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Lake Natron is an alkaline lake located in northern Tanzania. It is known for its red waters, as seen on this image:

enter image description here

Natural-color Landsat 8 image showing Lake Natron in March 2017. NASA, public domain.

However, there seem to be some confusion around the species responsible for this color. Many sources state it is due to "cyanobacteria". Some sources have mentions of "spirulina" and "algae":

the lake actually derives its color from salt-loving microorganisms that thrive in its alkaline waters. Spirulina, a blue-green algae with red pigments, passes its pigments along to the Lesser Flamingoes that feed on the algae and raise their young here. (NASA Earth Observatory)

I know there is a lot of confusion around the word "spirulina", as the main species constituting the food, Arthrospira platensis, is not classified in the Spirulina genus. I also know there is a lot of confusion between "cyanobacteria" and "algae". Then comes another challenger, from a source already cited, but saying a different thing (emphasis mine):

Small, salty pools of water can fill with blooms of haloarchaea—salt-loving microorganisms that impart the pink and red colors to the shallow water. (NASA Earth Observatory)

So I'd like to ask: what is (are) the species responsible for Lake Natron red waters?

Is it an algae? A cyanobacteria? A haloarchaea? And if so, which species? References to scientific articles containing biological analyses of the lake waters are very much welcomed.

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  • $\begingroup$ Welcome Jean-Marie. Excellent first post here, impressively researched. $\endgroup$ Commented Jun 10 at 13:56
  • $\begingroup$ @JiminyCricket. Thanks! I have a few years of experience asking and answering on the Stack Exchange network, mostly at Earth Science SE. ;) Happy to join Biology! $\endgroup$ Commented Jun 10 at 14:44

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As far as I can tell, no one has looked (at least not indexed on PubMed) in Lake Natron specifically for algal/bacterial/archaeal species. Lake Natron is a Soda Lake (high pH and high salinity) and is part of a chain of similar lakes in the East African Rift Valley, so there have been some general studies of these lakes, but mostly looking at zooplankton and flamingoes.

It seems that this is likely to be caused by members of the genus Dunaliella. These are strongly halotolerant green algae which have the ability to survive and thrive under salt conditions that most other species do not tolerate. This genus is cosmopolitan, so found world-wide and was first noted for turning evaporation pans for salt-making pink/red. The most likely species is Dunaliella salina, which is large and (apparently) turns more red than other species in the genus. The red colour is from accumulation of carotenoid pigments in the chloroplast and conversion of the green pigments into redder ones in response to extreme UV and saline levels.

There is another species that is possible called Salibacter rubra, but this has only been identified recently in Spain, so I don't know the global distribution.

I found a chapter The Microbiology of Red Brines1 by Oren in Advances in Applied Microbiology, 2020. to be of interest on this topic. As was Chapter 18 - African salt lakes: distribution, microbial biodiversity, and biotechnological potential2 in Lakes of Africa which had this to say in the section on halophilic eukarotes (sec. 18.6.5) (with my emphasis in bold):

Filamentous fungi of the genus Hortaea, Wallemia, and Aureobasidium, the yeast Debaryomyces, and microalgae of the genus Dunaliella and Asteromonas can be found in saline systems [65]. In addition, Dunaliella is the most ubiquitous microalgae in hypersaline environments [66]. It is a unicellular, eucaryotic, and green halotolerant microalga. The genus Dunaliella comprises almost 30 species, throughout fresh, marine, and hypersaline waters. The species Dunaliella salina is the most cultivated among the genus Dunaliella, for its high concentration (up to 10% of its dry cell weight) of β-carotene. When salt lakes dry out, salinity increases. These changes help the species D. salina to produce large amounts of β-carotene, which can turn the color of the lakes into red color.

You might want to check out references 66, 67 and 68 in that text, which are:

  1. A. Ventosa, A. Oren, Y. Ma Halophiles and Hypersaline Environments: Current Research and Future Trends, Springer (2011), p. 410

  2. M.A. Borowitzka Dunaliella: biology, production, and markets A. Richmond, Q. Hu (Eds.), Handbook of Microalgal Culture: Applied Phycology and Biotechnology, John Wiley & Sons, Ltd., Chichester, West Sussex (2013), pp. 359-368

  3. A. Hosseini Tafreshi, M. Shariati Dunaliella biotechnology: methods and applications J. Appl. Microbiol., 107 (1) (2009), pp. 14-35

Refs:

  1. Oren A. The microbiology of red brines. Adv Appl Microbiol. 2020;113:57-110. doi: 10.1016/bs.aambs.2020.07.003. Epub 2020 Aug 17. PMID: 32948267.

  2. Hafsa Yaiche Achour, Sid Ahmed Saadi, Chapter 18 - African salt lakes: distribution, microbial biodiversity, and biotechnological potential, Editor(s): Mostafa El-Sheekh, Hosam Easa Elsaied, Lakes of Africa, Elsevier, 2023, Pages 501-525, ISBN 9780323955270, https://doi.org/10.1016/B978-0-323-95527-0.00009-9.

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    $\begingroup$ This looks great! I will check the references you provided asap. Thanks! $\endgroup$ Commented Jun 11 at 8:07
  • $\begingroup$ @Jean-MariePrival Thanks. Most of those refs are likely to be paywalled unless you have some academic access. $\endgroup$
    – bob1
    Commented Jun 11 at 8:35
  • $\begingroup$ The chapter in ref 2 is available and contains some nice information: "High archaea densities in hypersaline environments can be observed with the naked eye due to the bright red, orange, or purple coloration. Archaeal strains are a promising natural source of carotenoids, which is why some water in hypersaline lakes such as the Magadi Lake in Kenya are colored bright red." Later: "These changes help the species D. salina to produce large amounts of β-carotene, which can turn the color of the lakes into red color." $\endgroup$ Commented Jun 11 at 12:42
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    $\begingroup$ And finally: "These water bodies are pink, orange, or dark red. This coloration is due to the presence of halophiles archaea, carotenoid-rich prokaryotes, and the microalga D. salina." So it looks like it could be both archaea and Dunaliella. I also kept looking on my side and found this article specific on Lake Natron doi.org/10.1038%2Fs41598-019-39935-3 which states: "Haloalkaliphilic archaea start to bloom under high salinities and give the lake an orange-pink-red appearance, especially observed in the upper (northern) half of the lake." With no reference to support this claim. $\endgroup$ Commented Jun 11 at 12:46
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    $\begingroup$ @Jean-MariePrival Refs 18-20 and 22 in your linked paper might be worth following up. Seems a lot of the literature on this is in books rather than papers, which makes it a bit more impenetrable. $\endgroup$
    – bob1
    Commented Jun 11 at 20:16

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