The term “Ocean acidification” is used to describe the decrease in ocean pH. This is brought on by an increase in COemissions in our atmosphere that are absorbed by the ocean, causing the ocean to become more acidic. This decrease in pH causes the chemical composition of the ocean to change, for example two substances known as calcite and aragonite decrease in saturation as the ocean’s become more acidic. unfortunately for many marine organisms they rely on calcite and aragonite to build a variety of body structures vital to their survival, such as molluscs, echinoderms and coral species. Both calcite and aragonite will potentially become undersaturated during the 21st century as CO2 emissions increase.  During the last 250 years, carbon emission levels have increased by 40% and is hypothesised to keep increasing unless Carbon emissions are reduced drastically. With atmospheric CO2 levels reaching over 400ppm (Parts per million) the earth is experiencing the largest level of CO2 emissions since approximately 800,000 years ago.

The YouTube video below gives a brief two minute overview on Ocean acidification and how it will affect marine life:

What is coral ?

Anatomy of a Coral polyp, diagram take from: NOAA – NOAA website

To understand how ocean acidification affects these organisms we must first understand what coral is. Coral, primarily stony coral (scleractinian) is composed of calcium carbonate, this is a material the organisms can produce from calcite and aragonite. The structure formed from this material encases the coral polyp (the organism itself) providing the organism with protection from predators and other marine stresses. Fully formed coral we see in the ocean can consist of a large number of polyps all living together in one calcium carbonate structure. Due to the effects of ocean acidification the substances coral need are rapidly declining meaning these organisms will find it harder and harder to produce their calcium carbonate skeletal structures.


Effect of Ocean acidification on coral

Cauliflower coral (Stylophora pistillata)    Taken from: commons.wikimedia, Photo by, User:Haplochromis

Recently major research has gone into how ocean acidification will affect coral species, one specific area being the calcification rate of coral. Calcification rate is the rate at which a polyp can produce calcium carbonate from calcite and aragonite, scleractinian coral being capable of producing a large amount of calcium carbonate making them perfect candidates for such research. Due to calcite and aragonite saturation decreasing with increased acidity, it is believed production of calcium carbonate in coral will decrease, decreasing calcification rates. This decrease in calcification has been monitored during scientific investigation in many coral species including Acropora.sp and Stylophora pistillata (Cauliflower coral).  Reports from the early 2000’s suggest within the 21st century we will start to witness decreased calcification of 40% due to ocean acidification.  With decreased calcification in coral we will also see a decrease in the size with coral species becoming stunted due to lack of calcium carbonate production. This was shown in a study performed in 1999 where homotypic coral Porites compressa was placed under two pH levels 8 and 7.2 to simulate increase ocean acidification. It was found that the growth rate of coral subjected to pH 7.2 was significantly lower by half the growth rate of coral subjected to pH 8, providing evidence towards ocean acidification negatively affecting growth rates.  Overall in the future we will see a decrease in coral diversity, population and overall fitness of coral reefs and individual coral species due to rising CO2 levels and acidity of our oceans.  Although this seems to be the case with the majority of coral species, recent findings have shown some species of coral may eventually be able to acclimate (Adapt) to the increased Ocean acidification over long term exposure, allowing certain species to thrive while others decline. Therefore, we may not lose all coral species, but the diversity of our reefs will surely decline. One recent study from 2012 suggests the cold-water coral species Laphelia pertusa can acclimate to long exposure to increasingly acidic environments. During the study high CO2 effects were tested on for both short term (1 week) and long term (6 months) exposures, short term exposure resulted in a decline of 26-29% in calcification rates with a pH decrease of 0.1, whilst long term exposure resulted in a constant considerably high calcification rate. This provide some fairly recent evidence to support the idea that some coral species may be able to acclimate to increased ocean acidification.

Coral Bleaching

Coral bleaching can be associated with ocean acidification due to the dramatic change in pH as this process occurs in coral species under any significant stress from changes in temperature, salinity and pH. Coral bleaching itself is the process in which coral colonies become severely stressed and lose their colouration becoming a white skeletal structure, either due to the loss of pigment in microscopic algae known as zooxanthellae or that the zooxanthellae has been expelled all together. Zooxanthellae have a symbiotic relationship with coral as they use the calcium carbonate structure for protection while providing the coral with nutrients via photosynthesis, so with them gone the coral they inhabited cannot use photosynthesis as a means of providing itself with nutrients. previously mentioned one environmental stress that can cause this process to occur is a change in pH notably a decrease known as ocean acidification.

Example of Healthy coral specimen (left) against fully bleached specimen (Right) Photo taken by: Smithsonian’s National Zoo

In general, the majority of coral species on this planet will be affected by ocean acidification either from understaturation of necessary materials to build their calcium carbonate structure or the loss of their symbiotic partners who are detrimental to providing colonies with enough nutrients to thrive. Some species may be able to adapt to the quickly changing environment, but this will be at the cost of the amazing diversity we have in our oceans if measures are not put in place to combat ocean acidification.



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