One of the most important factors threatening the ecosystem today is global warming resulting from human-induced climate change. Coral dominated ecosystems sensitive to such stressors are negatively affected by an increase in sea surface temperatures which can lead to the deterioration of coral reef communities. This occurs following the break-down of the symbiosis between corals and their endosymbiotic zooxanthellae which play an important role in the corals’ capacity to thrive in extreme conditions. Various Symbiodinium clades also exhibit differing resistance to such circumstances. The present study aimed to specify the zooxanthella types of coral reefs in northern parts of the Persian Gulf. For this purpose, different coral species were collected from several locations, i.e. from Larak, Farur and Hendourabi Islands. Throughout the study, partial 28S nuclear ribosomal (nr) DNA of Symbiodinium was amplified using polymerase chain reaction (PCR), and the resulting PCR products were analyzed.
Results showed that Symbiodinium clade D was the most abundant symbiont in the sampled corals. Clade C was the next most common Symbiodinium at the studied sites, with the exception of Larak Island in the Strait of Hormuz, where clade C was not found and the rarer clade A occurred. The frequency with which clade D, A and C were found was 70%, 18.3% and 11.7%, respectively. The prevalent abundance of clade D may be due to its physiology which is best adapted to the specific conditions of the Persian Gulf and its bleaching history. Hence, with global temperatures continuously on the rise, the capacity of the host-symbiont combination to mitigate the effects of global warming could result in fewer bleaching events.
Human-induced climate change poses significant threats to coral reefs worldwide (Obura and Grimsditch, 2009); the resulting global warming is one of the main causes of increasingly serious consequences for various marine ecosystems. Specifically, it is feared that an increase in sea surface temperature may further deteriorate reef communities and coral-dominated ecosystems (Hoegh-Guldberg et al., 2007).
Several factors affect the survival of coral reef communities of which sea water temperature is regarded as the most important (Downing, 1985). Continued research over the past couple of decades has shown that global warming will affect nearly every aspect of the biology and ecology of key coral reef builders (Jones et al., 1998). Under these conditions, there is also a risk of bleaching (Langdon et al., 2000; Hoegh-Guldberg, 1999).
Coral symbiotic algae of the genus Symbiodinium, the zooxanthellae, are photosynthetically active. They store solar energy and nutrients, providing up to 95% of the energy requirements of the coral host (Muscatine, 1990). The survival of coral reefs and their capacity to thrive in tropical waters is largely dependent on this relationship.
To date, nine Symbiodinium clades (A-I) have been genetically identified (Stat and Gates, 2010), six of which predominantly form symbioses with scleractinian corals. Within this group, clades A-D are the most commonly observed in waters worldwide (Baker, 2003; Goulet, 2006). The level of tolerance toward stress caused by temperature as well as other environmental factors is determined by the genetic make-up of these clades and is the reason behind the observed diversity (Iglesias-Prieto and Trench, 1997; Robinson and Warner, 2006).
Among the known clades, the photosynthetic mechanism of clade D Symbiodinium shows evidence of higher thermal tolerance than other clades (Rowan, 2004), and clade D corals often dominate in environments of high temperature (Baker et al., 2004), salinity (Fabricius et al., 2004), and turbidity (van Oppen et al., 2001; Chen et al., 2003). It has also been observed that before reaching critical temperatures, corals tend to shift and shuffle symbiont communities toward clade D to ensure their survival (Stat and Gates, 2010; Goulet, 2006; Baker et al., 2004; Berkelmans and van Oppen, 2006).
Higher than average temperatures during the summer can break down the delicate yet crucial symbiotic relationship between corals and their zooxanthellae. The significance of prolonged thermal stress is such that it has been recognized as the principle cause for 70% of the world's reported bleaching events (Goreau and Hayes, 1994). The highest temperatures seen within the past few decades were in 1998 when 16% of world's corals died, including 46% of corals in the western Indian Ocean (Wilkinson, 2000). Within the past twenty years, the Persian Gulf has experienced at least four coral bleaching phenomena, the first two of which have been linked to ENSO events (El Niño Southern Oscillation; Wilson et al., 2002).
