Paramecium bursaria, It's Easy Being Green!
Written and Photographed by Bruce J. Russell

Humm! What’s this? A jar of pond water in the closet? Now I remember. This was from a collection made about a month ago, unintentionally left on the closet shelf with the empty jars and other collecting equipment. Did anything survive in the dark?

P. bursariafrom my forgotten culture.
P. bursaria from my forgotten culture.

A few drops examined with hand lens shows a few whitish specks moving around. Under the microscope the specks looked like this-- The survivors are small paramecium, each containing some green cells that appear to be suspended in the cytoplasm. How to Know the Protozoa shows the organisms to be, Paramecium bursaria.

P. bursaria is normally packed with green cells --symbionts living within the paramecium, called “zoochlorelli” But the paramecia I'm looking at have only a moderate number of rather pale green bodies. Could these inclusions be zoochlorelli that have survived a month in the dark?

I decided to try a simple experiment. Divide the pond water sample into two small jars. Leave one in the dark and place the other on a window ledge that receives indirect daylight. Two weeks later here are the results:

Comparison – P.bursaria grown in the light and in the dark

P. bursaria from the ‘forgotten’ jar after two weeks in the light.
P. bursaria from the ‘forgotten’ jar after two weeks in the light.
P. bursaria from the ‘forgotten’ jar after two weeks in the dark.
P. bursaria from the ‘forgotten’ jar after two weeks in the dark.

The subjects receiving light have become green. High magnification shows that, indeed, the green bodies are Chlorella, a common single celled alga. The Chlorella are definitely multiplying. Also, the illuminated population of Paramecium bursaria has increased dramatically.

In the dark jar, cells are few and far between and viewed on the microscope, they are now virtually devoid of green symbionts. Study the cell for any remnants of its green guests.

Over the next month Paramecium bursaria continued to increase in the "light" culture and the green symbionts became more concentrated, creating swarms of bright green paramecia.

bright green paramecia
bright green paramecia

These observations pose questions about the relationship between Paramecium bursaria and its smell green guests:

Our animation mimics the fate of zoochlorelli when P. bursaria is kept in the dark for several weeks
Our animation mimics the fate of zoochlorelli when P. bursaria is kept in the dark for several weeks
What happened to the green cells in the original, closeted population of P.bursaria?

Were they eaten by their host?

Did they fail to reproduce? Perhaps as their host cells continued to reproduce, the algae simply became diluted with each generation.

What benefits might each partner derive from living together?

Is the relationship obligatory?

Obviously P.bursaria can survive, at least for a time, without its guests. To find out if these symbiotic Chlorella can live independently, one might squash the host cells, releasing the symbionts. Then try culturing these cells in an algae growth medium, and see how they do.

It would be fun to rear “clear” P.bursaria (by culturing them in the dark), introduce them to free-living Chlorella, and see if they can capture and reinstall the algae cells as symbionts.


Seasonal ponds often dry up so how does P.bursaria survive these disruptions? Occasionally I have seen P.bursaria cysts that include the symbionts. These cysts may be able to withstand drying, ready to hatch when the pond again fills with water. How the green symbionts manage to survive within the cyst is another question. It’s interesting that only one species of Paramecium (there are half a dozen commonly found species) has formed a relationship with zoochlorelli. A number of other ciliates have become algae farmers, including species of Stentor, Blepharisma, and Vorticella. But these hosts still enjoy a good meal, taking in smaller organisms which are then digested in food vacuoles. The waste products that result from the metabolism of ingested food become fertilizer for the green symbionts. It would be interesting to determine if green Paramecium bursaria could survive without feeding. Now there’s an idea for your next science fair project.

Dry-tolerant cysts of P. bursaria
Dry-tolerant cysts of P. bursaria

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