I recently found out that at there is such a thing as “millennial pink,” which refers to the color pink's rise in popularity coinciding with the dominance of the universally mocked millennial culture. Perhaps this explains why I was so thrilled and intrigued, and just tickled pink I guess, to learn that in Westgate Park, in Melbourne Australia, a lake turned hot pink! And this is not an isolated phenomenon: pink lakes exist all over the world. But why are they pink? This turns out to be a rather complex and fascinating question.
The key to the pink lakes has to do with the high salinity of these bodies of water and the microbial ecosystems that thrive in these conditions. Salt, in high concentrations, puts a lethal amount of osmotic stress on a cell – causing it to rapidly lose water and shrivel up and die. This is why saline solutions are often used as an antimicrobial measure.
So one would think that a body of water containing high concentrations of salt would be a terrible home for an organism. But as we have seen before, microbes are astonishingly adaptable, and numerous types of bacteria, algae and archaea, have evolved strategies to overcome the dangers of high-salt environments. These salt-loving (“halophilic”) organisms thrive in salty lakes because their salt-sensitive competitors have been killed off – leaving all the lakes’ bounty to the well adjusted. Natural selection at its finest!
The strategies employed by halophiles to overcome osmotic stress are numerous and diverse, but the general theme is this: osmotic stress is caused by a disparity in the concentration of a solute between the intra and extra-cellular sides of the cell membrane. Upon sensing high salt concentrations in their environment, halophiles either produce intracellular solutes (like amino acids or glycerol), or increase salt transport into the cell (using trans-membrane ion pumps). This allows them to match the concentration of solutes in their intracellular environment to that of their surroundings, decreasing osmotic pressure, and preventing water loss from the cell. This generalized strategy is employed by halophiles (ranging from bacteria to plants) – but our explanation here really only skims the surface of a complex and deeply fascinating area of research. In fact halophiles have even grabbed the attention of astrobiologists interested in adaptations that might allow an organism to survive in the extensive salt formations on Mars.
Many halophiles utilize photosynthesis to generate energy, and for halophiles this sunlight-induced metabolic activity is likely essential to generate the solutes or proteins required to maintain an osmotic balance in the cell. To maximize their sun exposure, some salt-lake dwelling halophiles float on the surface of the lake – and while this may increase cellular productivity, the unrelenting sunlight also increases their risk of UV-mediated DNA damage. Yet these resourceful microbes have come up with a chemical strategy to overcome this environmental danger! Some microbial photosynthesizers produce large quantities of anti-oxidants that scavenge the DNA-damaging free radicals that are generated upon UV exposure. In other words, these microbes can produce their own SPF!
So what does this have to do with the pink color of the lake? Well it just so happens that many of the halophiles residing in pink lakes produce a class of anti-oxidants called carotenoids, which are brilliant shades of orange and red. In fact, these molecules are often generated in such large quantities that the organism itself turns bright red, causing a pink-coloration of whatever broth they are growing in. One well-known example is that of Dunaliella salina - a halophilic unicellular green alga (so-called because it contains chloroplasts) that is commonly found in pink lakes. Researchers have shown that this microbe, when exposed to high salt concentrations, produces vast amounts of the carotenoid Beta-carotene. This well-studied anti-oxidant is a brilliant red-orange color and is a precursor to vitamin A, which is widely used in both the food and cosmetic industry (in the cosmetic industry, as retinol). In the 1960s D. salina became a paradigm for how halophilic microorganisms could be mass-cultured for commercial production of Beta-carotene.
But it turns out that while the commercial interests drove extensive study of D. salina, this organism is probably not the major contributor to the pink color of salty lakes. Sample collection from these lakes reveals diverse halophile populations that are often dominated by red archaea or bacteria, many of which are not well studied. These non-algal halophiles likely produce Beta-carotene or other carotenoids and could also be a source of previously uncharacterized red (or not red) molecules.
The study of the microbial population dynamics of the pink salty lakes is pretty neat. There are cool colors, fascinating environmental adaptations and potential for commercial production of useful natural products – but there’s still quite a lot to be determined. Scientific efforts like the Extreme Microbiome Project aim to classify and characterize the fascinating organisms that reside in Lake Hillier, a well-known Australian pink lake, as well as numerous other extreme environments such as the Door to Hell gas crater and the toxic hot springs in Ethiopia. Through extensive sample collection and genome sequencing, this type of research aims to expand our understanding of the different types of organisms that thrive in diverse environments, and to examine the strategies these microbes use to overcome seemingly hostile environments.