Q. Healthy individuals of Paramecium bursaria contain photosynthetic algal endosymbionts of genus Chlorella. When within their hosts, algae are referred to as zoochlorellae. In aquaria with light coming from only one side, P. bursaria gathers at well-lit side, while other species of Paramecium gather at the reverse 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:
(1) If kept in darkness, algae 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 these situations would be most positive to the re-establishment of resident zoochlorellae, assume compatible Chlorella are present in P. bursaria’s habitat?