The most severe case of bleaching occurred in the summer of 1998, leading to near-complete destruction of the reefs in the southern Persian Gulf, i.e. Saudi Arabia, Bahrain, Qatar, and UAE (Riegl, 1999; 2002, Sheppard, 2006).
Studies have shown that in addition to the duration and severity of thermal stress, different genotypic clades respond differently to harsh conditions (Rowan, 2004; Baker, 2001). This can explain the predominance of clade D in Persian Gulf waters where conditions are often extreme with broad seasonal fluctuations in sea surface temperatures (14–34°C), high salinity (up to 40 ppt; Coles and Fadlallah, 1991) as well as a history of coral bleaching events (Mostafavi et al., 2007). Other studies also suggested that SSTs in this region can also rise up to 38°C during the summer (Baker et al., 2004). Research conducted on Persian Gulf zooxanthellae since 2005 demonstrates the occurrence of Symbiodinium clades D and C in the shallow waters off Kish and Larak Islands, with clade D being the most abundant of the two (Mostafavi et al., 2007). This was also observed in the tidal pools of Hengam Island (Shahhosseiny et al., 2011).
This study aims to assess the extent to which different Symbiodinium clades are found in some corals of the northern islands in the Persian Gulf. This will provide some insight into the capacity of the region's reefs for survival at a time when global warming continues to threaten coral species worldwide.
From January 2007 to January 2011, live fragments of 13 different coral host species were collected on SCUBA from three islands (Larak, Farur and Hendourabi) and six sites in northern parts of the Persian Gulf (Table 1). The coral species, depth of sampling, location and number of colonies collected per species are listed in Table 2.
Coral fragments were preserved by way of dipping them into dimethyl sulfoxide buffer (20% DMSO, 250 mM EDTA, saturated with NaCl, pH 8.0, Seutin et al., 1991). The slurry was then removed using DNAB buffer ejected by a waterpic machine (0.4 M NaCl, 50 mM EDTA, pH 8.0) to isolate the tissue. The collected slurry was frozen at −20°C. DNA extraction was carried out on the defrosted slurries using the CTAB/chloroform method of Baker (1999).
The LSU rDNA (D1/D2 domains) was amplified using Symbiodinium specific primers. These were MOS Forward (5′-ATA TAA GTA AGC GGA GGA GGA AAA G-3′) and MOS Reverse (5′-CTT TCG GGT CCT AAC ACA CAT G-3′). All polymerase chain reactions (PCR) contained 1 ng of template DNA, 1 mM total dNTP, 7.5 pmol of each primer and 1.5 U of Taq DNA polymerase (Ampli-Taq, Cinnagen) in a total volume of 25 μl. Amplification was performed using a Corbett Thermal Cycler, following PCR cycling conditions of Mostafavi et al. (2007). Original PCR products from 180 samples of coral species were directly sequenced. Each sequence was determined in the forward direction of the rRNA gene using the dye-terminator sequencing facility at the Alpha sequencing Company in United State of America.
Following the sequencing and using BLAST N (Basic Local Alignment System Tools N, Altschul et al., 1997) at NCBI (National Centre for Biotechnology Information), the specific clades of zooxanthellae which formed the symbioses with each of the coral hosts were determined. This was confirmed by phylogenetic analyses using the PAUP beta version 4.0b10 (Swofford, 2002).
PCR amplification of Symbiodinium 28S rRNA genes resulted in products of approximately 780 bp from all 13 coral species. Direct sequencing of PCR products was carried out on 180 samples. Upon analysis of resulting sequences using the BLAST N, the highest identity (>99%) was found in clades D, A and C.
Symbiodinium clades observed at Larak Island were clade A in Stylophora pistillata and Goniopora djiboutiensis, as well as clade D on Favia pallida, Favia sp., Platygyra sp., Pocillopora damicornis, Sarcophyton sp., Sinularia sp. and Sinularia erecta.
