Passage III

Students grew 2 different species of paramecia (microscopic, unicellular organisms)—P. caudatum and P. aurelia—together for 18 days in a test tube containing a growth medium. A constant supply of bacteria (a food source for paramecia) was maintained in the growth medium. Figure 1 shows how the population density (number of paramecia per milliliter, mL, of growth medium) of each species changed over the 18 days.

The procedure was repeated except that P. caudatum was grown together with P. bursaria (see Figure 2).

Student 1

P. caudatum and P. aurelia can grow only near the sur­face of the growth medium. As the population densities increased, competition for food increased. Eventually, one species outcompeted the other.

Unlike P. caudatum and P. aurelia, P. bursaria harbor cells of photosynthetic algae, called Chlorella, in their cytoplasm. Chlorella produce O₂ for P. bursaria. This allowed P. bursaria to feed on bacteria at the bottom of the growth medium. Because P. caudatum and P. bursaria occupied different regions in the growth medium, there was little competition for food between the 2 species, and they were able to reach stable population densities.

Student 2

Initfally, both P. caudatum and P. aurelia fed on bac­teria. But P. aurelia can also feed on other paramecia. Thus, when the population densities increased, and compe­tition for bacteria increased, P. aurelia began to feed on P. caudatum.

P. caudatum. and P. bursaria are unable to ingest other paramecia, so P. caudatum and P. bursaria were able to reach stable population densities.

Student 3

P. caudatum and P. aurelia were able to find food when their population densities were low. When population densities became too high, P. aurelia released a toxin that instantly stopped the cilia (short, hairlike projections) on

competing paramecia from moving. Because the movement of cilia is necessary to ingest food, the population density of P. caudatum declined.

P. caudatum and P. bursaria were able to reach stable population densities because neither species can produce the toxin.