The Shifts in the Bacterial and Fungal Communities Associated with Dark Spot Syndrome in the Coral Stephanocoenia intersepta

Dark Spot Syndrome (DSS) is thought to be one of the most widespread coral diseases and/ or syndromes in the whole of the Caribbean, and can be seen as dark spots on the corals’ surface in varying colours and sizes.  The lesions that subsequently form become depressed into the skeleton, and the subsequent loss of tissue can lead to the successional colonisation by algae and/ or boring sponges. It has been suggested that communities of fungi and bacteria, rather than one single bacterium, may contribute to infection, particularly in the coral species Stephanocoenia intersepta, Stephanocoenia siderea and Montastraea annularis. However previous reports have indicated that many other coral species can also be affected, so it has significant implications for coral ecology and population biology within the Caribbean. 

Other studies have attempted to isolate one sole bacterium that causes a certain disease, and only use culture-dependent techniques, whereas this study undertakes culture-independent molecular analyses of both bacterial and fungal communities that are present, and so possibly associated with DSS, to determine any shifts in these communities. In addition, they compare the communities in different types of tissue in S. intersepta (healthy tissue with no signs of DSS (H); apparently healthy tissue >5cm from the lesion on a coral showing signs of DSS (AH); and the different sized lesions for DSS itself. This study has also attempted to determine the temporal changes of assemblages as the disease and/ or syndrome persists by comparing the different sizes of the lesions. The authors used three different sizes (Small, S-DL; Medium, M-DL; and Large, L-DL) of spots as they are thought to be an indicator for the age of infection, due to the fact that they progress ~3 mm per month. 

The 16S rRNA gene bacterial diversity of H, AH and the three different sizes of disease lesions (S-DL, M-DL and L-DL) varied significantly. Four ribotypes related to Corynebacterium sp., Acinetobacter sp., Parvularculaceae sp., and Oscillatoria sp. (also the causal agent of Black Band Disease (BBD)) matched the patterns expected from the potential pathogens, as they were absent from H, increased in abundance with AH, and were dominant in the disease lesions. Two Vibrio species were also dominant in the lesions only, although other conflicting results showed that other species of the same genus (i.e. V. harveyi and V. carchariae) were present in all/ most samples, which implied that some potential coral pathogens may exist in healthy tissues. In comparison, the fungal diversity was much lower, but one particular pathogen which showed the most significant difference between healthy and diseased tissues was the newly-identified strain of the ribotype, Rhytisma acerinum, which is a known causal agent of tar spot on terrestrial tree leaves. 

The authors therefore suggest that the darker pigmented state of the zooxanthellae caused by DSS is predominantly down to R. acerinum and the bacteria Oscillatoria, as similar previous results for BBD imply that bacterial communities vary with lesion size, so it is likely to be linked to the age of infection, as predicted. 

I particularly liked this paper because it focuses on the microbial community and which pathogens are involved, but is also able to indicate which microorganisms contribute more to the infection and the manifestation of DSS than others in this assemblage, such as R. acerinum and the bacterium Oscillatoria. It is by no means a holistic view of the cause of the disease however, particularly due to the fact that they are unable to establish whether DSS is actually a disease or if it is a syndrome, so it is clear that there is a paucity of knowledge and understanding concerning Dark Spot Syndrome. As they mentioned the three coral species that are normally affected in the introduction of the paper, I felt that they could have expanded this study out to compare the microbial communities in both healthy and diseased tissues between the different species. 

Sweet, M., Burn, D., Croquer, A., and Leary P. (2013) Characterisation of the bacterial and fungal communities associated with different lesion sizes of Dark Spot Syndrome occurring in the coral Stephanocoenia intersepta. PLoS ONE, 8(4): e62580. doi:10.1371/journal.pone.0062580