A STORY OF SUCCESSION: BLACK BAND DISEASE

It has long been known that black band disease (BBD) is a poly-microbial disease which affects corals, characterised by a darkly pigment microbial mat, leading tissue loss and necrosis. However, what is currently unknown is how these microbes cause the cause the black bands or how changes in the microbial communities lead to the onset of BBD.
In has been noted in previous research that distinctly different cyanobacterium patches (CP) sometimes developed into BBD.
With the threat of BBD only likely to increase over time, with the predicted trends in global warming, it is worthwhile knowing BBD transmission modes and the early developmental stages to help the advance of reef management strategies to mitigate the impacts of this disease.

In this paper the authors studied an inshore Montipora reef at Pelorus Island in the central Great Barrier Reef (GBR) between September 2006 and January 2009. CP infected colonies were individually tagged and monitored approximately monthly to determine if the CP lesions developed into BBD.

Not all CP lead to BBD, and not all BBD is derived from CP, as demonstrated by the findings that 18.7% of the 262 CP lesions recorded on Montipora developed into the visually characteristic BBD lesions, with the remaining 81.3% of CP lesions disappearing, but leaving behind exposed coral skeleton. Of the 263 BBD cases observed on Montipora 18.6% were derived from the CP, sources for the remaining BBD lesions are not known.
16S rRNA clone libraries showed distinct differences between CP and BBD lesions and terminal restriction fragment length polymorphism analysis revealed a shift in bacterial ribotypes during transitions from CP to BBD.
CP clone libraries were dominated with Alphaproteobacteria-affiliated sequences in comparison to BBD clone libraries which were dominated by Gammaproteobacteria-affiliated sequences. The percentage of cyanobacterial-affiliated sequences increased in the transition from CP to BBD, contributing to only 7.3% of the CP clone library at an intermediate stage compared to 42.6% at an advanced stage of BBD.
The maximum prevalence of CP was between November and December, with the maximum prevalence of BBD 40-50 days after, this, together with the fact 40-50days is the average recorded time for CP to develop into BBD, delivers evidence that CP can be an early successional stage for BBD.
The authors believe Trichodesmium species (the dominant cyanobacteria in CP) act in the same way as Hydrocoleum ssp. in that they are a huge contributor of nitrogen fixation. The authors therefore believe Trichodesmium spp. have a huge part in the successional changes from CP to BBD because they create new niches for others microbes.

The authors point out that further research which would help to discover which microbes are the key players in the BBD pathogenesis development is needed. This would involve key functional genes and their expressions at different stages in the CP-BBD development.

This paper was an interesting read; it offers a good insight into one way BBD develops i.e. through successional changes. However, as already mentioned above BBD does not only develop from CP, the other ways remain unknown. It offers an explanation on how the microbial community changes, but does not offer any way to stop these changes, meaning that in terms of reef management more needs to be done to stop corals reefs dying. I believe this paper is impourtant in its area and acts as a good stepping stone for further research to be carried out.

A review of;
Sato Y, Willis BL and Bourne DG (2010) Successional changes in bacterial communities during the development of black band disease on the reef coral, Montipora hispida. The ISME Journal. 4, 203-214.