Abstract
Scleractinian coral are experiencing unprecedented rates of mortality due to increases in sea surface temperatures in response to global climate change. Some coral species however, survive high temperature events due to a reduced susceptibility to bleaching. We investigated the relationship between bleaching susceptibility and expression of five metabolically related genes of Symbiodinium spp. from the coral Porites astreoides originating from an inshore and offshore reef in the Florida Keys. The acclimatization potential of Symbiodinium spp. to changing temperature regimes was also measured via a two-year reciprocal transplant between the sites. Offshore coral fragments displayed significantly higher expression in Symbiodinium spp. genes PCNA, SCP2, G3PDH, PCP and psaE than their inshore counterparts (p<0.05), a pattern consistent with increased bleaching susceptibility in offshore corals. Additionally, gene expression patterns in Symbiodinium spp. from site of origin were conserved throughout the two-year reciprocal transplant, indicating acclimatization did not occur within this multi-season time frame. Further, laboratory experiments were used to investigate the influence of acute high temperature (32°C for eight hours) and disease (lipopolysaccharide of Serratia marcescens) on the five metabolically related symbiont genes from the same offshore and inshore P. astreoides fragments. Gene expression did not differ between reef fragments, or as a consequence of acute exposure to heat or heat and disease, contrasting to results found in the field. Gene expression reported here indicates functional variation in populations of Symbiodinium spp. associated with P. astreoides in the Florida Keys, and is likely a result of localized adaptation. However, gene expression patterns observed in the lab imply that functional variation in zooxanthellae observed under conditions of chronic moderate stress is lost under the acute extreme conditions studied here.
Highlights
A decline in global coral populations as high as 50% has been recorded over the last 30–40 years, with strong links to coral bleaching [1,2,3]
There were no significant effects in gene expression between winter and summer observed, significant differences in zooxanthellae gene expression were found between subsamples from Acer24 and Birthday reefs (p
Relative to zooxanthellae from sub-samples originating at Acer24 reef, Symbiodinium spp. originating from Birthday reef and transplanted to Acer24 reef exhibited significant down-regulation of the genes protein I chloroplastic (PCP) (p
Summary
A decline in global coral populations as high as 50% has been recorded over the last 30–40 years, with strong links to coral bleaching [1,2,3]. Bleaching is a response to elevated seawater temperatures and high irradiance, and results when the density of symbiotic algae (Symbiodinium spp. commonly referred to as zooxanthellae) declines severely in the host coral allowing the white skeleton below to show [4,5]. In the Florida Keys, several studies found variation in bleaching susceptibility between inshore and offshore communities of P. astreoides [14,15,16]. The observation of variation in subclades between proximate inshore and offshore reefs prompts questions of the role of genetically associated functional variation in zooxanthellae to bleaching susceptibility [17]. The role of zooxanthellae in bleaching susceptibility variation in inshore and offshore P. astreoides of the Florida Keys is relatively unknown [15]
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