Do temperate corals rely on their microbes to the same extent as their tropical cousins?

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.