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This environmental global crisis is lead to the loss of up to one-third of coral reef areas in the world. There is great concern among major scientific, governmental and philanthropic entities propelling action to help coral reefs.  It has been well-established that environmental parameters affect the formation and growth of calcium carbonate structure in the scleractinian corals (Hennige et al., 2010). Light, water motion (Todd, 2008), nutrients (Atkinson and Bilger, 1992) and such factors as ocean acidification and increase in seawater temperature influence carbonate structures in the coral polyps and the morphological forms of their colonies (Foster et al., 2016). Interestingly, a recent study by Iwasaki et al. (2016) showed that even zooxanthellae may affect corals crystalline structure. Their results demonstrated that zooxanthellae play an important role in the vertical growth and formation of the skeletal structure in the branches.

   One of the most determinative parameters in coral calcification is temperature (Howe and Marshall, 2002). Calcification rate increases notably when water temperature elevates to 25–28  C (Coles and Jokiel, 1978; Kajiwara et al., 1995), but at above 28  C the calcification rate generally declines (Howe and Marshall, 2002). At only 1-2  C above the optimum temperature, which differs for different scleractinian species (Swart, 1983), the calcification process may stop (Coles and Jokiel, 1978). A possible explanation for the impact of temperature on calcification is that temperature affects the activity of calcium ATPase in coral epithelium cells (Marshall and Clode, 2004). A significant correlation has been seen between temperature and enzyme-catalyzed reaction in the coral Galaxea fascicularis (Marshall and Clode, 2004).

 

A study by Howe and Marshall (2002) showed that crystal formation in the skeleton of Plesiastrea versipora depended on the water temperature. They also discovered that in the presence of light, calcium carbonate deposits as small spheroidal crystals, while in darkness, it deposits in the form of an amorphous sheet-like cementation. At 21  C, it mostly looks like small needle-shaped crystals with some spheroidal crystals, while at 15 C, as spheroidal crystals (Fig. 5). In the study of Reynaud et al. (2007) on the effect of temperature on Mg/Ca ratio in the scleractinian coral Acropora sp, it was shown that the increase of temperature from 21 to 29  C increased the rate of calcification 5.7 times and the Mg/Ca ratio in the coral skeleton by 30%.

 

However, an earlier study by the same authors (Reynaud et al., 2003) on the coral Stylophora pistillata revealed that the increase of temperature (up to 28  C) with pCO2 can reduce the calcification rate by 50%. It has been accepted that coral reefs are threatened by changes in environmental conditions (Stolarski et al., 2016). The increase in sea surface temperatures (SSTs), as a result of global warming, is a major concern, threatening marine ecosystems, mainly coral reefs (Solomon et al., 2007). With the predictable extreme inconsistency in environmental parameters in the future, more investigations are needed to obtain a good understanding about the corals responses to the changing environmental parameters (McNeil et al., 2004). If there would not be a significant reduction in greenhouse gas emissions in the next two to three decades, the survival of coral reefs worldwide would be uncertain (Baker et al., 2008). Strict attention must be paid to the issue of protection of marine ecosystems in order to keep the Earth a habitable planet.

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Coral crystal formation at different temperatures. A and B show skeletal deposition at 10  C, C and D represent skeletal deposition at 15.5  C, and E and F show skeletal deposition at 21 C. Small spheroidal crystals (SP), areas of smooth cementation (CE), non-mineralized spaces (NMS) and needle-shaped crystals (N) are also shown in the figure. Scale bar = 3 µm (Howe and Marshall, 2002).  

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We studied the coral crystal morphology in a high temperature by SEM. 

The septa was selected as the coral region for further analysis of crystalline structure following comments in Isa, 1986 whereby in coral corallite, the spine, septa, thecal walls and costae are the most active places for crystal formation. The coral microstructures in the skeleton observed and compared micro-structures (i.e., crystal fibers/or coral fibers) in PociIlopora damicornis septa. The septa in all three coral fragments in each treatment were investigated in the apical and lateral views. Three types of crystalline were observed in the coral skeleton at the high temperature.  The only one crystal morphology of fibers in the treatments with normal temperature (25  C) was identified.

It seems that high temperature can cause critical crystalline structural deformities in coral skeletons. 

I will let our paper in my website as soon as published. Contact me for more information: mahdimoradi@id.uff.br

                                                                                                                                                       

   

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