It is without a doubt that climate change (or rather, 'global warming') whether anthropogenic or naturogenic, can have a major negative impact on coral reefs. This is not to say that it will last forever, or that it is not part of a climatic cycle, but nonetheless, it is clear that we are seeing increasing temperatures, on average. 'Coral bleaching' is an occurrence whereby corals 'white out' - i.e. when they lose their endosymbionts, the zooxanthellae. When this happen, corals lose their source of photosynthesized energy and various chemicals obtained from their mutualistic algae. For corals that are highly reliant on their symbionts, this can only spell death.
At the current global temperature range, the effects are already very clear across the world. Large swaths of reefs are disappearing, areas of immense biodiversity now nothing more than rocks and a few hardy scavengers here and there. Rising temperatures has more of an effect on corals than first seem, especially when rising temperatures also lead to increasing CO2 levels and ocean acidity.
However, not all corals are made the same, and recent studies have found something quite surprising - corals that actually fair better despite ocean acidification and warming.
The team at the Department of Marine Sciences, University of North Carolina has discovered something quite remarkable about a certain coral, Siderastrea siderea.
The full paper can be found here.
S. siderea
It is amazing news that something remarkable has been discovered about S. siderea. This coral is only one amongst many different species of coral that can be found in the Carribean sea. However, it is important in the fact that not only is it abundant, but is also hermatypic, which means that it contributes directly to building up the reef.
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S. siderea are the brown/tan corals. Not exactly the prettiest of corals, but who cares.
The genus (also called the 'starlet corals') include five species, with S. siderea and S. radians being the better known two. Respectively, they are called the 'massive startlet' and 'lesser starlet' corals, and no surprise in so. S. siderea can grow to a huge size, as much as two meters. Described by Ellis and Solander in 1786, they have yet to really enter the marine aquarium hobby. Though possibly good that they don't, for we need more of these in the wild.
The Experimental Results
Through subjecting the coral to various CO2 levels (pCO2 = 324, 477, 604, 2553 µatm) and temperatures (25, 28, 32°C), it was found that up to pCO2 of 604 µatm and/or a temperature of 28°C, calcification rate (rate of calcium carbonate deposition) increased, which is higher than what is tended to be suggested for corals. Above the aforementioned levels, calcification is past optimal and starts to decline.
However, it is interesting to note that regardless of the parameters tested, calcification still occurred - indicating an ability to produce their calcium carbonate skeleton. The research did indicate though, that this could just simply be a short-term effect, when corals can still work from their reserve.
What is most important here, is the fact that this coral's optimal calcification rate is at temperatures and acidity higher than that of many previous species of coral tested. In the end, we have to remind ourselves that corals are a diverse group of organisms, each unique in their own way, and therefore there is no way to 'catch all' with a simple parameter definition.
Calcification Rates from Photosynthesis
The author is this research paper provides an explanation for this unlikely news. Coral calcification is impacted by ocean acidication in two opposite ways. Whilst acidification reduces the seawater calcium carbonate concentration (and hence harder for calcification to occur), an increase in CO2 levels will increase photosynthesis, which aids in calcification.
This means that at optimal CO2 concentrations and temperature (604 µatm and 28°C respectively), the calcification rate as a result in the difference between the product of photosynthesis and seawater calcium carbonate concentration is greatest. In other words, despite everything, the increase in CO2 concentration and temperature drives, photosynthesis allowing for more calcium carbonate to be secreted. Of course, photosynthesis here is driven by the coral's endosymbiotic zooxanthellae, and therefore this poses good news in regards to that aspect.
If corals can be more dependent on the ability of zooxanthellae, in and of itself consisting of many species and variety, then there is all possibility that such a shift in ocean parameters can induce zooxanthellae to evolve to be more effective, in response to the need of their host corals, and hence better combat global warming and ocean acidification. However, there is every indication that even at higher temperatures and CO2 concentrations, zooxanthellae is already faring better - though the effects are being offset by seawater calcium carbonate concentrations.
How this balance plays out, it is hard to tell, but this gives a glimmer of hope, that even though global warming is still getting worse, hopefully the corals and their zooxanthellae can cope until warming is reduced.
Conclusion
Whilst it is far from known what the future may hold, or if this really does bode that well for corals and their survival in the future, it does offer hope - and another avenue for research. If it is possible to boost the photosynthetic capability of zooxanthellae and their ability to promote calcification, then it is definitely a worthwhile cause. Most research has been focused on corals and their adaptive capacity, but we always forget that there are two biological components in play here, and the lesser investigated - zooxanthellae - may just be the answer to the problems caused by global warming, the answer we seek to help extend the life of corals until global warming can be alleviated.
In the end, who knows if the current warming will go away by itself (as part of a natural cycle), or is something that we caused and must solve. Either way, the fact is that as of right now, average temperature is on the rise and corals must find a way to survive.
The Earth has seen average temperatures much hotter than now, and much colder than now. Organisms have always managed, so they may very well even now, without any help from us. Assuming it is even truly an issue for them, if corals indeed not mind an increase in temperatures or ocean acidification that much.
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