Symbiophagy: Coral Allergies?

Coral bleaching (defined by the loss of zooxanthellae) can be brought about by a variety of environmental stressors eg, temperature fluctuation, chemical pollutants, high light intensity and many others. However, other possible mechanisms for coral bleaching have also been recorded. The digestion of symbiotic unicellular algae was first documented in cnidarian corals as early as 1883, though it has since been disputed by the theory that coral zooxanthellae is expelled rather than digested.  The theory suggesting the digestion of unicellular symbionts by their host (termed symbiophagy by the authors) has since gained evidence and momentum through more recent investigation. The theory suggests a transformation of the coral vacuolar membrane from a site for nutrient transfer to a digestive organelle, due to a stress-related destabilisation of the mechanism that maintains symbiosis.

To gain more evidence for the symbiophagy theory, the authors conducted further research using the coral species Pocillopora damicornis. Nubbins from this coral were harvested and allowed to acclimatise in darkness at 25^oC for 8 hours before being tested in 6 situations: 1) low light at 25^oC 2) low light at 32^oC 3) high light at 25^oC 4) high light at 32^oC 5) darkness at 25^oC 6) darkness at 32^oC. All variables were carried out in triplicate and subjects were left in these conditions for 10 hours, 48 hours or 5 days.

The results showed that zooxanthellae experienced the initial stages of lysis by their coral host at higher temperatures and to a lower degree at high light levels, which suggests that heat and light can affect coral-zooxanthellae symbiosis independently of each other. High light and high temperature combined produced an amplified but similar affect on the zooxanthellae, meaning that the symbiophagic process was accelerated when stressful factors were combined. There has been no mention of any change found in the samples with time. The specimens kept in the dark showed no change to the coral-zooxanthellae symbiotic relationship, despite cellular morphological changes to the coral itself. The authors of this paper suggest that bleaching of coral in the dark is a longer process than time of the investigation allowed for, which would explain the lack of change.

It is possible that symbiophagy has evolved from other defensive techniques that coral are known to employ to prevent damage from foreign bodies. Xenophagy for example, is the process of coral lysing and consuming foreign bacterial cells. It is thought that symbiophagy may be the result of this becoming non-specific due to environmental stressors.

This paper has tried to underline the importance of understanding multiple mechanisms for coral bleaching, and a possible way that defensive strategies can go wrong in coral and other phyla. This subject would benefit from extensive further research to increase our knowledge innate defence mechanisms and the consequences it could have on some species of coral.

A review of Downs, C.A., Kramarsky-Winter, E., Martinez, J., Kushmaro, A., Woodley, C.M., Loya, Y., Ostrander, G.K. (2009) Symbiophagy as a cellular mechanism for coral bleaching, Autophagy, 5, 211-216