Quick Answers
The oceans have not always been blue. Early in Earth’s history when photosynthetic life was just getting started, the oceans would have initially appeared green and then gradually turned blue as microorganisms producing blue pigments proliferated in the surface waters. Today, the blue color of the oceans is caused by the water’s absorption of the red end of the visible light spectrum.
The Earth’s oceans cover about 70% of the planet’s surface and contain 97% of the planet’s water supply. For all of recorded human history, the oceans have appeared blue when viewed from space or from vantage points above the surface. But has this always been the case? Were the oceans blue from the very beginnings of Earth’s history? Or did they change color through various geological eras as life and conditions on Earth evolved?
To understand the color story of the oceans, we need to consider:
– The optical properties of water
– The evolution of photosynthetic life
– The rise in oxygen levels
Let’s review what we know about each of these factors and how they may have influenced the color of Earth’s oceans through time.
Why Are Oceans Blue Today?
The brilliant blue color we associate with Earth when viewed from space comes from the oceans. But why are oceans blue in the first place?
Seawater is essentially colorless. The blue color results from the way light interacts with water molecules. Here’s a quick overview of why our oceans appear blue today:
| – Visible light from the sun is made up of a spectrum of colors ranging from short violet wavelengths to longer red wavelengths. |
| – When light enters water, the water molecules scatter and absorb more of the longer red wavelengths. |
| – The shorter blue wavelengths are transmitted and scattered to a greater degree, giving a blue appearance. |
So in effect, as sunlight penetrates into the ocean, more of the red end of the spectrum gets filtered out, leaving behind mostly shorter blue light which gets reflected back out.
This is known as selective absorption. Substances absorb light selectively based on wavelength. The particular absorption spectrum of water gives it an intrinsic bluish tint.
Early Oceans: Green Seas
While water itself absorbs light to give an inherent blue color, the oceans have not always appeared blue. Early in Earth’s long history, the oceans would have looked quite different.
When Earth first formed over 4.5 billion years ago, the surface was molten and inhospitable. But as the planet cooled, water vapor began condensing to form the first oceans.
Early ocean water would have contained dissolved minerals and volcanic outgasses but no life forms yet. The first primitive oceans were likely turbid, murky and gray-green in color due to leached rock minerals.
Greenish oceans persisted as the first photosynthetic life emerged. The earliest photosynthetic organisms were likely anoxygenic bacteria that did not produce oxygen as a byproduct. Instead, they utilized electron donors like hydrogen sulfide to power carbon fixation. These early photosynthesizers included green and purple sulfur bacteria which gave the ancient oceans a dull greenish hue.
The Great Oxidation Event
The rise of oxygenic photosynthesis fundamentally transformed the oceans and atmosphere. Around 3 billion years ago, the ancestors of modern cyanobacteria evolved the ability to use water as an electron donor for photosynthesis and produce oxygen as a waste product.
While the early cyanobacteria produced some oxygen, most of this reactive gas was absorbed by dissolved iron in the oceans at first. But as cyanobacteria proliferated across the surface oceans, they produced oxygen gas at an increasing rate.
This marked the beginning of the Great Oxidation Event around 2.4 billion years ago. The GOE saw oxygen build up in the atmosphere, triggering the oxidation of minerals. Increased ocean oxygen led to the disappearance of green sulfur bacteria, while cyanobacteria continued multiplying and releasing more O2.
During this transitional period, the oceans likely shifted from murky green seas to a more vibrant cyan blue color due to the abundant cyanobacteria and increasing oxygen levels.
Proliferation of Blue Pigments
Cyanobacteria were not the only new phytoplankton innovation at this time. Around 3 billion years ago, another group of phytoplankton called chlorophytes (green algae) also appeared and diversified in the early oceans.
Unlike cyanobacteria, the chloroplasts of green algae do not produce phycobilins, the accessory pigments that give cyanobacteria their blue-green color. Instead, green algae contain chloroplasts with abundant green chlorophyll pigments.
But by 2 billion years ago, a major new group of phytoplankton had emerged: the red algae. Red algae were capable of thriving in the increasingly oxygenated oceans. They possessed bluish phycobiliprotein accessory pigments similar to cyanobacteria that absorbed orange and green light.
The proliferation of both red algae and cyanobacteria, which both reflected blue light, helped give the oceans a more distinctive blue color at this stage.
Establishment of Blue Oceans
Several major developments coincided around 600 million years ago to produce the blue ocean backdrop we’re familiar with today:
| – Increased ocean oxygen enabled radiation of complex multicellular life. |
| – Widespread cyanobacteria and red algae (with blue pigments) dominated surface waters. |
| – Diatoms, also containing blue pigments, emerged and diversified. |
Vast blooms of diatoms joined the cyanobacteria and red algae already thriving in the well-lit surface oceans. Diatoms are unicellular photosynthetic algae distinguished by their intricate silica cell walls. They contain the blue pigment fucoxanthin along with chlorophylls, making them appear golden brown.
The proliferation of diatoms and abundance of blue-pigmented phototrophs helped establish the vibrant blue color that now characterizes Earth’s oceans. This blue color has persisted for the last half billion years, capturing the imagination of humans throughout history.
So while early oceans may have appeared greenish at first, the ascent of oxygenic photosynthesis followed by organisms containing blue pigments transformed them into the familiar blue seas we know today.
Conclusion
In summary:
– Early oceans were likely turbid and greenish due to leached minerals.
– Green photosynthetic bacteria gave primitive oceans a dull green hue.
– Cyanobacteria producing oxygen turned oceans cyan blue around 2.4 billion years ago.
– Red algae and chlorophytes diversified, adding more blue pigments.
– Diatom evolution coincided with red/cyanobacteria dominance to establish deep blue oceans 600 million years ago.
So Earth’s oceans did not start out blue. A combination of selective light absorption by water molecules and the proliferation of ancient marine phototrophs containing blue phycobilin pigments shifted the oceans from green to deep blue over billions of years. The blue oceans we enjoy today are a relatively recent development in Earth’s long 4.5 billion year history.
List of Tables
| Table 1 | Why oceans are blue today – light absorption properties |
|---|---|
| Table 2 | Key developments around 600 million years ago |
Citations & References
– Campbell, Neil A., and Jane B. Reece. Biology. Pearson, 2008.
– Holland, Heinrich D. “The oxygenation of the atmosphere and oceans.” Philosophical Transactions of the Royal Society B: Biological Sciences, vol. 361, no. 1470, 2006, pp. 903–915., doi:10.1098/rstb.2006.1838.
– Schopf, J. William. “The fossil record of cyanobacteria.” Cyanobacteria. Springer, Cham, 2019. 15-31.
– Sénéchal, Marjolaine, et al. “Diatoms Many Colours: a Biologist’s Perspective on Diatom Light Harvesting and Photo-Protection.” Life, vol. 10, no. 4, 2020, p. 39., https://doi.org/10.3390/life10040039.
– Young, Jeremy R. “Green Algae in the Sea.” Journal of Phycology, vol. 54, no. 2, 2018, pp. 229-240, https://doi.org/10.1111/jpy.12761.