Nearly three-fourths of Earth is enveloped by oceans, presenting the planet as a pale blue dot when viewed from space. However, a groundbreaking study published in Nature by Japanese researchers suggests that the oceans of our planet may have once appeared green. This intriguing hypothesis stems from the chemistry of ancient oceans and the evolution of photosynthesis.
During my studies as a geology undergraduate, I learned about the significance of a rock deposit known as the banded iron formation in documenting Earth's history. These formations were created during the Archean and Paleoproterozoic eons, approximately between 3.8 and 1.8 billion years ago. At that time, life on Earth was limited to unicellular organisms inhabiting the oceans, while the continents were barren landscapes dominated by grey, brown, and black rocks.
Rainwater interacting with continental rocks dissolved iron, which was subsequently transported to the oceans via rivers. Additionally, volcanic activity on the ocean floor contributed to the iron levels in the seas. This iron would play a crucial role in the evolution of life and the transformation of ocean chemistry.
The Archean eon was characterized by an atmosphere and oceans devoid of gaseous oxygen. It was during this period that the first organisms capable of harnessing sunlight for energy emerged. These early life forms utilized anaerobic photosynthesis, a process that occurs in the absence of oxygen. One of the significant byproducts of this process was oxygen gas, which bound to iron present in seawater.
Oxygen began to accumulate in the atmosphere only when the dissolved iron in seawater could no longer bind with it. This early form of photosynthesis eventually led to the Great Oxidation Event, a pivotal ecological turning point that facilitated the emergence of complex life on Earth. The alternating layers of iron deposits in banded iron formations serve as a geological record of this significant transition, showcasing the shift from an oxygen-free Earth to one enriched with oxygen in both the ocean and atmosphere.
The research team's argument for the existence of green oceans during the Archean eon begins with a notable observation: the waters surrounding the Japanese volcanic island of Iwo Jima exhibit a greenish hue associated with oxidized iron (Fe(III)). This color supports the thriving populations of blue-green algae in these waters. Despite their name, blue-green algae are actually primitive bacteria and not true algae.
During the Archean eon, the ancestors of modern blue-green algae evolved alongside other bacteria that utilized ferrous iron as an electron source for photosynthesis. This indicates that iron levels in the ocean were likely very high. Photosynthetic organisms rely on pigments, primarily chlorophyll, to convert carbon dioxide (CO₂) into sugars using solar energy. Chlorophyll imparts a green color to plants and algae.
Interestingly, blue-green algae possess not only chlorophyll but also a secondary pigment known as phycoerythrobilin (PEB). The researchers found that genetically engineered modern blue-green algae containing PEB exhibit enhanced growth in green waters. While chlorophyll is efficient for photosynthesis in the visible light spectrum, PEB appears to be particularly effective under green-light conditions.
Before the advent of photosynthesis and oxygen production, Earth's oceans contained dissolved reduced iron. The oxygen released by early photosynthetic organisms subsequently led to the oxidation of iron in seawater. Computer simulations conducted in this study revealed that the oxygen produced by early photosynthesis resulted in a sufficient concentration of oxidized iron particles to give surface waters a greenish tint. Once all the iron in the ocean was oxidized, free oxygen (O₂) became present in both the oceans and the atmosphere.
This research implies that pale-green dot worlds, as viewed from space, are promising candidates for planets that could host early photosynthetic life.
The alterations in ocean chemistry were not abrupt; rather, they unfolded gradually over the course of the 1.5 billion years that comprised the Archean period—a time span that represents more than half of Earth's history. In contrast, the entire timeline for the rise and evolution of complex life constitutes only about an eighth of Earth's history. Consequently, the color of the oceans likely changed gradually during this era, possibly oscillating between different hues. This variability could explain the evolution of both chlorophyll and PEB as photosynthetic pigments in blue-green algae, as having access to multiple light spectra would confer an evolutionary advantage.
The findings from this recent study highlight the interconnectedness of ocean color with water chemistry and the influence of life. Speculating on future ocean colors is not purely the realm of science fiction. For instance, if sulfur levels rise significantly, purple oceans could emerge, potentially linked to volcanic activity and low atmospheric oxygen levels, favoring the proliferation of purple sulfur bacteria.
In addition, red oceans could arise under extreme tropical conditions when red oxidized iron is formed from terrestrial rock decay and transported to oceans by rivers or winds. This phenomenon could also occur if algae associated with red tides proliferate in surface waters, especially in coastal areas with high fertilizer concentrations.
As our sun ages, it will first increase in brightness, leading to heightened evaporation rates and intensified ultraviolet light. This shift may benefit purple sulfur bacteria residing in deep, oxygen-poor waters, resulting in more purple, brown, or green shades in coastal and stratified regions, while the deep blue colors typical of phytoplankton-rich waters may diminish. Eventually, as the sun expands, oceans may evaporate entirely. Over geological timescales, nothing is permanent, and changes in ocean color are therefore inevitable.
