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Research from Nagoya University in Japan suggests that Earth’s ancient oceans may have been green rather than the blue we recognize today. This intriguing possibility stems from studies indicating that for over two billion years, Earth’s oceans exhibited green hues, potentially transforming our understanding of the planet’s history and informing the search for extraterrestrial life.
Although Earth formed about 4.5 billion years ago, it wasn’t until approximately 800 million years later that the first life forms began to emerge. During that extensive period, vast oceans enveloped the planet, teeming with hydrothermal vent systems that released substantial quantities of ferrous iron into the water.
Cyanobacteria, a type of algae and among the earliest organisms capable of oxygenic photosynthesis, surfaced around four billion years ago. Unlike modern plants that utilize chlorophylls, ancient cyanobacteria harnessed solar energy with phycobilins. Their rise marked a pivotal moment about 2.4 billion years ago called the Great Oxidation Event, during which oxygen began to saturate Earth’s atmosphere, significantly impacting the evolution of life. The exact need for phycobilins within cyanobacteria remained a question for researchers until recently.
In a study led by Taro Matsuo, researchers evaluated the light spectrum’s behavior in the oceans during the Archean era, roughly spanning 4 to 2.5 billion years ago. Their advanced computational simulations revealed that the escalating oxygen production by cyanobacteria reacted with oceanic iron, shifting its form from ferrous to ferric iron.
Ferric iron, being insoluble, precipitates into rust-like particles, altering the way light interacts with the water. These sediments preferentially absorb blue and red light, allowing green light to refract, thus potentially giving the oceans a green appearance. Matsuo’s team suggested that this shift would have prompted cyanobacteria to evolve specialized phycobilins to effectively harvest light in these green-hued waters.
Matsuo highlighted the discovery of a particular type of phycobilin known as phycoerythrin, which aids cyanobacteria in absorbing green light. He stated, “This adaptation was crucial for their survival in the iron-rich, green oceans.”
Initially skeptical of the green ocean theory, Matsuo’s perspective shifted over years of ongoing research. A significant epiphany occurred during a field study on Iwo Island in the Satsunan archipelago, where the water’s distinctive green shimmer confirmed his long-held hypothesis.
Matsuo expressed the importance of this finding, saying, “The waters around Iwo Island exhibited a green hue due to iron hydroxides, similar to what I imagine ancient Earth would have looked like.”
The implications of this research extend beyond our planet. While blue oceans might suggest the presence of water on other celestial bodies, Matsuo encourages astronomers to broaden their criteria, incorporating green-hued oceans into their analyses.
He explained, “Data shows that waters with high iron hydroxide concentrations appear more vibrant than standard blue oceans, indicating that green oceans could be identifiable from greater distances, thus improving our detection capabilities in the search for extraterrestrial life.”
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