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Why Chlorophyll is Green (?)

  1. Dec 13, 2008 #1
    According to the History Channel documentary How the Earth was Made (DVD), Earth's early oceans were green, b/c of their (different) chemical composition. For example, they had more dissolved Iron, and CO2.

    CONCLUSION: Earth Early Lifeforms were confined to the Oceans, for lack of an Ozone Layer to shield against DNA-damaging UV. If those Oceans reflected Green light (hence, looked green), then marine Photo-Synthesizers well under the surface could not have absorbed any green photons. Instead, they would have evolved to absorb all other frequencies... and hence, look "green" just like the early Oceans they evolved in, as their Absorption Spectrum would resemble the Transmissivity (sp?) of the archaic Oceans.

    IMAGE of CHLOROPHYLL ABSORPTION SPECTRUM: http://bp0.blogger.com/_qiUMQxM9rCg...0GiQmIdyk-A/s1600-h/070410_light_graph_02.jpg
    Last edited: Dec 13, 2008
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  3. Dec 14, 2008 #2
    I see no reason why early marine photo-synthesizers deep underwater would have excluded green from their frequency absorption range. Basically, they didn't know about "green", so I don't see why they would have cared. Nothing would have changed if they could absorb all light, since green wasn't there anyway.

    A more likely assumption is that early marine life used various materials for their photo-synthesis. For example, some could have used material that only excluded blue or red light. Those organisms would have had a much harder time getting the required energy because they are taking a huge hit in the frequency they can absorb, while for the organisms with chlorophyll everything was dandy because nobody else could see green either, so they ended up surviving while others died off.
  4. Dec 14, 2008 #3
    Modern ocean water "strongly absorbs" red, orange, and yellow light. Conversely, green & (especially) blue light can penetrate to depths of 150' or more*. Since modern oceans appear blue, this suggests (?) that the same frequencies which get reflected, are also those that get transmitted. IOW, blue light is not absorbed, so some scatters off the surface, while the rest penetrates.
    * James F. Luhr. Earth: the definitive visual guide, pg. 384.
    Assuming that this was true for the early Earth, then those archaic oceans actually transmitted (or reflected) green. That would disprove my assertion, that early Photo-Synthesizers evolved to "ignore green" (as it were) b/c it wasn't present at depth. Rather, it now looks to me like there were more green photons at depth in the early oceans compared to today.

    Roughly speaking, today, we have Blue-Green Algae, b/c Blue & Green are transmitted. Back then, it "should" have been just Green Algae.

    Could anyone please provide more information about Earth's early oceans?

    Thank you very much in advance,

  5. Dec 14, 2008 #4
    First of all, why is an ocean blue or green? It's not the water. The water's been the same since forever and it's almost entirely clear for optical light. It's what's in the water that gives it color.

    So if your water is green, that means you have green stuff in your water. No kidding. What makes something green? When it can absorb all other optical light except green light. Which is exactly what your picture of chlorophyll absorption shows.

    What throws a wrench in the works is that water also happens to reflect light, but since you can see a pretty good reflection of yourself or anything else from water, it's safe to assume it doesn't discriminate and reflects all (optical) light equally. So water really isn't the issue here.

    What it boils down to is what you have in your water. I would assume algae used chlorophyll to gather energy from light for a different reason than some sort of limitation of light, i.e. making chlorophyll was easier than making something else, or chlorophyll didn't fall apart like something else, etc. Or that chlorophyll absorbed a broader frequency of light than anything else available. But it doesn't look like blue or green light had anything to do with it.

    Basically, chlorophyll being used by algae is what made the ocean green, NOT green oceans making algae use chlorophyll.
  6. Dec 15, 2008 #5
    That makes allot of sense.

    But just to clarify, the aforesaid documentary claimed, that something like Iron and/or CO2 was dissolved in the early oceans, and that that is what made them green. That is, they looked green b/c of dissolved chemicals, not algae.
  7. Dec 15, 2008 #6


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    Water is very nearly clear, with a slight bluish color that is only noticeable when you have perhaps a hundred feet to look through. Iron would give the water a red tint (which could easily overwhelm the blue of the water depending on the concentration). I don't know what color, if any, carbon dioxide has -- I think it's even more clear than water, and its concentration would surely be low. So I don't really think that water with dissolved iron and CO2 would be green.
  8. Dec 15, 2008 #7


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    Iron could give glass a green tint. Only Iron (III) ions (Fe3+) are red in color (e.g. giving rust its characteristic red color). Iron (II) ions (Fe2+) are green and, for example, give glass its greenish hue.
  9. Dec 16, 2008 #8
    I see. Then yeah, it looks like green light would not have penetrated deep underwater in those conditions, so is algae had used chlorophyll for photosynthesis instead of some material that absorbs ALL light, it would not have noticed a difference. Whereas algae that used material that absorbed all but red light (for example) would not have been able to get green light as well, so it is at a disadvantage compared to the chlorophyll algae.
  10. Dec 16, 2008 #9


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    At times we are discussing there was no oxygen in the Earths atmosphere, so iron was mainly in the form of Fe2+ - slightly green (although to be seen as green it needs to be in relatively high concentrations). Fe3+ is more yellow/red (although again, to be seen as such it needs to be in relatively high concentrations, much higher that those that can occur in natural waters).

