9:00 AM - 9:15 AM
[BPT05-01] A comparative view on the evolution of symbioses of chemosynthetic symbioses and photosynthetic symbioses in marine environments
Keywords:Symbiosis, Chemosynthesis, Photosynthesis
In chemosymbioses, the major host animals are protists (ciliates), sponges, molluscs (bivalves and snails), annelids (polychaetes and oligochaetes) and arthropods (crustaceans). In photosymbioses, the major host animals are, protists (ciliates and foraminifers), sponges, cnidarians, molluscs (bivalve and sea slugs) and ascidians. It is prominent that annelids are important hosts of chemisymbioses but not in photosymbioses, while they are also found in shallow euphotic zone. On the other hand, cnidarians are very important in photosymbioses in coral reefs but not in chemosymbioses in the deep-sea, though many cnidarians are dwelling there.
In the symbionts of chemosymbioses, all of them belong to prokaryots, most of which are bacteria. Especially proteoracterial (many of them belong to gamma-proteobacteria) sulfur oxidizer are important. On the other hand, in the photosymbioses, the major symbionts are eukaryotic microalgae. In marine environments, although some cyanobacterial symbionts are also known as ascidian symbiont, dinoflagellates belonging to a genus, Symbiodinium, are very important.
In both chemosymbiosis and photosymbiosis, inorganic carbon (Ci) is fixed and organic substances are made by using chemical or light energy. The heterotorphic host animals can not fix Ci and they have a system to oxidize organic substances and discharge Ci to the outside. As the host for chemosymbiosis or phtosymbiosis, they have to reverse direction but need this Ci intake system. Other environmental factors, e.g. pressure and temperature, are the common factors if they live in the habitats, either shallow water or deep-sea. Since, chemosymbioses use energy obtained from the oxidation of reduced chemical substances, they have to live at the boundary between oxic and anoxic environments. And the photosymbioses require light for the photosynthesis, they have to live in the euphotic zone. In addition, the partners of photosymbioses must tolerate the high oxygen concentration stress, which is derived from the photosynthesis. These two factors may be very important for the evolution of these nutritional symbioses.
In the various host animal lineages and also in various symbiont microorganisms, their respective chemosymbioses or photosymbioses seemed to be acquired independently. In the present presentation, I would like to discuss not only about the possible factors affecting the evolution of the symbioses but also about the possible effect (metabolism, organ structure, and genome modification etc) caused by the acquisition of the symbioses.