Healthy individuals of Paramecium bursaria contain photosynthetic algal endosymbionts of the genus Chlorella. When within their hosts, the algae are referred to as "zoochlorellae". In aquaria with light coming from only one side, P. bursaria gather at the well-lit side, whereas other species of Paramecium gather at the opposite side. The zoochlorellae provide their hosts with glucose and oxygen, and P. bursaria provides its zoochlorellae with protection and motility. P. bursaria can lose its zoochlorellae in two ways: (1) if kept in darkness, the algae will die; and (2) if prey items (mostly bacteria) are absent from its habitat, P. bursaria will digest its zoochlorellae.
-A P. bursaria cell that has lost its zoochlorellae is said to be aposymbiotic. It might be able to replenish its contingent of zoochlorellae by ingesting them without subsequently digesting them. Which of the following situations would be most favourable to the reestablishment of resident zoochlorellae, assuming compatible Chlorella are present in P. bursaria's habitat?
A) abundant light, no bacterial prey
B) abundant light, abundant bacterial prey
C) no light, no bacterial prey
D) no light, abundant bacterial prey

Question

Quizplus · Accepted Answer

Abundant light is necessary for photosynthesis by the zoochlorellae, but bacterial prey is also required for the survival of P. bursaria. Without bacterial prey, P. bursaria will digest its zoochlorellae, leading to aposymbiosis. Therefore, the most favourable situation for the reestablishment of resident zoochlorellae is abundant light and abundant bacterial prey.

Healthy Individuals of Paramecium Bursaria Contain Photosynthetic Algal Endosymbionts of the Genus