Abstract
Conservation of the blue coral Heliopora coerulea in Oura Bay is desired due to its size and peculiar shape, and its being representative of the coral reef ecosystem of the bay. The bleaching of H. coerulea in 2009 is believed to have occurred as a result of the thick growth of diatom Licmophora sp. on the surface of the coral, causing damages and health hazards. In this time, algal masses different from Licmophora sp. were discovered on H. coerulea. The algal masses were thought to be cyanobacteria because those masses resembled the genus Lyngbya which twisted around a gorgonian coral Annella reticulata around the Kerama Islands. In the coral reef, such cases are known that corals were killed due to cyanobacterial overgrowth on colonies in response to increase of nutrients and that a fatal sickness of coral was caused by cyanobacteria belonging to multiple genera. In this study, we aimed to observe the form of the algal mass specimens collected from the surface of the H. coerulea colonies and a surrounding bedrock under a microscope, examine the 16SrDNA sequences, clarify the algal species by comparing them with known cyanobacterial sequences, and presume influence of the algae on H. coerulea. As a result of the analyses, each of the algal masses collected from H. coerulea and the bedrock turned out to form a consortium of cyanobacteria consisting of several genera, including the genus Leptolyngbya. In those consortia, Lyngbya majuscula and Hydrocoleum lyngbyaceum, those might occur in outbreaks when the concentration of nutrients in seawater rises, were included. Although the influence of cyanobacteria on the H. coerulea colonies is thought to be small at the present stage, continuous survey on cyanobacteria growth is needed.
