A new study by Japanese scientists suggests that during the Archean eon, Earth’s oceans may have appeared green, not blue.

Key Findings of Study:

• Reasons for greening of ocean in earlier times
  • During the Archaean eon (4.0 billion to 2.5 billion years ago), there was no oxygen in atmosphere or oceans.
    • Iron from rocks and underwater volcanoes dissolved into oceans.
  • The first life forms, anaerobic bacteria, began performing photosynthesis, which produced oxygen as a byproduct.
  • This oxygen reacted with the iron in the seawater, forming oxidised iron (Fe(III)), which caused the oceans to appear green
• Evolution of Cyanobacteria:
  • These were photosynthetic bacteria (not true algae) that evolved in iron-rich, green oceans.
  • They used two pigments: chlorophyll (for normal sunlight) and phycoerythrobilin (PEB) (better for green light).
    • This helped them survive in varying light and ocean conditions.
  • These bacterias flooded Earth’s atmosphere with oxygen and also paved the way for complex life.

Other Possible Colors of the ocean

• Purple Oceans: High sulfur content from intense volcanic activity and low oxygen levels could lead to the rise of purple sulfur bacteria, turning oceans a deep violet.
• Red Oceans: Intense weathering of rocks in tropical climates or blooms of red algae (associated with "red tides") could stain oceans red. 
  • Today, such algae blooms are linked to nutrient runoff near coasts.