Coral reefs are a highly complex and diverse habitat, supporting a wide array of marine species. These particular structures also play an important role in the global economy through their involvement in different industries, especially fishing and tourism. However, over the past several decades, the influx of anthropogenic carbon dioxide (CO2) as a result of mass fossil fuel combustion has resulted in the global degradation of these economically and ecologically crucial habitats.
The key process that has resulted in this extensive damage to coral reefs is termed ocean acidification, which is defined as the increase in oceanic acidity as a result of the uptake of atmospheric CO2. Over the past 200 years, more than 30% of all CO2 that has been emitted to the atmosphere as a result of anthropogenic activity has been absorbed by the oceans (Anthony et al. 2008). Such drastic changes in oceanic pH are having multiple adverse impacts upon coral reefs, with many of these effects expected to propagate throughout a multitude of marine taxa.
Calcification As CO2 is dissolved by the oceans, a sequence of chemical reactions is initiated as a result. The CO2 that is taken up by the oceans readily combines with water molecules (H2O) to produce carbonic acid, which easily dissociates into two end products; bicarbonate (HCO3-) and hydrogen ions (H+). The concentration of H+ ions in a substance determines its acidity, therefore as an increasing amount of CO2 is dissolved by the oceans and the concentration of H+ ions rises, the acidity of the ocean increases.
A direct consequence of this chemical reaction is a reduction in concentration of carbonate ions (CO32-) available for calcification and corals located within the photic zone are hypothesised to expend more energy during calcification (Guinotte and Fabry, 2008). Increasing oceanic acidity has been proven to have an adverse impact upon the calcification rates of hermatypic corals (containing photosynthetic zooxanthellae). Evidence from experiments undertaken to date shows that calcification rates in tropical reef-building corals in double preindustrial CO2 concentrations decrease between 20-60% (Kleypas et al. 006). Such a drastic reduction in calcification rates will alter the structure and function of coral reef structures, as the rate of erosion of coral reefs is likely to exceed the rate of accretion (Guinotte and Fabry, 2008). The magnitude of reduction in accretion rates will become critical in the maintenance of photosynthesis in zooxanthellae, as light intensity is already a limiting factor towards the deepest reaches of the photic zone (Guinotte and Fabry, 2008).
Combining this reduction in accretion with global sea level rise, reef-building corals that reside towards the bottom of the photic zone may struggle to calcify and grow quickly enough to remain within this photosynthetic region of the ocean. Zooxanthellae may not be able to capture adequate light to maintain photosynthesis as a result, disrupting the symbiosis between the coral polyps and the dinoflagellates harboured on and within the polyp tissue. Despite being devoid of photosynthetic zooxanthellae and hence not bound to the photic zone, cold water corals are still suffering as a result of increased ocean acidity.
Hermatypic corals are experiencing the impacts of a combination of reduced accretion rates, increased erosion and sea level rise, however cold water corals are also experiencing reduced calcification rates as a result of reduced carbonate ion saturation. Carbonate ions occur in two polymorphs; calcite and aragonite, which are produced as a result of biogenic calcification (Guinotte and Fabry, 2008). The stability of these two compounds is heavily dependent upon the concentration of dissolved Co, in seawater, which is also partially dictated by temperature.
Since colder waters sequester Co, more readily than warmer waters, water column acidity increases with depth. The depth of calcite and aragonite saturation horizons is important to reef-building corals as water below the saturation horizon is under-saturated, therefore dissolution will occur at depths below this boundary (Guinotte and Fabry, 2008). The rapid influx of anthropogenic CO2 is causing the migration of calcite and aragonite saturation horizons into shallower waters, resulting in the under-saturation of calcite and aragonite at shallower depths than normal.
Consequently, reductions in calcification rates will occur in corals below these horizons. Some bioherm-forming scleractinian corals (e. g Lophelia pertusa, Madrepora oculata, Goniocorella dumosa, Enallopsammia profunda, and Solenosmilia variabilis) all form calcium carbonate skeletons using aragonite (Guinotte and Fabry, 2008) and the upward movement of the aragonite saturation horizons will pose major problems for calcification rates. As a result, it is likely that cold water corals will experience the effects of ocean acidification before the shallower hermatypic corals.
