Shading Reduces Progression of Coral Disease

Many studies attribute the global phenomenon of increased coral disease outbreaks to an increase in novel pathogens and their virulence in warmer waters. Alternatively, the compromised-host hypothesis (CHH) considers that coral disease outbreaks are becoming more frequent because environmental stressors are compromising the health of the coral host, leaving it more susceptible to infection from pre-existing pathogens as well as novel ones. In both explanations thermal stress is key as the CHH suggests that bleached corals (bleaching being a response to thermal stress) are far more susceptible to infection than those which still have their zooxanthellae; and pathogens generally rely on increased temperatures to increase their virulence. The paper makes this link between bleaching and infection with relation to high irradiance stress as this can upset the photosystems of the symbiotic zooxanthellae, consequently leading to bleaching. Therefore the authors hypothesise that when irradiance stress is reduced; the corals will be less prone to bleaching and so less prone to disease, thus disease progression will be slower.

The massive coral Colpophyllia natans was used to test this hypothesis and several colonies suffering from white plague were shaded. This shading reduced UV radiation by 80% and photosynthetically active radiation (PAR) by 40%, which was thought to reduce stress by allowing corals to down-regulate photoinhibition; a process which builds up damaging reactive oxygen microbe species of coral symbiont. Therefore, overall oxidative stress was reduced in the coral holobiont. I think it is important to mention here that the authors refer to the coral holobiont, not just the microbial community or the coral itself. Many of the papers I have read have focussed in on one aspect of the holobiont, which I understand in necessary for a detailed investigation, yet have failed to properly relate their findings back to the symbiosis as a whole.

Significant results were produced from this study as disease progression was markedly reduced in shaded corals, whereas it increased in unshaded corals. However, the authors note that in no case did disease progression stop all together. Even so, this is still a really important finding as it shows us that there could be a way of helping corals to recover and at the very least, reduce the effects of disease. Furthermore, the corals were only observed for 10 days after shading began so perhaps if the trials were run for longer, disease progression would have ceased and perhaps regressed.

The mechanisms behind these findings were unexplained although several ideas were suggested. The reduced radiation could have alleviated stress, in accordance with the compromised-host hypothesis, therefore reducing disease susceptibility. Following on from this, it may be that reduced irradiance reduced thermal stress also, and so the antimicrobial agents produced by the coral and its symbionts (which are usually retarded by heat) may have been able to keep being produced continue working. Finally, it may be that pathogen virulence is coupled with irradiance and therefore the lack of it reduced their activity.

This list of possible explanations demonstrates that far more work could be done to better our understanding of this topic as it could be replicated and run for longer and with other diseases. It could also ease our minds on the subject of rising sea levels damaging corals as if the water were deeper, less light would penetrate to the corals, perhaps mimicking the results found here.

A review of: E. M. Muller and R. van Woesik (2009) Shading reduces coral disease progression. Coral Reefs 28: 757–760