The Hot Life

Hot springs, colourful microbes: surviving in a world that’s unique, beautiful, and hostile

Yellowstone National Park is a self-proclaimed “living museum of natural history”. It contains 3,500 acres of rich landscape situated atop a volcanic hotspot. Home to grizzly bears and bighorn sheep, mule deer and timber-wolves, Quaking Aspen and Engelmann Spruce, Yellowstone’s life is as varied and remarkable as its geography.

But, what’s even more interesting than all the animals and plants that call Yellowstone their home? Perhaps the microscopic organisms that form rainbow rings in their hot springs.

Imagine an enormous, deep hole in the middle of a rocky landscape. Three hundred and seventy feet across, it is almost like a football-field dug into the ground, endzones and all. But, this is no empty hole. Even as you stand, you can feel the hot air blowing into your face from the scalding waters below. Temperatures there reach up to 70°C, the same as a slow oven.

This hole is called the Grand Prismatic Spring, and it’s one of the most notorious hot-springs in Yellowstone. At its cobalt blue center are gallons of superheated water, fuelled by the fiery depths of the Earth and rendered sterile by the scalding temperatures. This extreme heat, combined with the risk of earthquakes and eruptions, make the hot-spring an overwhelmingly hostile place.

But even through this hostility, life persists.

Millions of years ago, when most carbon was still locked in the ground and life was very young, the Cyanobacteria arose. These bacteria were the first-ever creatures to learn the secret of photosynthesis, generating energy straight from the light of the sun.

Cyanobacteria — or “cyano”, as I’ll call them from now on — are the oldest photosynthesisers. And they’re still at it today. Because they evolved in much harsher conditions than ours, cyanos well adapted to survive in extreme places: places that you or I might think impossible to live in. Each small but mighty green cell contains powerful, microscopic machinery that allows them to survive.

To start with, cyano follow the same age-old rule as many other microbes: don’t go it alone.

When living in extreme environments, often get themselves into microbial mats: spaces with everyone living together, woven into a matrix, and covered in a nutrient-rich, sugary film. Microbial mats are like complex, staffed communities of bacteria. They’re highly organized, with different species playing different roles within the system. The role of cyano was thought to be energy making factories, focusing all their efforts into the photosynthetic process, but in general, the byproducts of each microbe can be used as food for another.

Microbial mats are heavily studied by ecologists, who seek to understand how the microbes interact with each other and their environment. They’ve also gained the attention of scientists at NASA. These “exobiologists” study cyanobacteria to understand the life that existed on the early Earth, and figure out what traces they leave behind so we lookout for the same patterns on other planets.

If you’re not a scientist, you can still see cyanos by visiting their habitat. The microbes in Yellowstone’s mats are tiny, but because there are so many of them, the structures they form are visible to the naked eye Different microbes form different colours on the mat.

And in the midst of them, you can spot the cyanobacteria as expansive bands of emerald green.

Different places have different kinds of cyanobacteria. In Yellowstone, with lots of sunlight during the day, cyanos are adapted to make plenty of energy through photosynthesis. But what wasn’t clear until the early 2000s was how this particular cyano manage to find and use nitrogen.

Nitrogen is common on Earth — it’s the most abundant gas in our atmosphere. But, it exists in a form that most living organisms cannot immediately use: inorganic N₂. In order to use this atmospheric nitrogen, organisms must collect it, break it apart, and turn it into something that can be absorbed — or rather, assimilated into organic molecules such as the vastly important DNA and protein found in every living cell.

Some groups, like the soil cyanobacteria Nostoc, are able to fix nitrogen on their own. But Nostoc doesn’t have to cope with extreme heat the way Yellowstone’s cyanos do — which is why scientists once doubted the latter could even perform such a feat. After all, if these cyanos are so focused on photosynthesising, they’d hardly have time for anything else.

So, how do the most extreme cyanobacteria gain enough nitrogen to survive? The surprising answer: by changing their metabolism when the sun goes down.

