How do environmental conditions determine the type of symbiosis that can occur between two organisms? Use the Hydra-Chlorella (algae) system as an example.

Question

What will be an ideal response?

Answer 1

An example of a symbiotic interaction that can have a range of outcomes is present with hydras and algae. Hydras are small freshwater Cnidarians that live in ponds and slowly moving rivers. They feed on small animal rotifers present in the water column. Some hydra are green because they contain algae from the genus Chlorella inside their bodies.
The algae photosynthesize and they release substantial amounts of photosynthetically fixed carbon to the animal cells in the form of maltose (a sugar).

Green hydra can easily be cultivated in the laboratory, and they can also be "bleached" of their algae by incubation at very high light intensity. Both hydra containing algal cells (symbiotic hydra) and those deprived of their algae (aposymbiotic hydra) reproduce rapidly by asexual budding, so large numbers of genetically identical symbiotic and aposymbiotic hydra can be generated for experiments.

To investigate whether green hydra benefit from their symbiosis with Chlorella, the performance (that is survival and growth) of symbiotic and aposymbiotic hydra can be compared under different environmental conditions. When hydra are maintained in the light without food, the non or aposymbiotic hydra die within a few weeks. Green hydra benefit from the symbiosis, provided they are illuminated.

Well-fed aposymbiotic and symbiotic hydra grow at approximately the same rate when light is present. The algae release sugars, but these sugars play a minor role in their diet. Fed cultures of the hydra can also be maintained indefinitely in the dark. In this case, the algal population drops by about 60% but the animals remain pale-green. In the darkness, green hydra grow slower than non-green hydra, probably because nutrients derived from the food are diverted to maintain Chlorella, which in the absence of light, contribute nothing to the hydra.

This example makes it clear that the environment plays a major role in determining the outcome of symbioses. In this case, the hydra benefit when food is scarce, but Chlorella are of no discernible value to fed animals in the light and they are detrimental, reducing the growth of fed hydra, in the dark.