Friend or Foe? The Paramecium-Algae story

by SC

A relationship has been found in nature between Paramecium bursaria, a ciliate protist, and chlorella, a green algae, who have been known to help one another in certain situations (image shown below, the larger cell being P. bursaria and the green organisms inside the cell are chlorella). This is an endosymbiotic relationship in which Chlorella resides inside P. bursaria for food in exchange chlorella gives P. bursaria the by-products of photosynthesis. Symbiosis has been known to hold a huge role in biology; for example, mitochondria in eukaryotic cells are theorized to have once been free living bacteria that now serve as the eukaryotic energy producer. In this blog post we will learn how prey can become a beneficial part of the cell and what happens when chlorella stops benefiting P. bursaria in its endosymbiotic relationship.

Paramecium bursaria

P. bursaria provides nitrogen, carbon dioxide, and protection from viruses that target chlorella and in return chlorella provides a food source to the ciliate by producing sugars as a by-product from photosynthesis. It has been studied that not only does P. bursaria protect chlorella from viruses that want to infect it, but chlorella can also return the favor by protecting P.bursaria in a different way. The results from a study show that chlorella can protect P. bursaria from UV radiation from the sun [1]. UV radiation can cause lethal mutations in a cell’s genome, which can inhibit its ability to perform photosynthesis and inhibit growth. P. bursaria strategically places the chlorella near the edges of the cell and around its macronucleus, a large nucleus found in ciliates, to prevent the UV radiation from getting to the DNA and damaging the host’s genome. Evidence from the summerer paper et al. paper supported this conclusion by exposing symbiotic P. bursaria, P. bursaria with reduced chlorella, and P. bursaria with no chlorella to UV radiation. The results showed that when exposed to UV radiation both the symbiotic P. bursaria and P. bursaria with reduced chlorella showed growth in the presence of UV radiation while the P. bursaria with no chlorella showed a significant decrease in the number of cells that were still alive after the UV exposure (As seen in the figure below). Both of these organisms are capable of living independently of each other, they have been used in experiments to recreate the genesis of their symbiotic relationship.

Figure

Researchers have used two independent strains of chlorella and P. bursaria and found that they were able to create the symbiotic relationship found in nature in 30 minutes [2]. For clarification, Chlorella is found in the parialgal vacuole of P. bursaria, which is derived from a P. bursaria digestive vacuole. When Chlorella is ingested it enters a digestive vacuole and must prevent the fusion of the lysosomes to the vacuole. If they were to fuse lysosomal enzymes would cause the degradation of chlorella. The chlorella that succeeds in preventing fusion starts its new symbiotic relationship with P. bursaria . The first step is the phagocytosis of chlorella into the P. bursaria cell, which is placed in an acidic digestive vacuole that has the ability to bind to lysosomes for further digestion of the chlorella. At first, P. bursaria is simply trying to acquire a meal, but occasionally the algae is able to bud from the digestive vacuole before the lysosome binds to the vacuole. It would appear at this point that the chlorella has control of where this vacuole goes since it migrates right underneath the host cell membrane. Tests by Kodama et al. show that time of budding to the time it reaches the edge of the cell membrane the vacuole differentiates and is controlled by the chlorella. At this point the vacuole can no longer be combined with a lysosome and P. bursaria and chlorella start their new symbiotic life together. If chlorella stops doing its job it will be degraded and it will be digested by P. bursaria.

Another paper by Kodama et al. suggests that when the protein synthesis of chlorella stops working P. bursaria will break off its symbiotic relationship and digests chlorella. This reaction was studied by introducing a solution of cycloheximide to selective media that affects protein synthesis of chlorella but does not affect P. bursaria enough to be considered important. Nine hours after cycloheximide is introduced to the media, P. bursaria is able to fuse lysosomes to the algal vacuole and digest the chlorella. The results gathered show that when the proteins in chlorella stop working it can no longer maintain its chloroplast and nucleus structure; consequently these organelles are degraded and then digested by P. bursaria.

With this information, it can be said that P. bursaria has a selfish relationship with chlorella. When the two organisms first meet P. bursaria eats chlorella and places it in a situation where if chlorella survives it will live the rest of its life inside P. bursaria. If unsuccessful, P.bursaria will digest chlorella for food. This is a win-win situation for P. bursaria because either way it will acquire a meal; however, it is more beneficial to keep chlorella alive since it provides a food source to P. bursaria as long as the paramecium provides food for the algae. P. bursaria also benefits from the UV protection provided by chlorella. At the end of chlorella’s life the organelles will degrade and be digested by P. bursaria for food. For one final point it should be noted that during chlorella’s life it will reproduce so this abusive cycle will be passed down through its progeny, but since it is a cell it doesn’t think like this all it knows is that it is getting food from its captor... Can anyone say Stockholm Syndrome?

References:

1.     Summerer, Monika, et al. "Symbiotic Ciliates Receive Protection Against UV Damage from their Algae: A Test with Paramecium bursaria and Chlorella." Protist 160.2 (2009): 233-243.

2.     Kodama, Yuuki, and Masahiro Fujishima. "Timing of Perialgal Vacuole Membrane Differentiation from Digestive Vacuole Membrane in Infection of Symbiotic Algae Chlorella vulgaris of the Ciliate Paramecium bursaria." Protist 160.1 (2009): 65-74.

3.     Kodama, Yuuki, Isao Inouye, and Masahiro Fujishima. "Symbiotic Chlorella vulgaris of the Ciliate Paramecium bursaria Plays an Important Role in Maintaining Perialgal Vacuole Membrane Functions." Protist 162.2 (2011): 288-303.