Make reefs great again: Symbiodinium clade D trumps Vibrio

Coral are inhabited by a diverse consortium of microbial life, including Symbiodinium, a genus of symbiotic dinoflagellate. There are nine identified clades of Symbiodinium (A-I), some of which can be found in coral tissues as mono or multi-clade populations. Corals can associate with different clades, and many studies have looked at the effect of different clades on the ability of corals to deal with stressful environments and disease. Over the last 30 years the frequency of coral disease outbreaks and bleaching events has increased, so understanding how different members of the coral holobiont affect the coral’s resilience to disease or ability to acclimate is increasingly important.

Rouzé et al investigated the Symbiodinium assemblage in the Indo-pacific stony coral Acropora cytherea and measured how this affected the corals sensitivity to disease (broadly described as ‘white disease’ but comprising any disease causing chronic lesions) and whether the presence of disease was linked with the appearance of Vibrio spp. Several small fragments were taken from eleven colonies of A. cytherea every 2 months for just over a year, and total coral DNA was extracted (this excludes DNA from outside coral tissues i.e. in the mucus). Symbiodinium clades were detected using clade-specific primer sets for clades A-F, and presence of Vibrio spp. were detected using genus specific 16S rDNA primers. Clades A, D and C were found in the tissues of A. cytherea, but B and F were absent despite being present in the surrounding environment.

Roughly half of the colonies (five out of eleven) developed disease lesions consistent with white syndrome during the study, and four of those five colonies contained only clade A when symptoms were identified. The colonies that remained healthy throughout the study contained A and D simultaneously, except for one which also contained clade C.

Vibrio spp. were only detected in diseased corals, but no link between these results can be made because Vibrio spp. were detected before, during and/or after disease symptoms, suggesting that they are not the causative agent of the disease. This does however strongly suggest that the bacteria may be opportunistic. During Vibrio spp. infection, there was a switch from multi-clade association to mono-clade A, and vice versa if the bacteria disappeared. The results show that affected colonies of A. cytherea were either lacking clade D to begin with, or lost it over the duration of the study. The strong negative correlation between association with clade D and Vibrio spp. infection may reinforce the hypothesis that the clade has antibacterial properties.

Although this study has significant results showing that corals associated with mono-clade A are more susceptible to white disease and Vibrio spp. infection, the small sample sizes give the results less weight. From only four colonies containing mono-clade A, the authors suggest that this makes the coral sensitive to white disease. Infection mainly occurred during the hottest months, leading me to believe that the ‘detrimental, opportunistic clade A’ may only become a hindrance when the environment becomes very stressful. If clade A is indeed a detrimental member of the microbiome it is strange that it occurs most frequently out of all the clades in this species of coral. This study is however very in-depth and with more replicates, as well as studies on different species of coral to see if the benefits and drawbacks of different clades are consistent, it could help us to understand how corals react to stress and disease.

Rouzé H., Lecellier G., Saulnier D., Berteaux-Lecellier V. (2016) Symbiodinium clades A and D differentially predispose Acropora cytherea to disease and Vibrio spp. colonization. Ecology and Evolution. 6(2) 560-572