"Life on the Edge of Global Biosphere: Living on Multiple Symbioses"


Takeshi Naganuma

Associate Professor,

Graduate School of Biosphere Science, Hiroshima University


Tubeworms living around the deep-sea hydrothermal vents are the animals that do not eat. Instead, they depend on the nutrition produced by their symbiotic bacteria. The symbiotic bacteria "eat" the vent fluid chemicals such as hydrogen sulfide (poisonous gas) to produce organic material (nutrition) from inorganic carbon (= carbon dioxide). Production of organics from carbon dioxide is termed "autotrophy". This mode of dietetics is similar to the plants' photosynthesis (photoautotrophy) in which plants "eat" sunlight to produce organics. Therefore, the sulfide-eating autotrophy is termed "chemoautotrophy". The finding of chemoautotrophic symbioses in the lightless deep-sea indicates the occurrence of ecosystems dependent on Earth-derived energy and materials, not on sunlight. The importance of Earth-dependent ecosystems is more evidently demonstrated by the recent findings from deep subsurface biosphere. The deep subsurface biosphere is the regime of no-light and no-oxygen, and facilitates diverse "anaerobic (anoxic) respirations" that are only minor in the surface aerobic (oxic) regime we live in. The anaerobic respirations are inter-connected as if they are in symbioses. Extrapolating the deep-sea and deep subsurface symbioses to a global scale, we will understand that the global biosphere also comprises of a number of various, direct/indirect symbioses. Interactions of atmosphere-hydrosphere-lithosphere and biosphere are a sort of symbioses between Earth and life. We may refer it as "Planet-Life Symbioses", and then we will realize that the study of the "life on the edge of global biosphere" is the beginning of astrobiological exploration.



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