Effects of primitive photosynthesis on Earth’s early climate system

Abstract

The evolution of different forms of photosynthetic life has profoundly altered the activity level of the biosphere, radically reshaping the composition of Earth's oceans and atmosphere over time. However, the mechanistic impacts of a primitive photosynthetic biosphere on Earth's early atmospheric chemistry and climate are poorly understood. Here, we use a global redox balance model to explore the biogeochemical and climatological effects of different forms of primitive photosynthesis. We find that a hybrid ecosystem of H2-based and Fe 2+ -based anoxygenic photoautotrophs-organisms that perform photosynthesis without producing oxygen-gives rise to a strong nonlinear amplification of Earth's methane (CH4) cycle, and would thus have represented a critical component of Earth's early climate system before the advent of oxygenic photosynthesis. Using a Monte Carlo approach, we find that a photosynthetic hybrid biosphere widens the range of geochemical conditions that allow for warm climate states well beyond either of these metabolisms acting in isolation. Our results imply that the Earth's early climate was governed by a novel and poorly explored set of regulatory feedbacks linking the anoxic biosphere and the coupled H, C, and Fe cycles, with important ramifications for the sustained habitability of reducing Earth-like planets hosting primitive photosynthetic life.

Rethinking CO Antibiosignatures in the Search for Life Beyond the Solar System

“…We note that model results are generally similar for alternative configurations that employ alternative pathways of CH 4 production that do not involve acetoclastic methanogenesis (Ozaki et al 2018). Full model details and boundary conditions for individual model runs presented here can be found in Ozaki et al (2018). Figure 1 shows atmospheric pCH 4 and pCO as functions of the volcanic H 2 outgassing flux.…”

The Archean atmosphere

“…Anoxic Archean air could hold 1000 s of parts per million by volume of CH 4 if a microbial flux of CH 4 was comparable to today's (135,183,184). Phylogenetically, methanogens date back to >3.5 Ga ago (23).…”

“…Anoxygenic photosynthesis also consumes H 2 [e.g., (198)]. In models with methanogens and H 2 -based photosynthesizers, atmospheric H 2 mixing ratios depend on assumed H 2 outgassing and biological productivity but generally are ≤10 −4 (183,184). These levels preclude H 2 as an important Archean greenhouse gas.…”

Co-evolution of primitive methane-cycling ecosystems and early Earth’s atmosphere and climate

“…Early on, Earth's carbon cycle likely established and maintained temperate climatic conditions 4,5 in spite of a Sun being 20-25% dimmer than it is today 6 . The earliest microbial ecosystems evolving under these conditions, hundreds of million years before the first anoxygenic phototrophs 7 became actors of the Archean climate 8 , most likely involved chemolithotrophs (i.e., unicellular organisms that use redox potential as energy source for biomass production) producing methane as a metabolic waste. Phylogenetic analyses 2,7,9,10 combined with isotopic evidence 11 and the thentime predominance 12,13 of the electron donors H 2 and CO lend weight to a very early origin of H 2 -based methanogens (MG), CO-based autotrophic acetogens (AG), and methanogenic acetotrophs (AT).…”