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From Engelmann's experiment

Engelmann used this device to illuminate a strand of Cladophora (not Spirogyra) with light from the visible spectrum, exposing different sections to different wavelengths (or colors of light). He added the oxygen seeking bacteria B. termo to this setup and noted where they accumulated (Note: Four years later, Hauser concluded that B. termo had been mislabeled and was not one, but three species of bacteria of the genus Proteus [2]). Their clumping allowed him to see which regions had the highest concentration of oxygen. He concluded that the most photosynthetically active regions will have the highest concentrations of bacteria. The bacteria accumulated in the regions of red and violet light, showing that these wavelengths of light generated the most photosynthetic activity.

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It can be concluded that the photosynthetically active regions of light are present at the blue-violet region and the red region

What I am unable to understand is , if photosystems I and II are activated at light wavelengths of 680nm and 700nm (primarily at the red region of the spectrum) then what type of photosynthesis is happening at the blue region due to which there is oxygen evolution?

My attempt:

The antenna molecules (light harvesting complex) which absorb majorly at this wavelength are transferring energy to the photocentre due to which there is excitation of the photocentre and thus OEC gets activated.

Even if this were true, then should'nt blue light be as effective as red light in helping in generating the quantum yield?

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While it is true that strongest activity of Photosystem I and II happens with 700nm and 680nm light, these are not the only wavelengths that can be used for photosynthesis.

The green color of plants comes from chlorophyll, which is the main light absorbing component in the photo systems. If chlorophyll had only a single absorption peak at 680-700nm, it would have a blue(-ish) color instead of a green one.

Looking at the absorption spectrum of chlorophyll (which wavelengths it can absorb how well), you can see that there is a a second peak around 400nm:

Chlorophyll absorption spectrum

This both explains the results of Engelmann's experiment and the green color of chlorophyll (with both red and blue colors removed only green remains)

Picture taken from: http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/ligabs.html

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  • $\begingroup$ but this still doesn't explain why oxygen is evolved under blue light.The PSII is activated by red light and OEC coupled to this, splits water to reduce the photocentre... However, I am unable understand that even though blue light does not directly excite the PSII photocentre, how it is capable of causing oxygen production. As you can see there are oxygenic bacteria in the blue region indicating that there is oxygen evolution. $\endgroup$
    – user 33690
    Oct 31, 2018 at 12:44
  • $\begingroup$ You didn't quite understand my answer. the 400nm absorption peak of chlorophyll means that blue light does directly excite chlorophyll and thereby PSI / PSII. Also the bacteria cause oxygen production, not evolution. $\endgroup$
    – Nicolai
    Oct 31, 2018 at 13:28
  • $\begingroup$ blue light does directly excite chlorophyll and thereby PSI / PSII. Do you have any references to back this up? $\endgroup$
    – user 33690
    Oct 31, 2018 at 15:19
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Thanks to @theforestecologist for giving me the lead in this answer

Just to clear up, in short:

  • The photocentres, as it goes by their names $P_{680}$ and $P_{700}$ cannot be said to be activated by the blue violet light directly because they can directly absorb light only at the wavelengths 680nm and 700nm for PSII and PSI respectively.

  • The antenna molecules are the only pigments absorbing directly at this low wavelength (also called Soret's band) which makes these molecules important in protection against photooxidation.

  • However the chlorophyll a as pointed out in @Nicolai's answer still show absorption at the 400nm range. This could be explained by the transfer of energy from the antenna molecules (carotenoids) to the photocentre. The entire mechanism has been summarised in this paper

Thus it may account for the oxygen evolution in the presence of blue light

To the last part of my question on the effectiveness of red light over the blue light it can be said that:

  • Of the total blue light incident on the antenna molecules only a part of it's energy is transferred to the photocentre. Whereas in case of the total red light incident , almost all of it gets absorbed by the photocentre to be used in light reaction.

For further reference see theforestecologist's answer on the photosynthesis

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The answer you are looking for is most certainly here:

https://en.wikipedia.org/wiki/Cryptochrome

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  • $\begingroup$ thanks for the link. But how does the cryptochrome pigments couple to the OEC? The link basically states about photosensitivity but I cannot understand why there there is oxygen evolution in the presence of blue light. $\endgroup$
    – user 33690
    Oct 31, 2018 at 11:56

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