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The BBC News article Pink lake in Australia attracts and delights tourists describes an artificial salt lake in Westgate Park, Melbourne, Australia that tuns pink around February and then in "late Autumn, until cooler temperatures restore it back to blue."

I am not sure about the autumn part as it's in the southern hemisphere, but my question is about the color change.

  • Could they mean beta carotene rather than beto carotene?
  • And if so, does the pigment stay inside the algae or is it released into the water to produce this color change?
  • And why pink, rather than the orange color we associate with carrots?

enter image description here

In those conditions, the lake's algae produces a red pigment called beto carotene

enter image description here

The lake's pink colour typically lasts through to late Autumn, until cooler temperatures restore it back to blue.

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    $\begingroup$ It's definitely a typo and should be beta. $\endgroup$
    – Arsak
    Mar 29 '19 at 5:31
  • $\begingroup$ Carrots actually come in a variety of colors, orange carrots is just a fluke of political history and has little to due with beta-carotene. $\endgroup$
    – John
    Dec 31 '20 at 17:06
  • $\begingroup$ Note there are other ways to get pink lakes halobacteria dominated lakes (hypersaline) are often pink becasue they don't use chlorophyll but bacterioruberin. Salt itself tends to be pink which can contribute. Lake hiller in Australia is one such lake. en.wikipedia.org/wiki/Lake_Hillier $\endgroup$
    – John
    Dec 31 '20 at 17:11
  • $\begingroup$ @John see my comment below questioning if "Salt itself tends to be pink". $\endgroup$
    – uhoh
    Dec 31 '20 at 22:06
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The "beto" is a typing mistake. Beta is an organic chemistry label referring to a beta-ring of 6 carbons that exists at either end of a carotenoid polymer C40-Hx.

It is localized in the chloroplast. The plants don't really have a mechanism to release pigments, they can re-metabolize chlorophyll. The volvocalean algea Dunaliella salina is used to harvest beta-carotene in Karratha, Western Australia, from salt lakes. The can have 12% beta-carotene as dry-matter content, the highest of any known source. The squares are probably salt. enter image description here the algea afterwards: enter image description here

There are over 1,100 known carotenoids of which at least 700 have been identified in plants. They include carotene and xanthophylls, and carotene include beta-carotene. The plants and algea use it to protect themselves from intense sunlight and UV's. It is an anti-oxidant. enter image description here

Pink doesn't actually exist as a color. It is imagined when green and blue light is sensed by the eye, while red is at a maximum. In the lakes, we know there is 80% red and 20% green pigment in the algea. All that is required would be 20% blue to percieve pink. I don't know where the blue color comes from, the sky/water. Orange is complex for the eye, and so is pink, because we can only see red green and blue. enter image description here enter image description here

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  • $\begingroup$ Oh, this is a thoroughly wonderful and wonderfully thorough answer, thank you! $\endgroup$
    – uhoh
    May 19 '19 at 8:12
  • $\begingroup$ Hey thanks! People should be warned that the BBC takes 150 dollars per year in taxes from the 51st state and is keener on hiding state secrets than journalism, it's the same as RT in higher secrecy. $\endgroup$ May 19 '19 at 8:31
  • $\begingroup$ oh-kay.... I've been duly warned ;-) $\endgroup$
    – uhoh
    May 19 '19 at 8:35
  • $\begingroup$ worth noting that yellow and cyan are actually parts of the spectrum magenta is not. you can have pure yellow wavelengths, not so for magenta. $\endgroup$
    – John
    Dec 31 '20 at 17:57
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There are several combined reasons.

first salt itself in such lakes is often pink, so part of the color comes from the salt crust itself. this is more noticeable in more extreme salt lakes where the deposits can be quite thick. It can be due to iron impurities but can also be further dyed by trapped carotenoids in the salt as it is deposited.

Part is also coming from salt loving halo-bacteria which use are not algae but are photosynthetic. They use bacterioruberin and salinixanthin for photosynthesis but not chlorophyll. these are carotenoids and pink, and responsible for many pink salt lakes like lake Hiller. (there is an interesting link between these bacteria and why chlorophyll is so inefficient that is worth reading)

the last is indeed beta-carotene, produced by salt tolerant algae.

In the case of that particular lake algae appears to be the major factor. However there are many pink lakes each caused by a different mix of carotenoids produced by salt loving organisms which depends on just how salty it is.

As for why it looks pink instead of orange, the answer is carrots have other pigments that shift the color further towards orange, particularly yellow pigments. Also water is transparent so mix a pigment into it and you get a lot of subsurface scattering which can shift color towards blue, this is part of the reason water is blue. Purple and pink are artifacts of how the human eye sees color and not "real" colors so when you combine reds and blues you get pinks and purples.

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    $\begingroup$ Thanks for your answer! "...salt itself in such lakes is pink..." is surprising to me, I've never heard of pink salt. Your comment also mentions that the salt can be pink but I've never heard of at least an inorganic salt solution that has such a color. Can you support this with a source? I don't see it stated explicitly in the Wikipedia article linked in your comment that it's the salt itself that produces the color. $\endgroup$
    – uhoh
    Dec 31 '20 at 22:05
  • $\begingroup$ In solution? Can you find a source supporting that iron makes salt solutions pink? The optical properties of crystals are often affected by dopants (e.g. the variety of colors of diamonds and sapphires) but those effects are related to the crystal structure. Once dissolved, they crystal structure is destroyed and thse effects are no longer relevant. $\endgroup$
    – uhoh
    Dec 31 '20 at 22:19
  • $\begingroup$ trace minerals (mostly iron) turn sea salt pink. Not all sea salt is pink but many are. this is especially noticeable in thick layers of salt. Interestingly you can get darker pink and even red salt due to a build up of carotenoids in the deposited salt when it is deposited in isolated basins (allowing hyper-saline conditions to persist and the micro-organims to thrive). $\endgroup$
    – John
    Dec 31 '20 at 22:22
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    $\begingroup$ I was referring to pink lakes in general, I stated that the lake in question algae was the major factor. $\endgroup$
    – John
    Dec 31 '20 at 22:35
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    $\begingroup$ read the one about the algae which is a different orgaimsims, that survives in less salty lakes, said lakes are sill MUCH saltier than normal sea water. also look at salt farming in which halo bacteria often take over super saturated brine pools turning them pin, often getting pinker as they become more salty. you will rarely get all there at the same time but it is very possible to have two of them at the same time. $\endgroup$
    – John
    Dec 31 '20 at 23:42
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Not sure which kind of microorganism they talk about, but beta-carotene is a red-orange~ish pigment (as you pointed out, also produced by carrots). The pink coloration I would assume is the product of the color mixture of the pigment with green-blue from the water (not the water itself but the blue reflection from the sky + any other algae in the water). If they release it (the beta-carotene) to the water, again not sure which organism they talk about but it's probably not like it, since beta-carotene is a photosynthetic compound. Therefore is pretty likely they may want to keep it inside.

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    $\begingroup$ Do you have any sources? $\endgroup$
    – TanMath
    Apr 1 '19 at 3:01
  • $\begingroup$ It was pure guess, but here you go $\endgroup$
    – user50823
    Apr 1 '19 at 8:54
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    $\begingroup$ @N.P. the message "...requires additional references. Please edit to add citations to reliable sources..." should be interpreted literally. If you can edit your post to add your citation to your answer, along with a short description of how the reference supports your answer (usually a short block quote from it is sufficient) that would improve the quality of your answer. Thanks! $\endgroup$
    – uhoh
    Apr 2 '19 at 23:41

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