## Salar de Uyuni

I just learned about the Salar de Uyuni: the world’s largest salt flat, located in southwest Bolivia. It’s about 10,000 square kilometers in area!

It’s high up, near the crest of the Andes, 3,600 meters above sea level. Once there were permanent lakes here, but no more. This area is a transition zone: the eastern part gets rain in the summer, but clouds never make it past the western part, near the border with Chile. Further west comes the the famously dry Atacama Desert.

The Salar de Uyuni is high, but still it lives up to the name ‘salt flat’: its salt crust varies in height by less than one meter over the entire area. It’s so flat that people use it for testing equipment that measures altitudes.

Why is it so flat? Because the dry crust covers a huge pool of brine that is still liquid! This brine is a saturated solution of sodium chloride, lithium chloride and magnesium chloride in water. As a result, Salar de Uyuni contains over half of the world’s lithium reserves!

In the rainy season, the Salazar de Uyuni looks very different:

And when it’s wet, three different types of flamingos visit the Salar: the Chilean flamingo, the rare Andean flamingo, and the closely related but even rarer James flamingo, which for a while was thought to be extinct!

Flamingos eat algae that grow in the brine. This is why they’re pink! Newly hatched flamingos are gray or white. Their feathers become pink only thanks to carotene which they get from algae—or from crustaceans that in turn eat algae. Animals are not able to synthesize these molecules!

Carotene comes in different forms, but here is one of the most
common: β-carotene. I like it because it’s perfectly symmetrical. It has a long chain of carbons with alternating single and double bonds. Electrons vibrating along this chain absorb blue light. So carotene has the opposite color: orange!

It’s not just flamingos that need carotene or related compounds. Humans need a chemical called retinal in order to see:

It looks roughly like half a carotene molecule—and like
carotene, it’s good at absorbing light. Attached to a larger protein molecule called an opsin, retinal acts like a kind of antenna, catching particles of light. Humans can’t produce retinal without help from the foods we eat. Any chemical we can use to produce retinal is called ‘vitamin A’. So vitamin A isn’t one specific chemical: it’s a group. But beta carotene counts as a form of vitamin A.

Speaking of humans: people sometimes come to have fun in the Salar de Uyuni. There are hotels made of salt! And thanks to the featureless expanse of salt, you can take some amusing trick pictures:

Click on the pictures to find out more about them. For more on the Salar de Uyuni, try:

Salar de Uyuni, Wikipedia.

Puzzle: What kinds of algae, and other organisms, live in the brine of the Salar de Uyuni when it rains? How do they survive when it dries out? There must be some very interesting adaptations going on.

### 14 Responses to Salar de Uyuni

1. Jak Kornfilt says:

Fascinating. thanks!

2. dwinsemius says:

This webpage from an Australian researcher disputes the theory that the carotenoids in flamingo diet come from algae, and instead says they are synthesized by haloarchaea bacteria: http://www.haloarchaea.com/

• John Baez says:

Thanks! One of the websites I read said that carotene is a protein, so I guess one can’t really expect to get reliable information on biochemistry and microbiology from what are essentially touristic accounts. It makes a lot of sense to me that only archaea, not algae, are able to survive such tough conditions: extreme salinity alternating with complete desiccation!

By the way, archaea are no longer considered to be bacteria. They are now considered to be their own domain:

It would be very fun to learn more about haloarchaea and how they survive under these harsh conditions. They use bacteriorhodopsin rather than chlorophyll to get energy from sunlight.

3. Graham says:
4. Wikipedia says “It has a stable surface which is smoothed by seasonal flooding (water dissolves the salt surface and thus keeps it leveled)” – seems like a bit of different explanation for the flatness than you gave, so now I’m curious!

• John Baez says:

Wikipedia says there’s liquid brine underneath the surface, but I can’t remember why I thought that this is why the surface is so flat. Maybe I just guessed. So maybe I’m wrong… or maybe the other theory is just a guess too. Maybe both effects play a role.

5. John Baez says:

In email Jorge Pullin wrote:

But my friend, you missed the most important part, the train cemetery!

Click the links for more; the second explains why the trains are there.

6. Ivan Tarasov says:

Xerophiles/halophiles, that do various tricks with being able to dehydrate/rehydrate and/or control the salinity of their cells?

Uyuni is amazing. Lagunas near Uyuni are very outlandish and beautiful, especially Laguna Colorada. Trains are a bit overrated (and very overcrowded by tourists), but the story behind them (and places like Julaca) is the interesting part.

Puzzle: Looking at the map of Uyuni we can estimate that there’s probably, at least 32km of the salt flat expanse available in some direction. Knowing the radius of earth, the height of the spherical cap which is 32km big across its surface is (if I didn’t screw up the math)

$d = R (1 - \cos (\frac{L}{2R})$

for

$L = 32 \text{km}, R = 6371 \text{km}$

is $d \approx 0.02 \text{km}$ — around 20 meters! Why do we call it “flat”?

