Coral Microbiome Health: There’s no probiotic yogurt for that


The Paper: Ziegler M, A Roik, A Porter, K Zubier, MS Mudarris, R Ormond & CR Voolstra. 2016. Coral microbial community dynamics in response to anthropogenic impacts near a major city in the central Red Sea. Marine pollution bulletin 105:629-640.


The Expanding Importance of The Microbiome

It is nearly impossible to read an article or watch a program on human nutrition without hearing about how important it is to maintain a healthy microbiome. An individual’s microbiome is the community of microscopic organisms (bacteria, fungi, viruses, etc.) that thrive in and on our bodies and help us fight disease, digest food, and even remove waste products to allow more efficient use of the food we eat. A degraded microbiome has been associated with numerous diseases such as multiple sclerosis, diabetes (types 1 and 2), allergies, asthma, and even cancer. Because of this, scientist have worked extensively to accurately document the human microbiome, what makes it healthy, and how to use this information as an indicator of possible health problems before they become serious.

Figure 1. Healthy coral reef (Source: Zak Kerrigan)
Figure 1. Healthy coral reef (Source: Zak Kerrigan)

Only recently have scientists gone beyond humans to study the microbial world living in and on other organisms, and it is becoming increasingly evident that we have only scratched the surface of how important microscopic organisms are to our macroscopic world. In this case, a team of scientists from Saudi Arabia and the United Kingdom, led by Dr. Maren Ziegler, have conducted a study on coral communities in the Red Sea. They have studied the microbes (specifically bacteria) that reside on two different coral species under varying degrees of human impact (sewage discharge, municipal water runoff, and ongoing coastal construction) in order to determine what effect we may have on the health of coral reefs, and how to mitigate further loss of corals to disease and bleaching. Just as our microbiome is affected by the things we put into our bodies, a coral’s microbiome is affected by the water in which it lives and just as in humans, a degraded microbiome can be an indicator of something far more serious.

The Science

For this study, Dr. Ziegler and team selected two species of stony corals that are abundant in the Red Sea at three different locations near the Saudi Arabian city of Jeddah. Importantly, this is the first study to look at corals that appeared to be healthy, with no signs of disease or bleaching, among a community where other corals show signs of distress. The scientists were able to define the bacterial community that lives at these locations by analyzing the DNA from the corals and surrounding waters. Bacteria can only broadly be distinguished by looking at them under a microscope (also a very time consuming and painstaking process), but by analyzing the organisms’ DNA, it is possible to tell them apart down to a species-like level. Additionally, new techniques are allowing scientists to explore the ever-expanding microbial world, discovering hundreds to hundreds of thousands of bacterial species in very small samples.

Figure 2. Bacterial community of two stony coral species (Acropora hemprichii and Pocillopora verrucosa) under varying human impacts. Corals begin to lose their bacterial “specificity” under these conditions. (Source: Zeigler et al., 2016)

In general, a coral’s microbiome is species specific. This means healthy members of a single coral species will have very similar communities of bacteria across multiple geographic locations, especially the most dominant bacterial species. These bacteria aid the coral by performing metabolic functions such as cycling of nitrogen and sulfur, as well as aiding the coral in adapting to changing environmental conditions. In general, studies indicate that a properly functioning microbiome is essential for coral health and immunity to disease. Previous studies have found that on diseased coral, this community breaks down. The microbiome is no longer coral-species specific, and opportunistic bacteria are able to take over and push aside the beneficial organisms, leaving a very diverse microbial population that is similar across all diseased corals regardless of species and geographic location.

In this study, Dr. Ziegler and her team found that the bacterial communities of the otherwise “healthy-looking” corals in environments impacted by pollutants more closely resembled that of diseased corals rather than typically healthy ones. They had lost their “core” bacterial community and had been taken over by some of the more opportunistic species, that arguably provide no benefit to the coral. This was evident from the fact that the two coral species began to have similar microbiomes, whereas in the samples taken from sites not impacted by human pollutants, the corals maintain a high level of bacterial specificity (Figure 2).

Why Is This Important

Figure 3. Left is a healthy fire coral, while on the right is a fire coral that has “bleached” itself due to environmental stressors. (Source:
Figure 3. Left is a healthy fire coral, while on the right is a fire coral that has “bleached” itself due to environmental stressors. (Source:

Just as a shift in the composition of the human microbiome can be an indicator of something more serious, it appears that by looking at the microbiome of corals we may be able to predict declining health of the organism. Caught early enough, it may be possible to prevent widespread coral bleaching and disease (Figure 3) by minimizing human impacts to the affected areas.

This study has also shown how directly humans are impacting the world they live in. In some of the samples taken from areas affected by wastewater runoff, the scientists were able to find bacteria from both the human skin and gut infecting these corals. It is quite difficult to ignore or refute the impact humans are having on this planet against such strong, scientific evidence.

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