12 Questions
What was the primary composition of the Earth's atmosphere when life first arose?
主要是CO2和水蒸气
What was the source of reducing equivalents in nonoxygenic photosynthesis?
Other molecules with reducing power
What was produced as a byproduct of oxygenic photosynthesis?
O2
What was the outcome of the Great Oxidation Event?
The atmosphere and surface of the Earth were dramatically transformed
Where did O2 concentrations first accumulate?
In the oceans
What is toxic to cyanobacteria?
O2
What is a possible source of early O2 generation before the Great Oxidation Event?
Photolysis of H2O by high-energy ultraviolet radiation
What is a result of the oxidation of Fe2+ to Fe3+ on the Earth's surface prior to the Great Oxidation Event?
Insoluble ferric oxyhydroxides formed
What is a characteristic of the ferric ion that poses a threat to living organisms?
It is an inherently strong oxidant
What is a possible adaptation of early organisms to the environment before the Great Oxidation Event?
Adaptation to intermittent low levels of O2
What is a consequence of the oxidation of Fe2+ to Fe3+ for microbes?
They would not be able to obtain iron from insoluble ferric oxyhydroxides
What is a result of the abiotic photochemistry of Fe2+-containing minerals?
Conversion of Fe2+ to Fe3+ and production of H2
Study Notes
Early Earth and Anaerobic Life
- When life first emerged on Earth, the oceans and land were highly reducing environments, with redox metals existing in their reduced states and sulfur present mainly as sulfides.
- The atmosphere was anoxic, consisting of CO2, water vapor, N2, H2, and CH4, with O2 levels 10^(-5) of what they are today.
Photosynthesis and the Great Oxidation Event
- Early organisms acquired photosynthesis, but initially, it was non-oxygenic, using sources of reducing equivalents other than water.
- The Great Oxidation Event (2.4-2.0 billion years ago) transformed the atmosphere and Earth's surface due to oxygenic photosynthesis in cyanobacteria.
- Oxygenic photosynthesis uses water as a source of reducing equivalents, producing O2 as a byproduct, which is toxic to the cyanobacteria themselves.
Oxygenation of the Earth
- O2 concentrations from oxygenic photosynthesis rose slowly and unevenly during the Great Oxidation Event, first forming "oxygen oases" in the oceans and later accumulating in the atmosphere.
- The surface of the Earth and ocean minerals consumed much of the initial O2 produced, but eventually, O2 gas began to accumulate in the atmosphere.
Energy Sources and Anaerobic Life
- Early anaerobic life relied on chemical and photochemical reactions of electron donors and acceptors on the Earth's surface.
- Fe2+ was an abundant electron donor, and early anaerobic iron-oxidizing bacteria may have produced significant amounts of Fe3+ prior to the buildup of O2 in the atmosphere.
Abiotic Photochemistry and Oxidative Stress
- Abiotic photochemistry expanded the range of reduction potentials encountered by early life, potentially causing production of Fe3+-containing minerals and H2 without biological involvement.
- Conversion of Fe2+ to Fe3+ on the Earth's surface prior to the Great Oxidation Event may have triggered one of the earliest forms of biological oxidative stress.
Adapting to Intermittent O2
- Early organisms may have adapted to intermittent low levels of O2 generated biologically or abiologically prior to the Great Oxidation Event, despite the atmosphere remaining largely anoxic.
- H2O2 formed by ultraviolet radiation on the surface of ice may have been an abiotic source of early O2.
This quiz explores the early Earth environment, including the redox chemistry of the oceans and land, and the composition of the ancient atmosphere.
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