Coral symbiotic complex: Hypothesis through vitamin B12 for a new evaluation

Abstract

Historically, hermatypic corals are defined as a symbiotic system composed of an animal host, corals belonging to the taxa Cnidarian, and a photosynthetic organism, the dinoflagellate Symbiodinium spp., also known as zooxanthellae. The high gross productivity and stability of coral reefs have been explained by the efficiency of the coral-algal symbiotic system in using the low nutrient concentrations found in the surrounding water and their rapid recycling in the water. Although several studies have reported the presence of bacteria closely associated with corals, the mechanisms of the relationships among them, the host and the zooxanthellae, remain to be shown. In this study, evidence for the importance of coelenteric bacteria in corals as a component of the coral symbiotic complex was shown by using a new approach. Vitamin B12, which is produced only by prokaryotes, was chosen as a chemical tool to clarify the symbiotic relationships among bacteria, coral, and zooxanthellae, which require vitamin B12. High vitamin B12 concentrations (up to 700pmol l-1 compared with max. 20pmol l-1 in the surrounding water) and high bacteria abundances (100 times higher than surrounding water) were found in the coelenteron of live corals using a new sampling method. The results led to the hypothesis that prokaryotes are the drivers of internal processes, such as vitamin B12 production, occurring directly inside the coral, forming a semi-closed system. Furthermore, most maintenance of the symbiotic complex is due to internal processes rather than the supply from outside the coral.