At Farur Island, Symbiodinium clade D was found in Acropora downingi. Clade C was also observed in Porites compressa and Porites sp. at Farur and Hendourabi Islands, respectively; while clade D occurred in Acropora sp. at Hendourabi Island, as well. The overall frequency with which clade D, A and C were detected was 70%, 18.3% and 11.7%, respectively.
These findings were also confirmed by phylogenetic analyses. Clades identified during this research are listed in Table 3 based on sampling sites and coral hosts.
Discussion and Conclusions
Among coral species studied within the northern parts of the Persian Gulf, clade D was the most abundant symbiont. Research carried out at various sites of the Persian Gulf from 2005 to 2011, including the present study show that 77.4% of all coral species studied harbor clade D (Figure 1; Mostafavi et al., 2007; Shahhosseiny et al., 2011). Whilst clade D is relatively uncommon pandemically (Stat and Gates, 2010), it usually occurs in greater numbers where conditions are harsh in terms of exposure to heat and fluctuating temperatures, creating a stressful environment and negatively impacting the health of the coral host which often leads to bleaching (Baker, 2001; Rowan, 2004; Stat and Gates, 2010). Clade D is also often found to be the dominant coral symbiont after bleaching events (Thornhill et al., 2006; van Oppen et al., 2005). This is believed to be a coping mechanism for corals and lasts up to 2–3 years after the event (Baker et al., 2004; Goulet, 2006; Stat and Gates, 2010). Higher resistance toward thermal stress (1–1.5°C) in comparison with other clades (Toller, 2001) increases the chances of survival for clade D in a bleaching event (Buddemeier and Fautin, 1993; Baker et al., 2004; Goulet, 2006). It has also been noted that following the recovery period, corals often switch back to their original symbionts (Stat and Gates, 2010). Therefore, it may be concluded that the predominance of clade D in the Persian Gulf is not merely due to the bleaching events of the past decade, but may in fact represent the historical dominance of this clade in an area with little coral diversity (Mostafavi et al., 2007).
The detection of clade C as the second most dominant zooxanthella clade in northern parts of the Persian Gulf suggests that it is also thermally resistant, particularly when paired with some coral species such as Porites sp. (LaJeunesse et al., 2003). Clade A, however, was only observed in coral species off Larak Island which is located in the Strait of Hormuz in the transition zone to the Oman Sea. This suggests a change in the diversity of zooxanthellae clades located in the inner parts of the Persian Gulf toward the Oman Sea where oceanic conditions dominate (Mostafavi et al., 2007). Meanwhile, Stylophora pistillata was among corals that harbored clade A. The afore-mentioned symbiosis has also been reported to occur in the Red Sea (Shick, 2004). It may be noted that the overall abundance of clades C and A in all studied species in the Persian Gulf since 2005 is equal to 16.8% and 5.8%, respectively (Figure 1).
Hence, the predominance of clades D and C as the most tolerant dinoflagellate endosymbionts with regards to thermal stress implies an overall resilience of coral reefs situated in the northern parts of the Persian Gulf. Due to the symbiont clade composition, thermal hardiness will be more pronounced in northern parts of the Persian Gulf compared to the southern parts (Mostafavi et al., 2007).
Thus, it may be concluded that in the Persian Gulf where clade D is the most abundant of all coral symbionts, the coral host selects this algal genotype as the most stable symbiont in order to raise its tolerance toward thermally stressful conditions. This in turn enables the holosymbiont to mitigate the effects of global warming to a certain extent and provides hope for the survival of coral reef ecosystems.
The authors would particularly like to thank Mr. Ghadami Yazdi for field assistance, especially for sample collection off Hendourabi Island. Mr. Mohsenian is gratefully acknowledged for laboratory assistance during this research. Collection of coral samples complied with the environmental protection laws of the Islamic Republic of Iran.