    Interestingly, it was photosynthesis - with its main byproduct, oxygen - that was resposnsible for iron ore deposits. Iron got oxidized to Fe3+, and then precipitated, as solubility of Fe(OH)3 is very low.

    Chlorophyl is green, but it not the only photosynthesising pigment, there are several others, that differ in color. It just happens that chlorophyl is the most effective one in the light conditions present on the Earth surface, so it dominates what we see. In other places (like below water surface, where light spectrum is different), other pigments play crucial role. If - for some reason - light conditions would change - in a short time (short in geological terms) dominating pigment will also change to the one that will be the most effective. Thus early color of water can't play any substantial role in what we see now.
  11. Dec 16, 2008 #10
  12. Dec 16, 2008 #11


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    (Thanks for the corrections on Iron (III) vs. Iron (II), Ygggdrasil.)

    I always figured green was a good choice since it's the peak wavelength emission from the Sun -- but then again I also figured the choice of chlorophyll had more to do with its composition than the colors it absorbed.
  13. Jan 2, 2009 #12
    Chlorophyll is green because it wasn't the first photosynthetic pigment. The first successful photosynthetic organisms were probably archaebacteria rather like the present-day Halobacterium halobium, which lives in salt lakes and can be thought of as a "living fossil".

    It doesn't have chlorophyll. Instead it uses bacteriorhodopsin (a relative of our own visual pigment) which is a purple pigment and absorbs green light. This makes sense because this is the colour most copiously emitted by the sun and the organism would have lived close to the surface where it could get most sunlight. However, it cannot fix carbon dioxide, but uses sunlight to make ATP to give it energy to metabolise organic materials from its surroundings. It was therefore not self-sufficient and most of them have now died out.

    But there was another type of photosynthetic organism waiting in the wings, or rather in the sediments deep under water, which were rich in organic matter from the decay of organic debris falling from above. This debris would have been rich in sulphur compounds and these organisms used it as a source of electrons for the photosynthetic reduction of carbon dioxide. However, the presence of the purple organisms above meant that it had to use the left-over light at the edges of the spectrum; i.e. red and blue. This pigment was of course chlorophyll. Virtually all present day photosynthetic organisms evolved from these.

    The biggest step was the evolution of oxygen-producing photosynthesis where water replaced sulphur compounds as a source of electrons. This freed them from dependence on the sulphur in the sediments and they more or less took over the whole world. Being greedy, they then evolved the so-called accessory pigments that absorb in the mid region of the spectrum missed by chlorophyll and pass it on to chlorophyll by resonance transfer.

    The downside was that the oxygen they produced destroyed many organisms that had lived in the previously anaerobic atmosphere: green plants were the biggest polluters of all time. Still, without them and their oxygen the whole of the animal kingdom would not have evolved.
  14. Jan 8, 2009 #13
    When did Oxygenic Photosynthesis evolve ? Is there any evidence that it's older than ~2.7 Gy ?
  15. Jan 9, 2009 #14
    Heh, the animal kingdom as we know it maybe. Just makes you wonder what kind of life (and possibly intelligent) may have evolved in an oxygen deficient atmosphere.
  16. Jan 9, 2009 #15
    It's hard to say exactly when oxygenic photosynthesis began since it may have been around on a limited scale for some time before it took off about 2.7 Gy ago, when it destroyed much of the life in a hitherto anaerobic world.

    However, there have been several relatively brief periods in the Earth's history when the anaerobic organisms have struck back. Analysis of rock strata for biomarkers (breakdown products of chemical compounds in specific types of organisms) reveals at least five periods when the seas were rich in anaerobic sulphur bacteria. It is claimed that this is correlated with the mass extinctions of other forms of life brought about by the release of hydrogen sulfide from the oceans, which had become stagnant due to global warming (see http://www.newscientist.com/article...tions-the-microbes-strike-back.html?full=true ). It is ironic to think that present-day "intelligent" life may now be contributing to a similar mass extinction in the not too distant future.
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