Bleaching and Bacteria: (Not) A Microscopic Problem


Coral Reef at Palmyra Atoll National Wildlife Refuge” by Jim Maragos/U.S. Fish and Wildlife Service is licensed under CC 2.0

Coral bleaching in Chagos” by Mark Spalding/World Research Institute is licensed under CC 2.0








When we hear the phrase “coral reef,” the first thing that comes to mind is a rolling, rainbow expanse of life and color that shocks the eye (top image); we don’t imagine white, underwater boneyards blanketed in algae (bottom image). Unfortunately, due to widespread human-induced climate change, the former, bleak image may be all that’s left to see in the coming decades; that is, if we as a society do not take action. According to the National Oceanic and Atmospheric Association, this occurrence, known as coral reef bleaching, results primarily from increased water temperatures, a direct effect of humans on the environment (NOAA).

Why, you might ask, does something as naturally variable as ocean temperature have such an impact? The answer, scientists have found, lies in the coral microbiome; various algae provide food for the coral, and give it color (NOAA). Bleaching occurs when a stressor (such as heat) is introduced, which can expel the algae and turn the organism a shade of pale white. However, this does not necessarily mean that the coral will die. Recent studies have found that another microbial culprit plays the primary role in coral survivability: bacteria.

A June 2016 Science article by Drs. Tracy Ainsworth and Ruth Gates of James Cook University explicitly addresses the growing significance of bacterial diversity in the coral microbiome. They essentially assert that both during and after a bleaching event, the only way for coral to recover their symbiotic algae, restore coloration and consequently survive is with a healthy, functionally diverse microbiome (Ainsworth and Gates).

The bacteria inhabiting coral can be either positive contributors to overall health and a cohesive microenvironment, or pathogens that decrease the ability of the organism to respond to stress. While most living coral contain sizable populations of both, Ainsworth and Gates found in a sample population that survivors of bleaching had significantly fewer types of pathogenic bacteria present than their deceased counterparts; coral that were killed by bleaching had a disproportionate amount of pathogenic bacteria.

A combination of water pollution and rising temperatures, two consequences of the daily expenditures of humans, severely endangers the survivability of “good” bacteria in coral. Without their presence, or sufficient microbial diversity in general, reef ecosystems have no realistic chance to combat any degree of external adversity. Specific negative effects of imbalance include immune system deficiency and failure to maintain algal food production. Absence or malfunction of beneficial microbes also, as Ainsworth quotes, can even “have intergenerational impacts and affect ecosystem stability,” potentially more far-reaching concerns (Ainsworth and Gates).

These predictions, in conjunction with current climate trends, do not bode well for the long-term success of coral reef ecosystems. This is why I believe it is essential that we, as tenants of Earth, do everything we can to combat global warming on both a broad and local scale. Some argue that change is not feasible, that there is no one-step process; the complexity of the world ensures that stressors are constantly present, even once addressed. This line of thinking is part of the problem; every individual action we take has a quantifiable impact, and together we can build a movement toward noticeable improvement.

It is quite evident that coral endangerment on a microbial level can compound quickly, and ruthlessly. It is a cycle of sorts, in that warming water degrades the function and diversity of coral microbiomes, which in turn jeopardizes the ability of the organism to survive through future warming or hazardous conditions. This feedback loop lies at the root of recent, unprecedented wide-scale bleaching events, and furthering our understanding of bacterial and algal symbiosis with coral will aid in the search for solutions to this problem. A coral reef that is bleached cannot harbor the fantastically diverse ecosystem of fish and other marine organisms that we have come to expect and enjoy, and the opportunity and responsibility to preserve this majesty lies in the hands of the average citizen (YOU!). Whether it’s biking instead of driving to work, remembering to recycle or voting for pro-climate political candidates, we all can make a difference in the fight against bleaching. The coral microbiome needs all the help it can get, and limiting its stressors is the best way to start.



Ainsworth, Tracy D., Gates, Ruth D. “Corals’ microbial sentinels.” Science, 24 June 2016. Vol.

352, Issue 6293, pp. 1518-1519.

“What is coral bleaching?” What is coral bleaching? – Ocean Facts. National Ocean Service –

U.S. National Oceanographic and Atmospheric Administration, n.d. Web. 5 Sept. 2016.

This entry was posted in Conservation Biology Posts, Conservation Editorials 2016. Bookmark the permalink.

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