Biology of Blue

Blue-Bead Lily (Clintonia borealis)

Blue is very hard/expensive for biology to produce.

Blue light is higher energy than other visible frequencies, so chemistry to absorb everything else but pass/reflect blue light is ... tricky.

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The lovely blue feathers of the Blue Jay (Cyanocitta cristata) and Eastern Bluebird (Sialia sialis) have no blue pigment in them. The vibrant blue so often seen in dragonflies is produced without any blue pigment. Even the blue color we see in the eyes of some people has no blue pigment. These and many more examples of blue in biology all are referred to as structural colors instead.

Structural colors are produced by the presence of microscopically fine structures that interfere with light. The blue of some human eyes is caused by very small particles of melanin (a brown pigment), for example.

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Flavone structure.

Some blue berries, like those of Marbleberry (Pollia condensata) are blue due to a structural color. Others, like the Blue-Bead Lily (Clintonia borealis) I photographed in northern Minnesota (at top-left) have a blue flavinoid pigment. The flavinoid pigments are derived from or structurally similar to flavone (at right).

Some insects (like the Lycaenid butterflies) are blue due to flavoniods like kaempherol. Some of the more common flavinoid pigments are the anthocyanins responsible for the red/purple/blue colors seen in fruits and other plant tissues. Different compounds in the group have various modifications to the basic flavone structure. Those modifications impact the stability of the ionic form of the molecules at different pH levels, as well as the specific frequencies of light that are absorbed.

I don't have a solid grasp on the physics that leads to all the differences in color, but the following quote from this paper seems to be illustrative.

"Confining electrons to a smaller space makes the light absorbed bluer and if they move around in larger space the light absorbed is redder."

When the light absorbed is bluer, the light transmitted (and thus observed) is redder, and vice versa. Thus, when electrons are more de-localized, the molecule will have a more blue color. Conversely, when electrons are more restricted, the molecule will have a more red color.

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In the structure of lycopene, responsible for the classical red color of tomatoes, electrons are restricted to travel within very localized regions of the molecule.

[From Madu & Bello, 2018.]

Compared that to the anthocyains, where the electrons are de-localized into aromatic carbon rings. Here the electrons are much more free to occupy larger spaces. The molecules often absorb more red and appear bluer.

[Modified from Khoo et al, 2017.]

One of the anthocyanins that is more stable at higher pH is called delphinidin. It is responsible for the clear blue color found in delphiniums and can also be found in various purple plant materials. The exact shade it presents us with depends on the specific pH of the cell and the association of the delphinidin with various molecules or ions in ways far more complex than I've been able to become clear about with my readings so far.

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So. Why does biology often go with structural colors, when there are commonly available molecules which produce the color? Two partial answers that come to mind are:

1. It may be that the flavinoid blue pigments are more energetically expensive to produce than other pigment types.
2. It may be that structural blues are very easy to come by on accident (like iridovirus infection of isopods).

Evolution is a tinkerer, not a planner. It can be very hard to ever answer questions of "why" in biology. It is far easier to answer questions about "how" or "what". In the end, biologists often have to say, "I really don't know. Do you have any ideas about how we might find out?"


References:

• Structural color:
• https://en.wikipedia.org/wiki/Structural_coloration
• https://medium.com/@rawwerks/what-is-structural-color-536ee6fe46d4
• Bluejay color: https://dnr.wi.gov/wnrmag/html/stories/2003/feb03/jays.htm 
• Eastern Bluebird color: https://academic.oup.com/beheco/article/14/6/855/269116
• Dragonfly color: https://jeb.biologists.org/content/207/22/3999.long
• Human eye color: https://www.sciencealert.com/science-how-blue-eyes-get-their-colour
• Bluebead Lily:
• https://www.fs.fed.us/wildflowers/plant-of-the-week/clintonia_borealis.shtml
• https://books.google.com/books?id=YfpVC5aRRZcC&pg=PA69&lpg=PA69&dq=%22Clintonia+borealis%22+pigment&source=bl&ots=qOVVqSmgEi&sig=ACfU3U0QzbH4fUDoQwl2RO0pdEDb4xftZQ&hl=en&sa=X&ved=2ahUKEwio2NXZl7TlAhVJba0KHSmdB4YQ6AEwAXoECAkQAQ#v=onepage&q=%22Clintonia%20borealis%22%20pigment&f=false
• Butterflies:
• https://link.springer.com/article/10.1007/BF01880094
• Marbleberry (Pollia condensata):
• https://earthsky.org/earth/brightest-biological-substance-reveals-its-secret
• https://www.npr.org/2012/09/11/160894364/how-to-get-birds-to-pick-blue-berries-they-can-t-eat
• Anthocyanin pigments:
• http://www.micro-ox.com/chem_antho.htm
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5613902/ 
• https://www.tuscany-diet.net/2014/02/22/anthocyanins-definition-structure-ph/
• Chemistry of colors:
• https://www.ias.ac.in/article/fulltext/reso/006/03/0066-0075 
• Lycopene structure:
• http://ijsrch.com/IJSRCH18322
• Iridoviruses: https://link.springer.com/chapter/10.1007/978-3-642-70280-8_4