6.1.9 (HL)—Life and atmosphere evolution

Oxygenic photosynthesis lowered CO₂ and raised O₂, enabling ozone formation, aerobic respiration and new atmospheric conditions for life.

O₂ oxidized reduced minerals such as iron and some became stratospheric ozone, filtering harmful UV. The atmosphere changed life’s opportunities, while life changed atmospheric composition.

Photosynthetic microbes released O₂; oxidized mineral bands record the change, and later ozone reduced surface UV exposure.

O₂ supports high-yield aerobic respiration; ozone separately filters UV. Keep the two mechanisms distinct.

Photosynthesis does not directly make ozone; atmospheric chemistry converts some O₂ into O₃.