Bleaching and productivity Hermatypic corals can also experience a process known as bleaching if exposed to the appropriate stressors. Bleaching is a process by which the symbiotic zooxanthellae contained within coral polyps are expelled from the coral tissue. This process is widely characterised by a loss in colour of the coral reef system, with the coral tissue taking on a white appearance. Although coral bleaching is a phenomenon normally associated with thermal stress, an increase in ocean acidity has also been shown to instigate the expulsion of zooxanthellae in a study performed by Anthony et al. (2008).
Anthony et al. (2008) revealed that exposing the crustose coralline algae (CCA) Porolithon onkodes and the coral species Acropora intermedia to an intermediate dosage of CO2 (high category IV, 520-700 ppm according to the IPCC, 2007) resulted in a 30% and 20% increase in bleaching respectively. These results support the hypothesis that coral bleaching is not solely caused by an increase in temperature and that increasing oceanic pH may also contribute to this process. It is also possible that these two physical changes could act in synergy, hence increasing the potential for rapid coral bleaching on a global scale.
In addition to a loss of colour pigmentation, bleaching will also directly impact primary productivity in corals. With CO2 being one of the crucial reactants required for photosynthesis however, an increase in net primary productivity in corals could be expected. The investigation carried out by Anthony et al. (2008) revealed an increase in the productivity of A. intermedia under the intermediate regime (pH 7. 85-7. 95), however the primary productivity of both Porites lobata and CCA P. onkodes was suppressed. The increase in productivity of A. ntermedia could be the result of increased CO2, which is the primary substrate required for photosynthesis (Anthony et al. 2008), however this would not explain the suppression in productivity in P. onkodes and P. lobata as both of these organisms are also photosynthetic. Alternatively, a reduction in the symbiont population within A. intermedia may result in an increase in the efficiency of photosynthesis in the remaining zooxanthellae (Anthony et al. 2008). Fertilisation Generally, the primary method of coral reproduction is asexual.
However sexual reproduction is critical for the maintenance of genetic diversity, determining the structure of the reef system and rebuilding reefs after all manner of disturbances. Sexual reproduction in corals occurs in two ways; brooding and broadcast spawning. Brooding involves internal fertilisation within conspecifics using sperm that had been released into the water column, whereas broadcast spawning involves external fertilisation of gametes (Harrison, 2011). Many of the known coral species are hermaphroditic, releasing gamete bundles into the water column (Albright and Mason, 2013).
Since fertilisation generally occurs externally, this leaves gametes particularly vulnerable to the physical conditions of the water column, with dissolved CO2 being one of these variable factors (Albright and Mason, 2013). An experiment carried out by Nakamura and Morita (2012) shows a reduction in sperm motility in Acropora digitifera in response to an elevated dissolved CO2 environment, however the results generated by one species are not representative of coral species as a whole. The success of fertilisation under elevated dissolved CO2 conditions is highly variable and could depend upon a multitude of factors.
For example, corals that are sperm-limited are more likely to experience a reduction in fertilisation success in an elevated dissolved CO2 environment (Albright and Mason, 2013). Since ocean acidification occurs in conjunction with a rise in atmospheric temperatures, accurately predicting and documenting the impacts of elevated CO2 environments on the fertilisation success of coral species is difficult. Conclusion In hindsight, ocean acidification is having profound and adverse effects upon a multitude of processes that occur in coral species.
That being said, ocean acidification is often referred to as the “evil twin” of climate change as this particular process is associated with an increase in global atmospheric temperatures. Therefore it is widely unknown whether or not the impacts documented as being the result of ocean acidification are occurring synergistically with other changing physical conditions, namely temperature. Regardless, there is an overwhelming stockpile of evidence that proves that ocean acidification is having a wide range of negative impacts upon the physiology and process in coral reefs.