During the day, the cyanos are hard at work: photosynthesising and making the energy needed to grow and reproduce; staying active; maintaining the microbial mat. But when night falls, their metabolism switches like a light. Instead of photosynthesising, they begin turning nitrogen gas from the air into nitrogen compounds they can use.

When scientists monitored the cyanos over a 24-hour period, they saw that photosynthesis pathways shut down at night, while genes used for nitrogen fixation turn on. This answers a crucial question about how life manages to get enough nitrogen in extremely hot environments.

Cyanobacteria are the first organisms we know of that can make energy and fix nitrogen at such incredibly high temperatures. But they’re just one example of the spectacular skills organisms display Yellowstone’s diverse habitats. Perhaps even more hostile than their environment is the superheated and relatively dry soil that can be found throughout Yellowstone.

Take your eyes away from the colourful hot-springs for a moment, and look instead at the hot soil. They too are superheated, reaching up to 65°C which is just five degrees less than the hot-springs themselves. While there is an abundance of water in hot springs, in soils of this heat, life also has to balance extreme dryness. Most plants and fungi would dry, shrivel, and die.

Despite the odds, if you were to visit Yellowstone, you could see little clumps of dark-green grass rising up from the ground: the woolly-rosette grass Dichanthelium lanuginosum. Their thick leaves spread out like tiny rosettes to catch the Sun, but underneath the surface roots are at extreme heats. And to survive, they must employ some unique tactics.

Most of these tactics are chemical — like changing the structure of molecules in their membranes and DNA to make them more stable, which in turn will prevent the heat from tearing them apart. But, notwithstanding, these molecular adaptations, surviving the blistering heat sometimes relies on teamwork.

The panic-grass would never be able to survive if it wasn’t for the symbiotic fungus, Curvularia protuberata. While plants and fungi can’t survive soils of such extreme heat on their own, they can work together symbiotically to build up a tolerance for heat. It’s not clear quite how they’re doing it, but they’re definitely collaborating in some way or another.

And wait. The story’s not over yet.

In 2007, a new paper came out noticing how fungal viruses can influence the behaviour of the fungi they infect. Researchers were trying to sort out the fungi living in the plant-fungi system of Yellowstone, when they discovered a new virus. As luck would have it, that virus was living inside Curvularia, the same fungus that woolly-rosette grass D. lanuginosum collaborates with.

This virus, they found, is what gives Curvularia its heat-tolerance properties. Without the virus, there was no heat tolerance observed in the plants — suggesting that these viruses were not only involved, but necessary for the survival of the fungi and so the survival of the plant. That’s why it’s been tentatively named CThTV, short for “Curvularia thermal tolerance virus”.

CThTV is a pretty remarkable discovery. While two species often collaborate through symbiosis, this is the first time scientists have found a close, tightly-integrated partnership of three.

The exact mechanism by which fungi contribute to the plant’s heat tolerance is not yet understood. Maybe the fungi helps remove harmful compounds, which accumulate when the plant is stressed by heat. It will take more research to understand the exact role that CThTV plays, but what’s interesting is that such a setup exists at all.

It is, as the authors duly noted in their paper title, “a virus in a fungus in a plant”.

Yellowstone national park is truly a museum of natural history — because it’s a great space for grizzly bears and Engelmann spruce, yes, but also because it’s not the best for many creatures. The periodic earthquakes, rare but sudden eruptions, consistently present scalding water and scorching soil all make it an uncomfortable place to live.

So, the organisms that do stay on are the hardy adaptable one. They’re the ones that shatter our expectations of what life is capable of. The shimmering microbial mats, the patient grasses on the ground, and many other species I haven’t even heard of yet; they all have secrets to show, if only people are willing to look.

They teach biologists how important collaboration is in the natural world. They teach space agencies how life on other planets might alter its environment. And, most important of all, they tell us a story of resilience in the face of all odds, in places that are too frequently overlooked.

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Curious for more? Sources and references for this story can be found here.