Corals in Acidifying Oceans

By: Abbie Strater and Silje van Mierlo


Since the Industrial Revolution, ocean acidity has increased by 30%, 100 times faster than any acidity change that marine organisms have experienced for the last 20 million years. Ocean acidification is the result of increased atmospheric carbon input into the ocean. As more carbon enters the ocean, the chemical balance is shifted. There are fewer carbonate ions that can form calcium carbonate, the foundation of many organisms’ shells and hard parts, including corals. If corals cannot continue building their calcium carbonate skeleton, they face a higher risk of mortality.

“... ocean acidity has increased by 30%, 100 times faster than what has been seen in the last 20 million years.


Figure 1. Carbonate system and Ocean Acidification explained by the University of New Hampshire.  

Within coral’s calcium carbonate skeleton reside dinoflagellate algae, Symbiodiniaceae, which supplies the coral host with organic carbon, amino acids, glycerol, and oxygen. In return, the corals provide the symbionts with protection and nutrient-rich nitrogenous waste. In the event of bleaching, corals will expel these essential algae, which supply them with nutrients and could ultimately starve. Therefore, it is crucial to lower atmospheric carbon emissions to slow the bleaching process.

If this rate of ocean acidification continues, it is hard to imagine how our marine life will adapt to the warmer temperatures. The impact of acidification can be widely seen in coral reefs as they undergo the process of coral bleaching. Bleaching is a stress response to the warmer environment where the coral expels the zooxanthellae from their tissues, causing the coral to expose their white calcium carbonate skeletons.

Figure 2. Brett Monroe Garner / Getty Images 

According to NOAA, during 2014-2017, approximately 75% of the tropical coral reefs worldwide experienced heat stress that was severe enough to cause bleaching. We must understand how corals react to our changing environment and help fix it.

“... during 2014-2017, approximately 75% of the tropical coral reefs worldwide experienced heat stress that was severe enough to cause bleaching.

Figure 3. The effect of ocean acidification on corals. Left shows a healthy reef, while right shows the same reef after coral bleaching.  Credit: Hilke Fischer/Irene Quaile 

As ocean acidification rates rise, the likelihood that corals will experience bleaching rises as well. Along with coral bleaching, ocean acidification also decreases the amount of symbiont densities in coral populations. This reduction in symbiont densities will decrease the nutrient input of corals and make them more susceptible to bleaching. However, this increase in coral bleaching is not an instant death sentence, it will increase the mortality of coral reefs and change our environment as we know it.


In Jiang L et al. study, the researchers wanted to learn more about the physiological and biochemical responses of corals to fluctuations in carbonate chemistry under ocean acidification. To answer this question, they exposed the newly planted coral (Pocillopora damicornis) to the ambient pressure of carbon dioxide, steady and elevated pressures of CO2 (stable Ocean Acidification), and diurnally fluctuating pressures of CO2 under future ocean acidification scenarios (fluctuating OA). They also measured the photo-physiology, growth, budding rates and calcification, and the effect of Carbonic anhydrase, Ca-ATPas and Mg-ATPase effect on juvenile corals. The results from this experiment showed that the effects of coral bleaching could not be counteracted by active H+ pumping. The fluctuating OA still showed a 50% decrease in asexual budding. The research suggests that the diurnally fluctuating ocean acidification has an overriding effect on the success of growing new corals.

Figure 4. Effect size (Hedge’s d) of change in symbiont density per unit of coral surface area versus DpCO2 (the difference between the pCO2 of the acidification treatment versus the control treatment); n = 38. The line presented is a weighted least-squares regression line with the equation: d = -0.1 - 0.0007 9 DpCO2 (adjusted r2 = 0.24). Shows trend of CO2 presence to different coral surface areas tested in various studies. Some areas were affected by acidification more than others. Graph and mathematical values taken from Mason RAB 2018. 

In the Mason RAB study, the author found 38 different effect sizes to compare into one graph. These data points infer that symbiont densities decrease by 0.07 standard deviations for every 100 micro atmospheres increase in seawater pCO2. After putting a large number of studies into one place, it can be concluded that ocean acidification generates a reduction in symbiont densities in tropical and subtropical corals.


Learn More (Citation):

Mason RAB (2018). Decline in symbiont densities of tropical an d subtropical scleractinian

corals under ocean acidification. Coral Reefs. 37(1): 945-953.


Jiang, Lei, et al. “Diurnally Fluctuating pCO2 Modifies the Physiological Responses of Coral Recruits Under Ocean Acidification.” Frontiers, Frontiers, 22 Dec. 2018, doi.org/10.3389/fphys.2018.01952.




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