Black band disease – don’t blame it all on bacteria!

Black Band Disease (BBD) is an infection, hosted in corals, that is characterised by a darkly pigmented microbial consortium. It causes necrosis to underlying coral tissues exposing the bare skeleton as it migrates across colonies contributing to global coral decline.

The development of BBD was studied over time in situ on Montipora corals. Lesions dominated by cyanobacteria and aptly named ‘cyanobacterial patches’ (CP) precede the development of BBD lesions. Profiles of the microbial communities within both CP and subsequent BBD lesions were studied using two indepentant amplicon-pyrosequencing assays of 16S rRNA coding genes.

Overall, analyses consistently found both taxanomic composition and diversity of bacteria shifted within all coral colonies sampled with the progression from CP lesions to BBD. A shift from Trichodesmium spp. dominance of the cyanobacteria in CP to Oscillatoria spp. dominated BBD cyanobacteria  was indicated as the largest driver of alterations in community composition between the two disease stages.

Alphaproteobacteria were in significantly lower proportions in BBD samples that CP samples, though the opposite trend was true of Arcobacter sp. (within Epsilonproteobacteria).  Other bacterial species were found to be present in both BBD and CP lesions with no distinctive variations in relative abundances.

Successional changes in microbial communities during the disease transition from CP to BBD were concurrent with the shift to a typically anoxic, sulphide-rich microenvironment. Intensification of such conditions accounts for the increased pathogenicity of BBD and highlight the importance of the microbial consortium. Whilst these findings corroborate with previous studies, the potential contribution of non-bacterial groups to BBD has never previously been investigated.

Archaea are ubiquitous throughout many aquatic habitats, and account for large proportions of the biomass in extreme conditions. They are known to inhabit anoxic and sulphide-rich environments, a niche present within a BBD mat.

It was found that archaeal communities are not only present in the microbial consortia in both CP and BBD lesions, but that a similar shift in phylogeny to bacteria occurs with development of the disease. CP lesions were typically diverse assemblages whereas BBD lesions were found to be dominated by a novel archaeon, distantly related to other archaea usually associated with sulphurous and low-oxygen environments.

The dominance of such an archaeon makes it highly interesting for future study. Whilst its metabolic modes cannot be inferred from other archaea due to phylogenetic distance, its capacity to inhabit such environment could infer a potential role in methanogenesis, chemo-organotrophy and anerobic methane oxidation. Testing these and identifying the biological capabilities of the novel archaeon warrants further study.

The distinct shift in bacterial assemblages with the progression of Black Band Disease was evidenced, and has been noted for other coral species and diseases alike (see Rachel, Roberto, and Ellie's blog posts). However it is clear that further research is required to assess both the significance and the potential functional roles of the archaea.

The findings of this focal study have enhanced our understanding of Black Band Disease and may aid in understanding other diseases contributing to coral decline. The application of these findings to other diseases, where archaea may not have previously been tested for could perhaps highlight the importance of archaea in influencing disease virulence and the associated microbial ecology.

Y. Sato, B. Willis and D. Bourne (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