About this Video

• Video Title: That Time Oxygen Almost Killed Everything
• Channel: PBS Eons
• Speakers: The narrator (name not specified in transcript)
• Duration: 00:05:44

Introduction

This video explores the Great Oxidation Event, detailing how the rise of photosynthetic bacteria and the subsequent increase in atmospheric oxygen drastically altered Earth's environment, leading to a near-extinction event and ultimately shaping the planet as we know it. The video explains how oxygen, while essential for life today, initially caused a global crisis.

Key Takeaways

• The Archaean Eon: Earth 3 billion years ago was vastly different, with most of its surface covered by iron-rich, green oceans due to the lack of oxygen. Life was primarily anaerobic.
• The Rise of Photosynthesis: Photosynthetic bacteria (cyanobacteria or halobacteria) emerged, releasing oxygen as a byproduct. This dramatically altered the planet's chemistry.
• The Great Oxidation Event: Oxygen reacted with oceanic iron, turning the oceans red with rust (banded iron formations). Rising oxygen levels poisoned anaerobic life, while oxygen-tolerant organisms thrived.
• Climate Change and Glaciation: Decreased atmospheric CO2 (due to photosynthetic activity) and the removal of methane (through reaction with oxygen) caused significant global cooling and a major ice age (the Huronian glaciation), lasting 300 million years.
• Near Extinction and Recovery: The changes caused a near-extinction event, impacting even the photosynthetic bacteria that initiated the changes. Life that survived adapted to the new, oxygen-rich environment, leading to a more complex biosphere and the formation of the ozone layer.

Here are notes on the video, focusing on aspects relevant to General Biology 2 and photosynthesis:

I. The Archaean Eon ( ~3 billion years ago):

• Primordial Earth: The Earth was vastly different. Most of the surface was covered by oceans, extremely rich in dissolved iron. Oxygen levels in both the atmosphere and oceans were very low (2-3% surface dry land).
• Anaerobic Life: Life existed, but it was anaerobic (did not require oxygen). The green color of the oceans was due to iron reacting with other elements, not algae or other photosynthetic organisms.

II. The Great Oxidation Event (~2.3 billion years ago):

• Emergence of Photosynthesis: Photosynthetic organisms (likely cyanobacteria or halobacteria) evolved. These organisms used sunlight, CO2, and water to produce energy, releasing oxygen as a waste product. This is a crucial point for General Biology 2, highlighting the evolution and impact of oxygenic photosynthesis.
• Impact on Oceans: The released oxygen reacted with the dissolved iron in the oceans, forming iron oxides (rust). This is visible in geological formations called Banded Iron Formations (BIFs). The oceans changed color from green to red. This shows the dramatic geochemical impact of oxygen.
• Impact on Atmosphere: Eventually, oxygen accumulated in the atmosphere. This was a significant turning point in Earth's history, creating a completely different atmospheric composition.

III. Consequences of Increased Oxygen:

• Anaerobic Extinction: The increase in oxygen was toxic to many anaerobic organisms, leading to a mass extinction event. This is a critical point illustrating the selective pressures of environmental change.
• Climate Change: Photosynthesis consumed atmospheric CO2, a greenhouse gas. Oxygen also reacted with methane (another greenhouse gas). This reduction in greenhouse gases caused significant global cooling and the onset of the Huronian glaciation. This connects photosynthesis to climate regulation and reinforces the concept of environmental feedback loops.
• Huronian Glaciation: This ice age lasted for an incredibly long period (300 million years). It shows the profound impact of even small changes in atmospheric composition on global climate.
• Impact on Photosynthetic Organisms: Ironically, the increase in oxygen also negatively affected some of the photosynthetic organisms that produced it. The decrease in CO2 limited their ability to photosynthesize.

IV. Long-Term Effects:

• Survival and Adaptation: Some life survived the dramatic environmental changes. The organisms that survived were better adapted to the new oxygen-rich environment.
• Ozone Layer Formation: The accumulation of oxygen in the atmosphere led to the formation of the ozone layer, which shields the Earth's surface from harmful ultraviolet (UV) radiation. This allowed for the evolution of more complex life forms.

V. Key Concepts for General Biology 2:

• Photosynthesis: The video highlights the importance of photosynthesis in shaping Earth's atmosphere and environment. It demonstrates how a biological process can have profound geological and climatological consequences.
• Evolution and Adaptation: The rise of oxygen created strong selective pressures, leading to the extinction of some organisms and the evolution of others adapted to the new conditions. This is an excellent example of evolutionary change driven by environmental factors.
• Geochemical Cycles: The video illustrates how biological processes (photosynthesis) can profoundly impact geochemical cycles (e.g., the carbon cycle, the iron cycle).
• Climate Regulation: The link between atmospheric composition and global climate is made clear. The role of greenhouse gases and the impact of biological processes on greenhouse gas levels are significant.
• Mass Extinction Events: This is a real-world example of a major extinction event caused by environmental change.

This detailed outline highlights the points relevant to General Biology 2, focusing on the evolutionary and environmental implications of the Great Oxidation Event, all within the context of photosynthesis and its impact on Earth's history.

This video details the Great Oxidation Event (~2.3 billion years ago), a pivotal moment in Earth's history driven by the evolution of oxygenic photosynthesis. Initially, Earth had anaerobic life and iron-rich, oxygen-poor oceans. Photosynthetic bacteria (likely cyanobacteria or halobacteria) began releasing oxygen as a byproduct, causing a cascade of effects. Oxygen reacted with oceanic iron, creating red rust (visible in Banded Iron Formations), and poisoned many anaerobic organisms, resulting in a mass extinction. Simultaneously, decreased atmospheric CO2 (consumed by photosynthesis) and methane (reacted with oxygen) led to the Huronian glaciation (a 300-million-year ice age). Even the photosynthetic bacteria were initially negatively impacted by the reduced CO2. Life that survived adapted to the oxygen-rich environment, eventually leading to the ozone layer and more complex life forms. This event illustrates the profound impact of photosynthesis on Earth's atmosphere, geochemistry, climate, and the evolution of life. Key concepts for General Biology II include the evolution and impact of oxygenic photosynthesis, adaptation to environmental change, geochemical cycles, climate regulation, and mass extinction events.

• Archaean Eon: Anaerobic life; iron-rich, oxygen-poor oceans.
• Great Oxidation Event: Oxygenic photosynthesis evolves; oxygen released.
• Oceanic Changes: Oxygen reacts with iron, forming rust (Banded Iron Formations); ocean color shifts.
• Atmospheric Changes: Oxygen accumulates; CO2 and methane decrease.
• Climate Change: Global cooling; Huronian glaciation (300 million years).
• Extinction Event: Anaerobic life largely dies off; even photosynthetic organisms initially stressed by CO2 reduction.
• Long-Term Impacts: Oxygen-tolerant life thrives; ozone layer forms; more complex life evolves.