonas euglenid grows a clear shell for protection; its single flagellum emerges from a tube at one end. Its green plastids originated in an ancient symbiosis with cyanobacteria



Euglenas live partly on photosynthesis from chloroplasts that originated in an ancient symbiosis with cyanobacteria. They are called mixotrophs (mixed eaters).

This ocean acoel flatworm lives partly on photosynthesis from its symbiosis partners zooxanthellae, also found in corals
Image credit Chris Loban

Nutrition and Defense:Protists and Bacteria

The toxic dinoflagellate protozoan Ostreopsis lenticularis hosts a variety of bacterial symbionts. These bacteria not only provide nutrients but also create the toxins that poison fish that eat the tiny dinoflagellates. When they have a sudden upwelling of nutrients, their numbers surge to create dangerous "red tides."


Azolla is a floating fern that is nitrogen-fixing because of its symbiont cyanobacterium
Anabena, to the right.

Anabena azollae, the cyanobacterium symbiont of the floating fern Azolla. It lives in spaces between the cells of Azolla, to the left
The ciliate protozoan Ophryoscolex is one of the largest in a cow's rumen. There are millions of such protozoa in the rumen. Each of them contain millions of bacteria that digest cellulose, the cow's primary food.
Image credit Sharon Franklin. USDA
Cattle depend on bacteria inside protozoa inside their rumen stomachs to digest their food
Sheep depend on bacteria inside protozoa inside their rumen stomachs to digest their food
Deer, like all grazers, depend on bacteria inside protozoa inside their rumen stomach to digest their food.
Water Buffalo, like all grazers, depend on bacteria inside protozoa inside their rumen stomach to digest their food.
The ciliate protozoan Entodidiums beginning to divide by fission in the rumen (cud stomach) of a cow. Without such symbiotes, that themselves contain bacterial symbiotes, cows and other grazers could not digest cellulose. Image credit Sharon Franklin. USDA
so-called primitive Termites eat wood, which is mostly cellulose, which they cannot digest without their symbiote gut protozoans, which cannot digest cellulose either, so their own symbiote bacteria digest it for them, and for the termites. Image credit M. Kramer
Inside the termite gut, a multitude of protozoans swarm among wood particles.
The ciliate Trychonympha showing its many cilia that it uses to swim in the termite gut. It is full of wood particles and helpful bacteria.

 

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