• John Baez says:

Good puzzle! I’ll answer a puzzle with a puzzle:

Puzzle. If you stood on a perfectly spherical Earth on a very clear day, how would it look different than if you stood on a perfectly flat Earth on a very clear day? Would the visual appearance be the same, or can you tell whether the Earth is round just by looking at it, under these idealized conditions?

7. Steve Wenner says:

I notice that the dry season photo shows the salt flat tiled by polygonal plates of a certain size. This reminds me of drying cracked mud flats and other similar phenomena. I always wonder what determines the characteristic scale of the patterns. Is this known, or a mystery?

• John Baez says:

Very nice question—this is something a good physicist would wonder about? Curie’s principle implies that “it takes a length scale to create a length scale,” so there must be some length scale at work that answers your question: for example, the depth to which the top layer of mud dries out.

The Wikipedia article on mudcracks sheds no light on your question, though it has some nice pictures:

You’ll notice there are several length scales here: big cracks and smaller cracks. One could even imagine cracks that form a fractal (ha-ha, a pun!) with no characteristic length scale. But I don’t believe this is a fractal, even approximately: I believe it would look noticeably different if we zoomed in or out even a little bit.

All fractals in nature are only approximate: the self-similarity only extends for a certain ratio of length scales, as small- or large-scale phenomena become important and break the rescaling symmetry. But in some cases the approximation of rescaling symmetry is a useful one; here it seems only very slightly useful.

In fact there’s a certain very famous crackpot (ha-ha, another pun!) who got his start studying fractal structures in cracks, but I’d rather not name him here since he once threatened to sue me.

According to Wikipedia, this book proposes a classification of mudcracks:

• J. R. L. Allen, Sedimentary Structures: Their Character and Physical Basis, Volume 2, Elsevier, Oxford, 593 pages.

I wonder how long volume 1 is! This guy is deep into mud.

Finally, let me add that Curie’s principle is not always true: it’s not always true that symmetrical causes lead to symmetrical effects: ‘spontaneous symmetry breaking’ can occur. There are even examples of spontaneous breaking of rescaling symmetry, such as in quantum chromodynamics, where it’s called dimensional transmutation. However, mudcracks seem to have a fairly replicable, not randomly chosen, length scale, so I don’t think spontaneous symmetry breaking is relevant here!

8. Scott Hotton says:

Here is an interesting paper about bacteria and salt:

• José Marıa Gómez Gómez, Jesís Medina, David Hochberg, Eva Mateo-Martí, Jesús Martínez-Frías and Fernando Rull. Drying Bacterial Biosaline Patterns Capable of Vital Reanimation upon Rehydration: Novel Hibernating Biomineralogical Life Formations. Astrobiology, Volume 14, Number 7, 589–602. (2014)

The authors performed some laboratory experiments involving the dehydration and rehydration of E. coli in NaCl solutions on plastic surfaces. E. coli are eubacteria not archea but their observations may be relevant to wild haloarchea in salt flats.

They observe E. coli-NaCl aggregates develop as water was removed. The aggregates formed dendritic crystalline structures that somewhat resemble snow flakes except they have fourfold symmetry instead of sixfold symmetry. The fourfold symmetry is not surprising since pure NaCl crystals have a face centered cubic structure.

I like this supplemental video of theirs

http://tierra.rediris.es/erica/media/Supplementary_file-Movie_3.avi

which shows a branching tree structure juxtaposed to the dendritic crystalline structure. Both types of patterns are common in diffusion limited aggregation processes.

The authors say that the E. coli-NaCl aggregates have a two layered morphology. They write:

A thin filmlike top layer formed by NaCl conjugated to, and intermingled with, ‘mineralized’ bacterial cells covers a bottom layer constructed by the bulk of the nonmineralized bacterial cells; both layers have the same morphological pattern. In addition, optical microscopic time-lapsed movies show that the formation of these patterns is a kinetically fast process that requires the coupled interaction between the salt and the bacterial cells.

They propose that this two layered morphology can help protect the bacteria in the inner layer during periods of dehydration. The structure easily dissolves when water returns and the bacteria become active again.

• John Baez says:

That’s really cool! If even the humble E. coli can survive such sailinity, perhaps many bacteria can.

I’ve never seen four-pointed ‘saltflakes’ growing in a supersaturated solution of salt, but now I’m wondering why not.

9. Akshay Hotkar says:

These are Halophiles.
Halophiles can be Bacteria, Blue Green Algae,or Simply Algae. As the Flamingoes feed on Algae which are rich in Beta Carotene, the Beta Carotene is processed by body and the orange color of Beta carotene is reflected through the keratin.
These extermophiles adapt to harsh environment making them one of the few surviving organisms in such conditions.