[Hank] This episode is sponsored by Awesome Socks Club, a sock subscription for charity. Click the link in the description and sign up between now and December 11th to get a fun pair of socks every month in 2021.

[upbeat electronic music]

The color blue is kind of special. It has a reputation among biologists for being rare in nature, except you've got birds, insects, flowers, sea creatures, even the eyes of some humans. Blue is everywhere. So what's the deal?

Well, it is actually pretty hard for living things to make blue, blue pigments at least. Many of the natural blues you see are really made using structural color, microscopic structures that interfere with light to reflect color back to our eyes. But blue pigments are a whole different game. To make those, life has to jump through some surprisingly big hoops.

Pigments are molecules that selectively absorb and reflect visible light, and they get their color from molecular structure and how the atoms are bonded to one another. So to understand why natural blue pigments are so tricky, we need to talk both biology and physics.

A lot of the problem with making blue pigment comes down to how molecules can have colors in the first place. Surprisingly complicated. The very basic idea is that a molecule gets its color by reflecting and absorbing different kinds of light. If it mostly reflects red, it's red. If it reflects everything, it's white. If it absorbs everything, it's black. But to understand the problem with natural blue, you need to go a little deeper than that.

So imagine some generic molecule, one with multiple atoms sharing some of their electrons in the form of chemical bonds. Those electrons usually exist in what's called a ground state, with a set amount of energy, and it tends to be as little as they can get away with. But, if you add some energy, like if an electron absorbs a photon of light, the electrons can jump to a higher, so-called excited state. 

It's pretty straightforward so far, but one of the weird yet fundamental rules in physics is that electrons can only absorb light if it has exactly the right amount to actually get them to a new energy state. If it's too much or too little, the photon just bounces off the molecule. It has to be just right.

And this is the reason why different things are different colors. Molecules have all kinds of different energy levels, and photons of different colors of light have different amounts of energy. Shorter, bluer wavelengths have the most energy, while the longer, redder wavelengths are more laid back. They're just chilling. Less energy. Low energy red.

So the shape of the molecule really matters, because different structures within the molecule create those different energy levels. If its electrons can only jump to an excited state with high-energy light, it's going to absorb blues, and the opposite is true as well. If it needs to absorb low-energy light, it will absorb reds.

And it turns out it is easier for life to make molecules that absorb blue and reflect red than the other way around. Blue rocks and minerals can hit that sweet spot thanks to either very complex crystal structures or the involvement of metals like copper and cobalt. This works fine for us humans when we're making things like paints, but metals like cobalt can wreak havoc in living things at high enough concentrations, so they're not necessarily something you want to, like, pack into your living cells so that you can have a nice bright blue color.

Instead, life is generally working with carbon as a starting material. So to make an organic or carbon-based blue, we need something that a living thing could both make and keep around without poisoning itself. And it turns out the kinds of carbon-based structures that lead to blue are often really complex, but you can do it.

Carbon's specialty is joining up in long chains or rings, and certain arrangements allow electrons to move throughout the chain. This arrangement is known as a bond conjugation. As the number of conjugated bonds gets higher, the amount of energy between the molecules' energy states decreases. That means it takes less energy to boost the electrons into the excited state, so the molecule will absorb lower energy red photons and reflect blue ones. So the more conjugation you have, the bluer the molecule appears.

Problem is, the molecules that can accomplish this can be pretty tricky to use. Part of that may be that they're big and weirdly shaped, but the relatively shorter gap between the electron states can also make it easier for these molecules to react with other molecules floating around, which ruins all your lovely conjugation and destroys the pigment. So blue pigments are hard, but they are not impossible.

If you want to find one, you can ask your liver about it. After all, there is one group of, if not super blue, at least blueish molecules that animals, including us humans, make and use on the regular. They're called biliverdins, and they're bluish-green bile pigments. Bile pigments are a group of compounds that come in several types and are all made from the breakdown of certain other compounds, often in the liver or spleen. And bile pigments come in all sorts of colors, including reds, yellows, and, yes, blues.

Now biliverdins, the blue ones, never reach a real vibrant royal blue, but they do give organisms some color. Like, they're good enough to color birds' eggs, butterfly wings, and some corals. But they're not normally used for their colorful properties, and doing so might actually come at a cost. Scientists haven't really examined the connections here all that much, at least when it comes to coloration, but we do know, at least in terms of health, you might want to be careful when you're tweaking the bile pigment production pathway.

See, biliverdin is often thought of as a waste product like other bile pigments, but turns out, it appears to also function as an antioxidant. That's a molecule that can block other reactive molecules from damaging DNA or other parts of the cell. And biliverdin itself is usually turned into a yellow pigment called bilirubin, which is an even more powerful antioxidant, one that may have a role in protecting the cardiovascular system.

There's some evidence to show that giving up biliverdin to color your body or shells could come at a cost to overall health. Researchers studying birds that make blue eggs have suggested  that mothers with bluer egg shells make them by depleting their own biliverdin supplies, giving up those valuable antioxidants.

