Effects of Ocean Acidification on Deep-Sea Corals
As humans continue to release greenhouse gases, notably CO2, our oceans are becoming increasingly acidic. While all marine organisms are affected by ocean acidification, organisms with carbonate shells, like corals, are particularly vulnerable. Scientists often look to coral reefs found in shallow, coastal waters as a convenient way to study the effects of acidification. However, new research suggests that deep, cold-water corals are even more vulnerable to acidification than their shallow-water counterparts.
But first, what is ocean acidification? At its root, ocean acidification is caused by an excess amount of carbon dioxide (CO2) in the atmosphere. Human activities like cutting down trees, producing cement, and burning fossil fuels all emit lots of CO2. This atmospheric CO2 mixes with the surface of the ocean and produces a weak acid known as carbonic acid (H2CO3). This weak acid then further dissociates and releases hydrogen ions (H+) which renders the water more acidic. This series of reactions also produces bicarbonate ions, and uses up carbonate ions, which corals need to build their shells. And even though the carbon enters the ocean through the surface, it makes its way down to the very depths of the ocean, so not even deep-sea corals are safe. In fact, deep-sea corals are even more vulnerable to acidification than their shallow-water counterparts.
Now, let’s understand the similarities and differences between shallow-water corals and deep-water corals. Both build carbonate skeletons and can form reefs, and therefore rely on oceans to have a delicate carbonate chemistry balance. But that is where the similarities end. The most obvious difference is in terms of appearance. Deep-water corals are often bright white in coloration, unlike the colorful reefs we see in shallow waters. This is because shallow, reef-building corals have a symbiotic relationship with photosynthetic algae called zooxanthellae, which live in their tissues. This algae is what gives corals their vibrant colors, so when the ocean gets too warm and the corals get stressed, the algae is expelled, resulting in coral bleaching. So while deep-sea corals are exempt from coral bleaching because they don’t possess this photosynthetic algae, they are greatly affected by climate change’s evil twin, ocean acidification.
Temperature and sunlight is not the only difference between shallow and deep waters, however. Deep waters also contain more carbon dioxide, because respiration becomes dominant over photosynthesis. This means that deep waters are naturally more acidic than surface waters, and less carbonate is present. So while shallow-water reefs typically don’t dissolve, because the surrounding water is already saturated with calcium and carbonate, deep-water reefs dissolve easily due to the lack of carbonate. This means that in the deep ocean, there is a depth at which shells dissolve and the water is known to be undersaturated with respect to the mineral form of carbonate, aragonite. This depth is known as the aragonite saturation horizon (ASH), which is the depth at which the saturation state ΩArag = 1. Ocean acidification is causing this saturation horizon to become shallower, therefore putting deep-sea corals first in line to experience the consequences. So while ocean acidification is causing both shallow-water reefs and deep-water reefs to experience reduced aragonite saturation, only deep-water reefs will be permanently exposed to corrosive waters.
Another unique component of deep-sea corals is that they build their structures off of dead skeletons, which are common because of the harsh environment at depth. Undersaturated waters weaken coral structures, causing them to be more porous and fragile. This phenomenon has been coined “coralporosis,” as it mimics osteoporosis in human bones. This can cause collapses of entire coral reefs and the ecosystems that rely on these corals.
The collapse of deep-sea corals doesn’t only affect marine organisms, however. New research has shown that these corals can be beneficial to humans, helping to treat diseases such as cancer, arthritis, Alzheimer’s and skin conditions. Additionally, scientists recently discovered that two sponges growing in deep-sea coral ecosystems have compounds with anti-inflammatory and anti-viral properties. Another deep-water sponge was found to have anti-tumor properties that can be used to fight human lung and breast cancer.
Yet as CO2 emissions continue to increase, ocean acidification continues to directly harm deep-sea corals. What’s more, is that ocean acidification isn’t even the only anthropogenic cause of deep-sea coral deterioration. Organisms on the seafloor, including deep-sea corals, are largely affected by harmful fishing practices, like bottom trawling, which can wipe out entire ecosystems at a time. Deep-sea corals are also not exempt from plastic pollution, as microplastics make their way down to the deep sea and get lodged into all kinds of organisms. Deep water oil drilling is another way humans interfere with deep-sea corals, as these corals are often found near areas where oil and gas naturally seep out of the sea floor.
Ocean acidification has profound effects on all marine organisms, but especially deep-sea corals. Though they are often thought of as “out of sight, out of mind,” they present a multitude of benefits to marine and terrestrial beings alike. What’s more, studying these creatures can provide valuable insight into how ocean acidification works in the deep sea and how it affects deep-corals differently than shallow-water corals. This insight can then be used by scientists to determine the timeframe we have before the entire ocean collapses as a result of human activity.
References
Hennige, S. J. et al. Crumbling Reefs and Cold-Water Coral Habitat Loss in a Future Ocean: Evidence of “Coralporosis” as an Indicator of Habitat Integrity. Frontiers (2020). Available at: https://www.frontiersin.org/articles/10.3389/fmars.2020.00668/full.
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