How small creatures recognize the direction of light (Professor Noriko UEKI Noriko, Department of Law, Faculty of Law)

The "eye" of the unicellular organism Chlamydomonas.

As its green color indicates, Chlamydomonas lives by photosynthesis. Unlike trees and grasses that grow on land, Chlamydomonas lives by swimming underwater, and when it needs light, it swims swiftly to the brighter side. How do we know which direction the light is coming from at this time? It is hard to imagine for those of us who have 37 trillion cells alone, but Chlamydomonas is a single-celled organism with only one cell per animal. It is not possible to create specialized cells and divide labor as is the case with the human eye. However, Chlamydomonas can produce a protein that absorbs light, much like that used by the human eye. When light strikes this protein, it changes its shape slightly to inform the cell that it has been illuminated. However, since proteins are mere substances, they cannot recognize the direction of light. Therefore, Chlamydomonas has collected its photoreceptor proteins in one place on the surface of its body (i.e., cell). It created a small "eye" on the surface of the round body. In this way, when illuminated from one direction, there is a slight difference in the amount of light striking the photoreceptor proteins depending on whether the eye is facing the light or the opposite direction. This is because when you are facing away from the light, the light hitting your eyes is weaker because of the shadow of your own body. If there is a difference in the amount of light you feel when you turn your head that way and look this way, you will know that the side you feel more strongly is the light source side. If they are constantly moving from one side to the other, they will always know the direction of the light and can move to the right place for photosynthesis. This is why Chlamydomonas always swims while spinning around.

• Two Chlamydomonas. The diameter of the cell is about 10 micrometers (1/100 mm). They swim by moving two flagella extending from the cells. The orange part is the "eye spot," or the part that corresponds to the eye.

Role of the light shield

However, it turns out that this is not enough. Because Chlamydomonas is almost perfectly round, its body itself acts like a lens, collecting light. The light is collected on the opposite surface of the body. Even if the eyes are facing away from the light, the round body acts as a lens and collects the light to the eyes. This means that the light feels bright whether you look that way or that way, and you cannot tell which direction the light is coming from. Even though the eyes are shaded by the body, the body is actually quite transparent, so the effect of the lens is lost and the eye mistakenly perceives the light as coming from the opposite direction than it actually is. So Chlamydomonas decided to place a light-blocking plate just behind the eyes. This way, even if the round body collects light, the light is blocked by the board and does not reach the eyes. When Chlamydomonas spins around and its eyes turn toward the light, it feels much brighter because there is nothing to block the light. Chlamydomonas then reflects the plate like a mirror, so that when the eye turns toward the light, it perceives an even brighter light. This makes the difference even greater, and the direction of the light becomes clear. It's a very clever trick.

Proof of the cellular lens effect

This light shield, called the carotenoid granule layer, used to be thought of only as a reinforcement for shading the body, but we have found a positive meaning in counteracting the cellular lensing effect, as described above. This is because we found that Chlamydomonas that had lost this light shield could not correctly perceive the direction of light and would go in the opposite direction to the light. Assuming that the cells collect light as a lens, this behavior could be neatly explained. However, there was no evidence for this. We then thought that if the cells had the property of lenses, they should form an image, so we shone light on them with the letter P, the first letter of the word "photo" for "light," in the optical path. The result is shown in the award-winning movie. It clearly shows that the letter "P" forms an image near the surface of Chlamydomonas cells. Thus, we were able to prove the above story correct.

• The winning entry in the intellectual category of the "ABiS Image Contest" at the 2019 Annual Meeting of the Biophysical Society of Japan, "Swimming P - Evidence of Chlamydomonas Cell Lens Effect" (Kenichi Wakabayashi).

• The paper is here.