On an island off the south coast of Western Australia lies Lake Hillier, which is famous for its stunning pink colour.
There are many pink lakes in Australia, but Lake Hillier is special because it's permanently pink, so pink it doesn't look real —but it is.
"It's certainly not fake," says molecular biologist Ken McGrath, who has studied the lake.
"It is that pink in person. I've been there, I've seen it. It is quite phenomenal to look at… it's like a pink milkshake."
But you wouldn't drink it, he tells Mo Hamde inthe ABC Science program Weird Australia.
"Lake Hillier is 10 times saltier than the ocean seawater is."
In 2015, Dr McGrath was working as part of the Extreme Microbiome Project, which uses genomic sequencing to study organisms that live in extreme environments, like the ultra-salty Lake Hillier.
Something surprising in the water
Previous studies of other pink salt lakeshad found the salt-loving algae called Dunaliella salina in the water.
The algae contains a pigment called beta carotene —also present in carrots —which has long been thought to be what gives pink lakes their colour.
But at Lake Hillier, Dr McGrath found something surprising.
"We confirmed that D. salina, the algae, was there, but it was there in low amounts. And really one thing stood out to us and that was this bacteriumcalled Salinibacter ruber," he says.
"More than 20 per cent of every piece of DNA we recovered from the lake was from the bacterium, whereas less than 0.1 per cent of DNA recovered was from the algae."
The bacteriumcontains a pigment called bacterioruberin, which is actually much pinker than beta carotene, Dr McGrath says.
After his research trip, Dr McGrathhad a souvenir bottle of water from Lake Hillier on his desk and noticed something quite telling.
There was a window on one side of his desk and he noticed the algae — whichhave little tails and can wriggle towards the sunlight —moved to the side of the bottle the light was on.
"But the rest of the water remained pink," he says, adding this was a visible demonstration that it was the bacterium, not the algae, causing Lake Hillier'spinkness.
A pink that comes and goes
Lake Hillier has been pink for centuries, but most pink salt lakes change colour and this can depend on rainfall, Dr McGrath says.
Take a lake near the centre of Melbourne, which is currently dark green but was once a much rosierhue.
A spell of hot dry weather seems to turn it pink, says Martin Watts from Parks Victoria.
"It's a natural phenomenon that happens when the temperatures are right … evaporating the freshwater elements out of the saltwater.
"The last time it went pink was … April 2019, and we just haven't seen it return with the cooler summers that we've had."
Australia has hundreds of pink lakes – but why?
According to environmental scientist Tilo Massenbauer, pink lakes are created by a combination of factors including the climate and hydrology of this old continent we call home.
"Underneath those lakes, sit saline water tables and with … drying climate, and that stable hydrology for millions of years, the evolution has just occurred," Mr Massenbauer says.
Back in Western Australia near Esperance, Mr Massenbauer studies another lake that used to be pink, and is in fact called Pink Lake.
But its heyday of pinkness and popularity was backin the 1980s, when all the businesses in town borrowed the lake's name.
"You had the 'Pink Lake Drive-in', 'Pink Lake Road', 'Pink Lake Butchers,'"Mr Massenbauer says.
But since the late 1990s, the lake hasdisappointedtourists by no longer living up to its name at all, thanks to too many years of people harvesting salt from it.
Mr Massenbauer is currently working on a project to return the lake to its original colour, which he hopes will be a win-win for the environment since it will involve pumping salt from a nearby lake that has been contaminated with salt by agricultural practices.
He says the bacteriumS. ruber that giveLake Hillier its pinkness hasn't been found in Pink Lake, but he is counting on beefing up the presence of the beta-carotene-containing D. salina algae to do the job.
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Dr McGrath says it's possible this could work, although it will depend on what caused Pink Lake's colour in the first place.
"A key question is whether the right organisms are going to still be there, or whether they've gone extinct."
He suspects all pink lakes are caused by S. ruberand not D. salina, but bacteria areharder to detect than algae.
"Metagenomic sequencing of DNA is the only way to accurately profile a complete microbial community, including the bacteria, fungi, algae and archaea," Dr McGrath says.
Mr Massenbauer has recommended gene sequencing of sediment cores from Pink Lake to see if the bacteriumwasthere in the past.
"One of the underlying uncertainties is whether Salinibacter ruber was present before the landscape became too disturbed," he says.
