Question

What process first began producing oxygen about \(2.7\) billion years ago? Why did the oxygen concentration remain low for 300 million years?

Short answer

Photosynthesis by cyanobacteria began producing oxygen, but it remained low due to absorption by iron and other elements.

Step-by-step solution

Identifying the Process

About 2.7 billion years ago, photosynthesis began with the emergence of cyanobacteria. These microorganisms used sunlight to convert carbon dioxide and water into glucose and oxygen, a process known as photosynthetic oxygen production.

Understanding the Oxygen Accumulation

Even though cyanobacteria started producing oxygen, it took approximately 300 million years for oxygen levels to rise significantly in Earth's atmosphere. Initially, the oxygen produced was absorbed by iron and other elements in ocean water and the crust, which delayed its accumulation in the atmosphere.

Explaining Retention Mechanism

As oxygen continued to react with volcanic gases and reduce substances like iron, the rate of retention in free form was minimal. It was only after these sinks became saturated that oxygen could accumulate in the atmosphere, leading to the Great Oxidation Event around 2.4 billion years ago.

Key concepts

Photosynthesis

Photosynthesis is an essential biological process that transforms light energy into chemical energy. This transformation occurs in plants, algae, and specific bacteria, including cyanobacteria. Using sunlight, these organisms convert carbon dioxide and water into glucose, a type of sugar, and oxygen. This process not only sustains the life of the organism conducting photosynthesis but also produces oxygen as a byproduct, which becomes available for other organisms.
In the case of cyanobacteria, their photosynthetic activity was groundbreaking. Millions of years ago, they were among the first organisms to perform photosynthesis using the sun’s energy. This pivotal development marked the beginning of biological oxygen production on Earth.

Cyanobacteria

Cyanobacteria, often referred to as blue-green algae, are microscopic organisms found in various water bodies throughout the world. These bacteria are pioneers of oxygenic photosynthesis, which means they use sunlight to produce oxygen as a byproduct.
Cyanobacteria played a monumental role in altering Earth’s early atmosphere. Before cyanobacteria, Earth's atmosphere was rich in methane, ammonia, and other gases, but devoid of oxygen. Their emergence around 2.7 billion years ago marked a turning point in Earth's history by gradually introducing oxygen into the environment.

• This change allowed for the development of aerobic organisms, which require oxygen to live.
• It also led to the eventual evolution of more complex life forms.

Oxygen Accumulation

Although cyanobacteria began producing oxygen through photosynthesis billions of years ago, it wasn't until roughly 300 million years later that oxygen levels in the atmosphere rose significantly. Initially, the oxygen produced reacted with minerals in the ocean and Earth's crust, notably iron.
This reaction led to the formation of iron oxides, consuming significant amounts of the newly produced oxygen. It was only once these 'sinks' became saturated that free oxygen began to accumulate in the atmosphere. This period marked the start of the Great Oxidation Event, a time when the atmosphere saw a dramatic increase in oxygen concentration.

• These changes set the stage for aerobic organisms to thrive.
• They also marked a significant shift in Earth's environmental conditions.

Earth's Atmosphere History

The history of Earth's atmosphere is one of dynamic change, influenced by geological and biological processes. Initially, Earth's atmosphere was composed of a mix of hydrogen, methane, and ammonia, lacking free oxygen.
The introduction of oxygen by cyanobacteria marked a significant transformation, setting the stage for modern atmospheric conditions. This shift from a reducing atmosphere (lacking oxygen) to an oxidizing one allowed for several critical developments in the history of life on Earth.

• The accumulation of oxygen began to form an ozone layer, which shielded the planet from harmful solar radiation.
• The presence of oxygen enabled the evolution of aerobic metabolism, which is more efficient than anaerobic processes.

Oxidation of Iron

The interaction between oxygen and iron illustrates the geochemical processes that delayed atmospheric oxygen accumulation. The early oceans were rich in dissolved iron, much of which originated from volcanic activity.
As cyanobacteria started producing oxygen, this oxygen would react immediately with the abundant iron, forming iron oxides, which are insoluble. These oxides settled out of the water, reducing the free oxygen available to enter the atmosphere.
This process is known as iron oxidation, and it played a crucial role in delaying the Great Oxidation Event. Over time, as more iron was oxidized and removed from the ocean, the sinks that had trapped oxygen were gradually depleted, allowing for an increase in atmospheric oxygen.