Tropical coral reefs have had a lot research investment to understand
the complex relationship between eukaryotic host and the microbial community.
It is generally accepted that tropical corals host a species-specific microbial
community (Rohwer et al 2002: Bourne & Munn, 2005), which play vital roles
in nutrient gathering/fixing in the oligotrophic waters, along with antibiotic
production and niche filling to prevent disease (Rohwer et al, 2002).
Temperate corals have had much less attention and little is
known about whether they harbour specific communities or why disease prevalence is becoming more apparent?
Here, the author has compared the microbial community of the
temperate coral Eunicella verrocosa during
summer and autumn, as well as between healthy and diseased states to give some
indication to what roles microbes play in this species of temperate coral. Ransome
et al (2014) used denaturing gradient gel electrophoresis (DGGE) and clone
library constructions to build a ‘fingerprint’ of the community structure, and
compared the community’s at different sites, season and health to see if the
microbial community changed. DGGE is an effective semi quantitative method that
is benefitted by being culture-independent, as culture dependent methods are
often limited as few marine microbes are culturable.
Ransome et al (2014) found significant differences in the
community composition between season, and between health states. It is also
documented that a distinct community was found in the water column compared to
the corals; however the water samples were taken eight miles from the coral
site, and no coral were present at this site, meaning the water sample was not
representative. It would be expected that the microbial community at a
different location would be distinct from the coral community. For that
reason I find it difficult to draw any conclusions from this comparison. I
think the author is trying to illustrate that the coral community is often
distinct from water column community, because the corals selects a beneficial
community. This point was well documented in tropical corals by Bourne and Munn (2005) and Richie
(2006).
The author documented a change microbial community, with increased diversity when diseased,
similar to findings in tropical corals reported by Pantos et al (2003) &
Cooney (2002).
During autumn the microbial community was more abundant, and
disease occurred more frequently, autumn (n =6), summer (n=1). It is unknown
what causes the increase in disease, but one mechanism might be increased microbial
activity due to higher nutrients content and dissolved organic carbon during
autumn, highlighted by Smith et al (2006).
The author used three study sites in this paper, which were
very different to one another, with differences in light intensity,
sedimentation and substratum type; without accounting for disease there was a
significant difference in community composition between sites. I think this paper
would have benefitted from having more similar sites to make any differences
caused by season clearer. Or to have had
more repeats to address the variables: light, sedimentation or substratum, and not
used time of year as a variable too.
This paper was the first paper to gain molecular data on the
bacterial community associated with Eunicella
verrocosa in healthy and diseased states, which suggested that Eunicella
verrocosa might harbour a species-specific community. There was a different
and more diverse community found on the diseased coral, which is also observed
in tropical corals, indicating a possible similarity between temperate and
tropical corals, in causative agents of disease.
Some interesting questions have arisen from this paper such
as how does the microbial community change seasonally in this temperate coral,
and is the prevalence of disease in autumn linked to increased temperature or
due to higher nutrient content? If you were to consider the substantial
differences in their environmental conditions, one might expect a substantial
difference. For example, in the tropics
the nutrient content is always low and corals rely on the microbes to
accumulate limiting nutrients. In temperate seas variations occur seasonally,
with nutrient concentrations high in the winter and depleted in the summer. Their
dependence on microbes may vary seasonally, related to the changes in
nutrients.
Primary Reference :
Ransome, E., Rowley, S. J., Thomas, S., Tait, K., & Munn, C. B.
(2014). Disturbance to conserved bacterial communities in the cold‐water
gorgonian coral Eunicella verrucosa. FEMS microbiology ecology.
http://onlinelibrary.wiley.com/doi/10.1111/1574-6941.12398/abstract
Other references:
Bourne, D. G., & Munn, C. B. (2005). Diversity of bacteria
associated with the coral Pocillopora damicornis from the Great Barrier
Reef. Environmental Microbiology, 7(8), 1162-1174.
Cooney, R. P., Pantos,
O., Le Tissier, M. D., Barer, M. R., & Bythell, J. C. (2002).
Characterization of the bacterial consortium associated with black band
disease in coral using molecular microbiological techniques. Environmental Microbiology, 4(7), 401-413.
Pantos, O., Cooney, R.
P., Le Tissier, M. D., Barer, M. R., O'Donnell, A. G., & Bythell, J.
C. (2003). The bacterial ecology of a plague‐like disease affecting the
Caribbean coral Montastrea annularis. Environmental Microbiology, 5(5), 370-382.
Ritchie, K. B. (2006). Regulation of microbial populations by coral surface mucus and mucus-associated bacteria. Marine Ecology Progress Series, 322, 1-14.
Rohwer, F., Seguritan, V., Azam, F., & Knowlton, N. (2002). Diversity and distribution of coral-associated bacteria. Marine Ecology Progress Series, 243(1).
Smith, J. E., Shaw, M.,
Edwards, R. A., Obura, D., Pantos, O., Sala, E., ... & Rohwer, F. L.
(2006). Indirect effects of algae on coral: algae‐mediated,
microbe‐induced coral mortality. Ecology letters, 9(7), 835-845.