Identification of Genes for Synthesis of the Blue Pigment, Biliverdin IXα, in the Blue Coral Heliopora coerulea

Heliopora coerulea is the only species in the subclass Octocorallia that has a crystalline aragonite skeleton. The skeleton has been reported to contain the blue pigment, biliverdin IX, which is formed by heme oxygenase (HO) during heme decomposition. There is little information regarding gene expression in H. coerulea; therefore, the biosynthesis pathway for biliverdin IX is poorly understood. To identify the genes related to heme synthesis and degradation, metatranscripts of H. coerulea and its symbiont Symbiodinium spp. were sequenced and separated from the host- and symbiont-derived sequences. From the metatranscriptome analyses, all genes for heme synthesis and three HOs were isolated from the host and symbiont. From our phylogenetic and amino acid analysis, we noted that one of the HO isoforms in the host coral was predicted to possess HO activity. However, biliverdin reductase, which reduces biliverdin to bilirubin, was not identified in the present study. Similarly, biliverdin reductase was not identified in the transcripts of the red coral Corallium rubrum, a species that also belongs to Octocorallia. However, genes related to heme synthesis and HO were found in C. rubrum. We speculate that Heliopora coerulea can produce biliverdin and accumulate it in the skeleton, while red corals and other Octocorallia species cannot. Further information from molecular studies of H. coerulea will provide insights into the synthesis of biliverdin IX, the blue pigment in the hard crystalline aragonite skeleton, and will be fundamental to future ecological and physiological studies.