By Anthea Phalosa –  

Coral reefs are a major source of attraction in the tropics, in places like Indonesia or Australia’s Great Barrier Reef. However, these biodiversity hotspots are facing a threat ultimately linked to human activity [1]. Anthropogenic activity such as the increasing use of fossil fuels have contributed to global scale warming and rising temperatures as atmospheric carbon dioxide (CO₂) concentrations continue to increase [1].

Coral seascape in Wakatobi, Indonesia. Fakhrizal Setiawan, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Now what does the air have to do with coral reefs? Well for starters, our oceans are a major store of CO₂, storing around 30% of atmospheric CO₂ [2]; CO₂ in the atmosphere dissolves in the ocean to form carbonic acid and undergoes a series of processes resulting in the increase of hydrogen ions. More hydrogen ions in the water indicates increased acidity associated with a lower pH level, otherwise known as ocean acidification [2]. 

The effects of ocean acidification are greatly felt by organisms that rely on the process of calcification for their structures, such as corals or molluscs. In short, calcification in corals involve using calcium carbonate (CaCO₃) to form their skeletons. However, with increasing atmospheric CO₂, the carbonate ions that would have been used to form CaCO₃ for corals’ skeletons are now reacting with the excess hydrogen ions previously mentioned [2]. Research by Foster et al. suggests that increasing ocean acidification leads to structural deformities in young coral skeletons. 

Foster et al.’s article investigated the link between the CO₂ involved in gas exchange with the water (PCO₂) and ocean temperature with the quality of coral skeletons [4]. The quality of coral skeletons under different conditions were examined using microscopic tools: high-resolution 3D X-ray microscopy and scanning electron microscopy. It was found that the corals were unable to build durable skeletons under high-PCO₂ conditions, its density was affected, resulting in porous and fragile structures, some were even missing components [4]. For example, compared to the 6 tertiary septas found in control conditions, high-PCO₂ conditioned corals were found to have asymmetrical, missing, stunted, or overgrown septas. Additional fractures and gaps were also commonly found in high-PCO₂conditions [4]. These findings reinforce the detrimental effects of ocean acidification on calcifiers.

Despite this, the effects of temperature increase have varying results in other studies investigating adult corals, which may instil uncertainties on its validity [4], although some have reported reduced skeletal density in adult corals [5]. Juvenile corals are at their early life stages, which translates to a more sensitive and vulnerable system. However, the growth of juvenile corals is significant in that it ensures that the species continue to flourish and avoid extinction. A weakened skeleton may even compromise corals’ ability to adapt to disturbances such as storms or coral bleaching events [4].

Coral reefs are a beacon for marine biodiversity, with many organisms relying on its structures as habitats, indicating the urgency of its conservation. With current “business-as-usual” scheme, atmospheric CO₂ is expected to rise at an alarming rate [1,4], which corresponds to increased ocean acidification, raising greater concern over disappearing reefs and biodiversity.

References and further reading

[1] Masson-Delmotte, V., Zhai, P., Pirani, A. & Connors, S. L., 2021. Climate Change 2021 The Physical Science Basis Summary for Policemakers, Switzerland: IPCC.

[2] NOAA, 2020. Noaa.gov. [Online] Available at: https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-acidification [Accessed 28 November 2021].

[3] Ford, P., n.d. Australian Online Coastal Information. [Online]  Available at: https://ozcoasts.org.au/indicators/biophysical-indicators/water_column_partial_pressure/ [Accessed 1 December 2021].

[4] Foster, T., Falter, J. L., Mcculloch, M. T. & Clode, P. L., 2016. Ocean acidification causes structural deformities in juvenile coral skeletons. Science Advances, 2(2).

[5] Mollica, N. R. et al., 2018. Ocean acidification affects coral growth by reducing skeletal density. PNAS, 115(8), pp. 1754-1759.