Hydothermal vent relative causes chaos for corals!

Black band disease (BBD) is what is known as a polymicrobial disease. As the name suggests, a black band progresses across the surface of the coral, leaving white exposed skeleton behind.
This disease is rapid and can spread across a coral surface at 2cm day^-1. So far it is understood that an unknown trigger begins the onset of a less virulent lesion, containing a high abundance of cyanobacteria, which then progresses to BBD. This sequence of events presents a novel opportunity to study the disease as it progresses. Previous literature states that as progression occurs, the microbial community shifts to a community abundant in sulfur reducing bacteria. This creates an anoxic sulfur rich condition, undesirable for most holobiont regulars, and tissue necrosis attracts all sorts of heterotrophic bacteria.

What is less studied is the role of archaea in this consortium. Sato et al (2013) recognised that the anoxic, sulfur rich conditions were similar to extreme environments, where archaea often dominate the population. To investigate what role achaea played in the progression of BBD, they used 16s rRNA pyrosequencing to amplify archaea and bacteria in the consortium.

What was interesting was that within both the CP and BBD lesions, 70-90% of the total sequences were archaeal. Suggesting a key component of the polymicrobial community had previously been overlooked.

A Shannon diversity test showed that the BBD lesions were less diverse than CP, and although both lesions had a large number of archaea, the communities were very distinct. The CP lesions had quite a diverse assemblage of archaea, from three clusters within the Euryarchaeota and Thaumarcheota. Whereas only one dominant species was present in the BBD lesion, making up 92% & 99% of the sequence reads. This archaea was a novel ribotype with only 88% similarity to anything previously documented. The closest relative to these archaea were retrieved from a hydrothermal vent and oxygen depleted cold-water seep.

The bacterial community also shifted as the lesion developing into BBD, and once again the community was more diverse in the CP than BBD. There were a high number of cyanobacteria in both lesions, but the dominant genus’s differed. The BBD had an abundance of Trichodesmium sp., and CP a large proportion of Oscilatoria sp. Both lesions contained a similar diversity and abundance of sulfur reducing bacteria.

This research by Sato et al (2013) identifies the abundance of previously overlooked domain, suggesting it could be important in understanding this coral disease. What I thought was good about this study was the results came from two separate assays, with two primer sets, both designed for archaea 16S amplification. When compared, they shared 362 nucliotides over 362 positions. This reduces any doubt that this was an affect from the primers. And of course there were three repeats. Sato et al (2013) also highlights, that although there appears to be a large abundance of one archaea, the role it plays is still unknown, and so is the trigger. Future work using temperature treatments as well as UV irradiance could help to understand what initiates this disease. Further genomic analysis looking at what other functional genes this archaea has, might help to understand what this organisms role is in BBD.

Reference :

Sato, Y., Willis, B. L., & Bourne, D. G. (2013). Pyrosequencing‐based profiling of archaeal and bacterial 16S rRNA genes identifies a novel archaeon associated with black band disease in corals. Environmental microbiology, 15(11), 2994-3007.