古代の環境とシアノバクテリア
Hello everyone and welcome to SCIEN-SPOT. This podcast shines the spotlight on the latest in
science and technology from Japan. Your host is REN from SCIEN-TALK. So today's topic is the color
of sea. Have you ever heard the Earth's environment was completely different billions of years ago?
This episode takes us back to the Archean eon, which lasted from about 4 to 2.5 billion years ago.
Back then, the atmosphere had almost no oxygen, just a million today's levels, and
harmful UV rays polluted the Earth's first surface. In those ancient oceans, a form of iron
called ferrous iron was dissolved in the fusion mount. It's a strange thought. Oceans are full of
iron instead of the blue sea we know today. Then, revolutionarily, a life form appeared, cyanobacteria.
These microbes performed oxygenic photosynthesis, meaning they used sunlight to produce oxygen.
First, they changed the chemistry of the oceans, and eventually they began filling the atmosphere
古代海洋と光の環境
with oxygen too. This led to what's known as the Great Oxidation Event, about 2.4 billion years ago.
That event paved the way for more complex life like aquarii and animals.
This research focuses on how these tiny cyanobacteria managed to survive in those early iron-rich oceans.
The key to this mystery lies in light. The research is focused on the underwater light
environment of the ancient Earth, when oxygen from photosynthesis reacted with dissolved ferrous iron,
and it formed iron oxide. This is a reddish-brown substance that doesn't dissolve well in water.
These particles would float in the ocean and absorb UV and blue light very efficiently.
Meanwhile, water absorbs red light. That means the light left behind in the water was mostly
green. So imagine this. Billions of years ago, after the oceans weren't blue, they were glowing
green. But here's the puzzle. Most modern plants and algae use a pigment
called chlorophyll, which absorbs red and blue light, not green. If ancient oceans were
シアノバクテリアの光合成のメカニズム
filled with green light, how did cyanobacteria perform the photosynthesis?
To find the answer, researchers studied the light-harvesting structures of cyanobacteria
called light antennas. These are made of pigments and proteins that capture light and transfer
its energy to chlorophyll. They discovered that cyanobacteria developed a set of special pigments,
including bilium pigments and three types of light-harvesting complexes. That allows them
to absorb green light effectively. It's as if these microbes built custom antennas
to catch green light in the ancient era. To that, this idea, called the green ocean hypothesis,
the term used multiple approaches. They stimulated evolution in the lab and showed that green light
selects for organisms with green light-absorbing pigments. And then they built molecular
phylogenetic trees to trace the evolution of these pigments and confirm that there are
cyanobacteria like we had them. They run quantum chemistry calculations to reveal how the antenna
緑の海の仮説
system efficiently transfers green light to chlorophyll. They even explore the modern-day
environment similar to early Earth, such as Iwo Jima in Japan, and their iron-rich waters filter
light in the same way. And sure enough, they found many photosynthetic organisms that thrive in green
light. This study proposes the oxygen released by early cyanobacteria oxidized iron in the ocean,
filling them with green light. This green light, in turn, drove the evolution of specialized
light-harvesting systems, allowing cyanobacteria to flourish. This is a beautiful example of
co-evolution, where life transforms its environment and that transformation shapes the future of life.
The green ocean hypothesis doesn't just deepen our understanding of Earth's history.
It also has implications for the search for life beyond Earth. NASA scientists are now considering
whether green light-rich planets might be home to similar photosynthetic life.
古代地球の可能性
And the researchers described Earth as a pale blue dot. Maybe ancient Earth was once a pale green dot.
This research was published in Nature Ecology and Evaluation on February 18, 2025.
研究の発表
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