Evidence of a shift in bacterial community structure from healthy to diseased corals, maintained across species

Evidence of a shift in bacterial community structure from healthy to diseased corals, maintained across species

White Plague Disease (WPD), investigated here, has been linked to coral reef declines globally however it is largely unclear whether a) the disease reported is actually WPD or an alternative disease with similar phenotypic characteristics or b) whether there is a definitive causative agent.   Thalassomonas loyana has been proposed as the pathogen causing WPD (or White Plague-like disease – WPL) in the red sea and Auranimonas coralicida the causative agent in the Caribbean, however neither of these species has been identified in many subsequent studies, including this focal study.  It is now thought unlikely that one single cause can be identified, but more a shift in community likely due to environmental or host condition, providing a niche for opportunistic pathogens.  This study sought to compare bacterial assemblages associated with two species of coral displaying WPL disease, in both diseased and healthy samples, to assess bacterial community changes.

The corals were collected from the same reef (Gulf of Thailand) to minimise any environmental effect and 16s rRNA gene microarrays (PhyloChip G3) were used to ascertain Operational Taxonomic Units (OTUs) in each sample.  Additionally, 16s rRNA gene sequences were used to conduct a search of comparisons with clone libraries.  Overall, analyses found the bacterial assemblages were species specific but also distinct in both diseased and healthy corals of both species.  The results indicate a differentiated pattern of community structure in diseased and healthy corals that is maintained inter-species, with a higher abundance of taxa that have known coral pathogens including Pseudomonadaceae, Rhodobacteraceae, Vibrionaceae and Alteromonadaceae , found in diseased corals.

Additionally, the results show a marked increase in bacterial diversity in diseased corals of both species, which reportedly corroborates findings in other similar studies.  This however, is in contrast to another recent study using the same PhyloChip G3 techniques (Kellog et al., 2013) looking at WPL in a coral species from two Caribbean locations, which found the complete opposite, even after applying analyses comparative to this focal study.  Why would this be?  As addressed in the lectures, many diseases displaying phenotypic characteristics akin to WPD, may be one of a variety of diseases, including WPD Type I, II & III.  It is possible that each ‘strain’ of disease harbours a unique bacterial assemblage that may explain the differences in diversity associated with diseased corals between studies.  Furthermore,  Kellog et al., (2013) argue that many clone libraries are biased toward Gram negative sequences, which would naturally exclude many Gram positive bacteria that may be associated with the coral microbiota.  There was no mention of either Gram negative or Gram positive gene sequences by Roder et al., which may affect the assemblages found in this study.  Clarification of this point is necessary to speculate further.   

Nevertheless, irrespective of whether the there is an increased or decreased abundance in bacterial diversity, it is apparent there is a distinct shift in community structure from a healthy to diseased state in corals, which appears to be common across species (Roder et al., 2014) and habitats (Kellog et al., 2013).   The PhyloChip  G3 may be effective in assessing coral health states in providing a more detailed picture of the bacterial community than has been previously possible, however it is clear that further clarification and research must be attained to provide a framework for assessment.

Roder, C., Arif, C., Bayer, T., Aranda, M., Daniels, C., Shibl, A., Chavanich, S. & Voolstra, C. R. (2014). Bacterial profiling of white plague disease in a comparative coral species framework. The ISME journal. 8, 31-39

Kellogg, C. A., Piceno, Y. M., Tom, L. M., DeSantis, T. Z., Gray, M. A., Zawada, D. G., & Andersen, G. L. (2013). Comparing Bacterial Community Composition between Healthy and White Plague-Like Disease States in Orbicella annularis Using PhyloChip™ G3 Microarrays. PloS one, 8(11), e79801.