Site-specific variation in gene expression from Symbiodinium spp. associated with offshore and inshore Porites astreoides in the lower Florida Keys is lost with bleaching and disease stress

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.

Fig 1. Graphical representation of reciprocal transplant sampling design.

A 16x16 cm fragment of P. astreoides was collected from each reef. This fragment was sectioned into two 16x8 cm small fragments and allowed to recover. Coral was then placed back onto a reef with one small fragment being placed at its reef of origin, while the other fragment half was transplanted to the companion site. For the above, fragments colored in blue originated from Acer24 Reef, while the fragments colored in green originated from Birthday Reef. The above was repeated ten times for each reef, for a total of n = 20 P. astreoides sampled and n = 40 small fragments placed back on to reefs.

Fig 2. By-gene plot of transcript abundance for zooxanthellae genes obtained from sub-samples of Porites astreoides taken from Birthday reef and Acer24 reef during winter and summer sampling efforts.

“AcerAcer” represents non-transplanted sub-samples that originated at Acer24 reef, whereas “AcerBirthday” represents transplanted sub-samples that were collected at Acer24 reef and transplanted to Birthday reef. “BirthdayBirthday” represents non-transplanted sub-samples that originated at Birthday reef, whereas “BirthdayAcer” represents transplanted sub-samples that were collected at Birthday reef and transplanted to Acer24 reef. Sub-samples collected in summer months are represented in orange, while sub-samples collected in winter months are represented in blue. Whiskers denote 95% credible intervals.

Fig 3. By-gene plot of normalized log₂-transformed expression values (±SEM) of experimental genes from all samples.

Orange lines represent fragments exposed to control treatments (28°C), green lines represent fragments exposed to heat treatments (32°C) and blue lines represent fragments exposed to heat+LPS treatments (32°C+LPS from Serratia marcescens). Whiskers denote 95% credible intervals.