Now, theoretically, an animal that uses biliverdin for coloration could just, like, make extra. But that would mean having extra bilirubin down the line, and bilirubin can be toxic in large doses. And then to get around that you could theoretically tamp down on the activity of the enzyme that changes biliverdin to bilirubin, but it turns out that that enzyme also helps control a lot of other important functions in cells.

So this part is a bit speculative, but there might be some evolutionary pressure to just not mess around here. So while some organisms do use biliverdin for its color, like those butterflies and corals, it doesn't seem like evolution has opted for it all that often. Again, though, we need more research to say that for sure and also know for sure why this is or isn't happening.

Now, this isn't the only pathway that can lead us to a blue pigment, and, for this next part, we need to turn to our friends the plants. Plants can make a type of pigment called anthocyanins. These are pigments found in blue flowers, and they have enough conjugated bonds to do the job. But they are, unsurprisingly, also super unstable.

In order to work, they need very specific working environments, ones that might be harmful for the cells as a whole, like special acidities or the use of dangerous compounds. So, to keep them stable, plants have to put them inside special compartments within the flower's cells called vacuoles. That way, the plant can adjust the pH, add metals or extra molecules, or do whatever it needs to keep those blue pigments happy without applying those same tweaks to the whole cell. Unfortunately for any would-be painters, that also means, if you take the pigments out of the flower, they tend to lose their color really fast.

But there are some solutions to the blue dilemma, like chromoproteins. These are proteins with color. Anthocyanins and biliverdins aren't proteins. They're complex molecules, but, in the scheme of things, relatively small. Proteins are made up of chains of amino acids and are much larger, more complicated molecules than our other options. 

But turns out they can be a good way for life to, you know, "im-blue-ve" itself. The most notable blue chromoprotein life makes may be hemocyanin, a molecule that some invertebrates use to carry oxygen in their blood. It's the equivalent of our red-colored hemoglobin. And the difference is partly thanks to metals. Hemocyanin has an atom of copper, while hemoglobin uses iron. Horseshoe crabs, for instance, have hemocyanin in their blood, and it is a beautiful baby blue color.

Another example of a chromoprotein is found in lobsters shells. Lobsters get their dark bluish hue by taking two molecules of the normally red pigment astaxanthin and sticking them together. They are then bound up in a protein to create beta-crustacyanin, which is deposited in the lobster's exoskeleton. The protein parts form weak chemical bonds with the pigment, which act kind of like conjugation in that they lower the energy needed for the electrons in the normally red astaxanthin to jump states, and that makes the complex blue. 

Fun fact, this is also why lobsters turn red when you cook them. The astaxanthin is freed from the protein as the protein part heats up and falls apart, and voila! You have a red shell. Crabs have a similar deal, and other blue chromoproteins exist in some mollusks and jellyfish. 

While there are a few ways organisms have solved the blue pigment dilemma, it ends up being a lot of hoops to jump through. Compare this to structural blues, which are complex in their own right but often made of relatively easy-to-make building blocks like collagen. So you don't have to use toxic substances, nothing that could potentially interfere with the functioning of a cell.

To be clear here, we're not saying organisms, like, considered their options and went with structures over pigments. Rather, over the course of evolution, they were just more likely to stumble on a blue structure rather than a blue pigment given all of the constraints on blue pigments, which actually brings up a pretty decent question.

Why go to the trouble of having pigments at all? Why do we have so many red and yellow pigments? After all, the molecules in structural colors are cheaper to make and more stable since they aren't absorbing light energy all the time. This is literally why colors fade in sunlight. The light energy molecules absorb can sometimes break them, splitting the molecules apart or twisting them into new colorless forms, so you also don't have to worry about structural colors fading.

But, in a poetic turn of events, while it's easy to make structural blue and hard to make chemical blue, it turns out it's actually really hard to make structural reds. You can get reds with iridescence, where the color changes depending on the viewing angle. Think of a blackbird's wing or an oyster's shell. But it's much harder to create a pure red, because blue light creeps in and pollutes it. Yellows and oranges are really hard too. So blue is actually kind of special in that you can employ structural color to get vibrant blues. 

So, to recap, it is hard for life to make blue pigments, because it is hard for life to make the right kind of molecules. They're often big or need a lot of help or require some cellular shenanigans to work right. Scientists are continuing to look for new blues, of course. We've found a lot of exciting blues in the ocean, for instance. In the marine life, not in the water, to be clear. Like, we know the water is blue. That's structural actually, not pigment. And we're not going to open that whole can of worms, because I think we already have an episode on it.

And finding and understanding these pigments could help us create better or more environmentally friendly paints and dyes. But also, it's amazing to look at the rainbow of colors life gives us and then be able to point to just one of them, just the blues, and say, "You know, this one's special."

Before we go, I wanted to talk about the Awesome Socks Club, a thing that my brother John and I are starting which will definitely not give you the blues, unless some of the socks are blue, which I think that they will be. This is your way to dress up your feet with 12 unique, snazzy designs, each created by a different designer, every month of 2021. And 100% of after-tax profits will go to decrease maternal and child mortality in Sierra Leone. So your fancy feet will be helping other people, too.  But the catch is you have to order by December 11th so we know how many socks to make. So go to awesomesocks.club/SciSchow to learn more.

[upbeat electronic music]