[B-PT04] Biotic History

[BPT04-02] Green sea hypothesis: coevolution of photosynthetic organism and habitat

*Kumiko Ito-Miwa^1,2, Yuri I. Fujii³, Kanno Satomi², Taro Matsuo^1,2 (1.Graduate school of Science, Nagoya University, 2.Institute for Advanced Research, Nagoya University, 3.Graduate School of Human and Environmental Studies, Kyoto University)

Keywords:coevolution, oxygenic photosynthesis, great oxydation event, light-harvesting antenna

Cyanobacteria induced the first great oxidation event around 2.4 billion years ago, leading to the promotion of biodiversity. However, the birth and prosperity of cyanobacteria on the Earth are still shrouded in mystery. To tackle this question, we need to understand why cyanobacteria developed their own light-harvesting antennas containing phycobilin pigments for photosynthesis, which are largely different from those of the other photosynthetic organisms such as the light-harvesting complex I (LHC I) containing chlorophyll. Notably, the two pigments have complementary absorption spectra: the phycobilin pigment for absorbing green light and chlorophyll-a for the blue and red light. Here, we propose a scenario for developing the own light-harvesting system in cyanobacteria, named the "green sea hypothesis." Focusing on the fact that the available light spectrum for photosynthetic organisms is characterized by the degree of oxidation on the surface of the Earth, the Earth's history is mainly divided into three eras from the perspective of the coevolution of photosynthetic organisms and its habitat: The birth of photosynthetic microorganisms in fully reduced water. The evolution of cyanobacteria from its common ancestor in a partially oxidized photic zone. The formation of the modern light-harvesting system on the fully oxidized surface. After the photosystems consisting of the chlorophyll pigment have been completed in the first era, the green sea formed by oxidization of the photic zone cultivated the phycobilin pigments in the second era. Because the green light is not necessary for photosynthesis after the first great oxidation event, the modern type of photosynthetic organisms does not apply the pigments related to the green light to the light-harvesting system. This big picture of the photosynthesis evolution is matched to the characteristics of the common ancestor based on the evolutionary tree analysis of cyanobacteria and the cultivation experiments of ancestor cyanobacteria under a simulated green sea.