Amplification of the methane cycle by anoxygenic photosynthesis countered the faint young Sun
Overview of the press release
The Earth has been continuously habitable with large amounts of liquid water at its surface for billions of years. However, the factors regulating the stability of Earth’s early climate in the face of the “faint young Sun”, which would provide basic knowledge for understanding the long-term maintenance of habitability on Earth and Earth-like planets, have been a matter of debate for several decades in the Earth and planetary sciences. This “faint young Sun paradox”, which remains somewhat enigmatic, is further exacerbated by the severely restricted primary productivity of the primitive photosynthetic biosphere.
In this study, we focus on a novel mechanism for stabilizing the climate system before the advent of oxygenic photosynthesis via a hybrid ecosystem of primitive anoxygenic photosynthetic organisms. Using a biogeochemical Earth system model, we show that the evolution of different photosynthetic organisms with different photosynthetic metabolisms (i.e., H2-based and Fe-based photoautotrophs) on the early Earth would have played a critical role in regulating climate stability and keeping the young Earth habitable. Our results indicate that multiple forms of photosynthetic organisms working together in the same ecosystem significantly (nonlinearly) amplify the methane cycle, hence methane flux to the atmosphere, which results in warming the climate of the early Earth, while an ecosystem including isolated microbial metabolisms cannot sustain the warm climate of the early Earth (Figure).
Many insights emerge from our biogeochemical model analysis, including the previously unrecognized role of the Fe cycle in stabilizing warm climate states. Our results also imply an intriguing series of unexplored climate feedbacks within the coupled H-C-Fe cycles, which will provide better understanding of the evolution of Earth’s early climate system more broadly. Given the complexity of the enzymatic machinery of oxygenic photosynthesis, it may well be that Earth-like exoplanets with primitive photosynthetic life could be more common than planets hosting oxygenic photoautotrophs. The Archean anoxic biosphere is, therefore, expected to be a useful analogue for the primitive biospheres of other Earth-like planets.
Figure 1: The methane degassing rate from the ocean to the atmosphere as a function of outgassing flux of reduced gases.
Journal Nature Geoscience Title Effects of primitive photosynthesis on Earth’s early climate system Authors Kazumi Ozaki, Eiichi Tajika, Peng K. Hong, Yusuke Nakagawa, and Christopher T. Reinhard DOI 10.1038/s41561-017-0031-2 Paper link https://www.nature.com/articles/s41561-017-0031-2
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