The coral holobiont is a complex, diverse community of
organisms including symbiotic zooxanthelle and a diverse range of bacteria.
Many organisms within the holobiont have positive impacts on the community such
as sun protection, nitrogen fixation and photosynthesis. Increased
anthropogenic activities and ecological stressors including pollution, fishing
and diving, temperature increases have increased the incidence of coral disease
significantly. Yellow band disease (YBD) is a slow growing coral disease in
reef building corals. YBD is the slow growth of single or yellow/ white
radiating bands on the coral surface causing the exposure of bare skeleton. Four
species of Vibrio spp. have
previously been identified as the cause of YBD.
This study used a combination of molecular tools (bacterial
taxonomic profiling, Symbiodinum genotyping
and, host transcriptome response) to further understand the mechanisms by which
YBD acts on the coral holobiont and investigate how the holobiont responds to
the disease. Skeletal tissue cores from colonies of healthy coral (HH),
diseased coral (DD) and healthy tissue adjacent to diseased tissue (HD) of O. faveolata were collected from the Puerto
Morelos coast, Mexico in September 2008.
PhyloChip hybridisation showed bacterial coral community
structure changed dramatically between healthy and diseased colonies with the
highest bacterial richness observed in HD which may represent a change in equilibrium
or a transitional community phase between healthy and diseased. The most
abundant operational taxonomic units (OTU’s) were from sequences previously
identified from coral, mammal guts and sediments. Vibrionaceae were more abundant in healthy samples which contradicts, but does not negate previous studies suggesting Vibrio’s pathogenicity
to this species. Here, Vibrio is reported at family level, not the species
level which have been identified as the cause of many diseases. Anaerobic Firmicutes were abundant in diseased
corals suggesting that disease is linked with oxygen limitation; Peptostreptococcaceae, which has been
observed in other coral diseases including black band and white plague, was
most abundant family in diseased samples. Population shifts between HH and DD
corals showed a shift from predominantly gram-negative bacteria in HH corals to
gram-positive in DD corals, HD showed a combination of the two.
Diverse Symbiodinium
assemblages were observed in diseased corals, this may be due to the detection
of lysed cells, or represent a changing community in response to microbial
interactions. Host gene transcriptomic responses showed a distinct down
regulation of mitochondrial- associated genes and up-regulation of immune and
inflammatory responses in diseased corals which could be resultant of nutrient
limitation and competition for resources further suggesting nutrient fluxes may
impact coral disease.
Closek, C. J., Sunagawa, S., DeSalvo, M. K., Piceno, Y. M., De Santis, T. Z., Brodie, E. L., Weber, M. X., Voolstra, C. R., Andersen, G. L., Medina, M. (2014) Coral transcriptome and bacterial community profiles reveal distinct Yellow Band Disease states in Orbicella faveolata. ISME. 8: 2411- 2422. doi:10.1038/ismej.2014.85
Hi Kat, very interesting study, do you think that if more replicates had been collected, you would see a different result? It is fascinating that the healthy corals shifted from gram-negative bacteria to gram-positive, did the author happen to mention why that may be?
ReplyDeleteHi Lucy, I don't think the results would necessarily change with more replicates but, they might. I think further yellow band disease research with more samples at a higher resolution is important to: give it more statistical significance; make it more comparable to previous studies and; increase scope for the discovery of specific species interactions.
ReplyDeleteFrom what I can see, the authors do not give a clear explanation of why the assemblage changed. I can only suggest from my own understanding the gram-negative bacteria may have been out competed by the gram-positive or cellular responses from the coral which both reduced nutrient production and suppressed the coral immune system made conditions less favourable to gram-negative and more favourable for gram-positive.
I thought most interesting aspect of this study was the inclusion of the coral transcriptomic responses, a tool which I feel could be really useful in coral research. For example, if the bacteria found in this study are different to previous research could yellow band disease actually be a qualitative trait of the corals symptomatic response to bacterial attack, rather than a response to specific bacteria?
Thank you for answering my questions, that technique does sound extremely useful, especially when looking at many different aspects of the entire coral holobiont as well. Do you think then that this technique could be used before and after the disease takes hold?
DeleteYes, I think in order to fully understand mechanisms of disease we first need to understand how the coral holobiont functions. Presently, we appear to understand what the coral holobiont is relatively well, we're starting to understand some of its functions, next we need to investigate the mechanisms it uses to achieve these functions. A big job in organisms with such a diverse symbionts.
DeleteUnderstanding host function isn't just helpful to disease research it can also be used to identify novel traits with potential bio-applications, such as drug discovery, or as an indicator for responses to climatic changes.