Mutualisms

3.1 Coral mutualism

Reef-building corals have a mutualistic relationship with photosynthetic zooxanthellae.  When these corals are placed in the dark, they expel the zooxanthellae from their tissues.  What does this tell us about the controls on this relationship?  What does this tell us about the costs and benefits to each partner?

Model answer:

Reef-building corals have a mutualistic relationship with zooxanthellae.   These unicellular algae are found in the outer tissues of the coral where they can photosynthesise.  They pass on some of the carbon they have fixed through photosynthesis to their hosts.  The cost to the zooxanthellae is that they lose some of their fixed carbon which they could otherwise have used to reproduce and increase the size of their population.  The zooxanthellae benefit by receiving nitrogen and phosphorus from the coral; these nutrients are scarce in seawater where corals grow.  In the dark, the zooxanthellae do not photosynthesise and are thus unable to provide carbon to the coral, which ejects them.  This suggests that there is a considerable cost to the coral in providing nutrients to the zooxanthellae.  It also implies that the coral is in control of the mutualistic relationship.

The University of Aberdeen is a charity registered in Scotland, No.SC013683