How Microbial Community Composition Regulates Coral Disease Development.

 

Reef coral is in rapid decline worldwide with the predominate cause being bleaching (the expulsion of photosynthetic symbionts) and infectious disease. An important factor associated with coral bleaching and disease is a shift in the microbial community composition in the coral mucus layer. The resident microbial community of coral plays an important role in the healthy functioning of the coral host and provides antibiotic resistance to pathogens. During environmental stress such as increased temperatures the resident host bacterial flora can become vulnerable to competition and overgrowth by pathogens in particular Vibrio spp.

In the study by Mao-Jones et al (2010) mathematical models were used to explorer the microbial community dynamics and how the surface microbial community responds to changes in environmental conditions and under what circumstances is it vulnerable to pathogen growth and whether it can recover. In the experiment the antibiotic activity and competition between beneficial and potentially pathogenic microbes such as Vibrio spp are key in understanding the community dynamics. A number of different models were run to assess how different factors influence the dynamic and whether they promote growth by pathogenic microbes or not. For full details of the methods and models you will need to read the materials and methods section of the paper.

The study’s key findings showed that under competition for a single limiting substrate, control of pathogens through antibiotic activity of the beneficial bacteria is the primary defense for the exclusion of pathogens. Under the assumption of the known effect of heat to reduce antibiotics effectiveness the model predicted a rapid switch from beneficial microbe dominance to pathogen dominance during temperature increases. The study also showed that when normal temperature conditions return the pathogen remains dominant and for the beneficial bacterial to return a conditions must be unfavorable for the pathogens. This means that brief rises in ocean temperatures above the normal can change the microbial composition in coral for long periods of time consequentially leading to coral bleaching and disease. These observations are consistent with observational studies on corals and explained in the example below.

 

 

 Effects of a brief thermal anomaly on microbial population dynamics in the well-mixed SMC model.

(A) NOAA sea surface temperature record for Glover's Reef, Belize (from coralreefwatch.noaa.gov/satellite/data_n​rt/timeseries/all_Glovers.txt).The open circles show temperatures considered high enough to elevate the risk of coral bleaching; the dashed curve is the fitted seasonal trend (a periodic smoothing spline) used to simulate the model (B). Simulations of the model using the seasonal temperature trend plotted in panel (A), but with a 2-wk thermal anomaly (indicated by the vertical dashed lines) during which temperature was elevated by 1 degree C, and antibiotic activity by beneficials was eliminated.

The graphs show how a brief 2-week thermal anomaly of a rise of just 1 degree c can decrease the antibiotic effect and allow the coral microbial community to change to predominantly pathogens. The graph also shows how once the normal temperature is restored the beneficial bacterial are not able to regain their dominance until unfavorable conditions for the pathogens occur.

Overall the paper presents models that give insights into the current problem in the decline of coral cover. In particular high lighting the importance in the microbial community of the coral. The findings are particularly important in regards to the expected temperature rise in the world’s oceans. Further research needs to be done to prevent the shift to pathogen dominance when stress factors occur. Ideas such as improving water quality or treating coral waters could be potential ideas that could reduce the risk.

Mao-Jones J, Ritchie KB, Jones LE, Ellner SP (2010) How Microbial Community Composition Regulates Coral Disease Development. PLoS Biol 8(3): e1000345. doi:10.1371/journal.pbio.1000345