Galdieria sulphuraria in Reykjavik.
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Credit: Christine Oesterhelt
Galdieria sulphuraria in Reykjavik.

How the Lord of the Springs Survives Where Most Things Die

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Yellowstone volcanic spring, by Andreas Meyer

Throughout Yellowstone National Park, volcanic springs bubble up through the rock. Their sulphurous water is hot, salty, extremely acidic, and laden with toxic metals like arsenic, mercury and cadmium. But even in these inhospitable conditions, there is life. The lord of the springs is a species of algae called Galdieria sulphuraria, and it has come to dominate this hostile world through genetic theft.

It’s usually simple microbes that thrive in the world’s most hostile environments—bacteria, and a group of hardy, single-celled life-forms called archaea. But G.sulphuraria is neither of these. It’s one of the so-called red algae, which is confusing because it’s not red. There’s a green version that uses sunlight to make its own nutrients like a plant, and a yellow version that breaks down chemicals in the rocks. Both live as single cells, but they can carpet Yellowstone’s rocks in colourful, slimy films. They account for 90 percent of everything living near these springs.

G.sulphuraria is also found in hot springs around the world, including in Reykjavik (in the image above) and Sicily’s Mount Etna. To understand how it survives in these difficult environments, Gerald Schönknecht from Oklahoma State University sequenced its genome.

He found that the alga has committed no fewer than 75 independent acts of genetic thievery, and at least 5 percent of its genome comes from either bacteria or archaea.

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Galdieria sulphuraria, by Gerald Schönknecht

Bacteria and archaea are well known for their ability to swap genes between one another, just as you or I might exchange business cards or email addresses. This “horizontal gene transfer” allows them to rapidly adapt to new environments by raiding the entire genetic larders of nearby species for useful traits.

But G.sulphuraria belongs to neither group. It’s a eukaryote—part of the lineage of life-forms with complex cells that includes us, other animals, plants and fungi. We tend to evolve new genes by duplicating existing ones and putting the copies towards some new use. In our trunk of the tree of life, horizontal gene transfer (HGT) is meant to be a rarity. When it’s found (and there are more and more examples of this all the time), it’s rare that anyone knows what the appropriated genes are doing. Are they actually playing useful roles in their new hosts?

In the case of G.sulphuraria, Schönknecht certainly thinks so. In fact, he thinks that these transferred genes have bestowed the alga with many of its extreme survival tricks.

Consider a group of enzymes called ATPases. Living cells use a molecule called ATP as a sort of energy currency—they build it to store energy, and cash it in to release energy. ATPases drive that second part—the breakdown of ATP. They’re utterly essential. G.sulphuraria has many famlies of them, some of which seemed to have come from archaea. Many archaea can grow at extremely hot temperatures, and Schönknecht thinks that their borrowed genes conferred the same ability to the alga.

Schönknecht also found that the alga has picked up bacterial genes that: pump sodium out of its cell and allow it to thrive in salty water; get rid of unwanted arsenic; and break down mercury.

So much for pumping stuff out—the alga’s genome is also packed with instructions for building transporters that smuggle chemicals in from the environment. Many of these transporters came for bacteria and archaea, and they allow the alga to absorb common nutrients around it, like acetate or glycerol. That’s why the yellow colonies can grow on the springs’ bare rocks, without making their own food like the green ones. They just suck it up.

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Galdieria sulphuraria in Reykjavik, by Christine Oesterhelt

But how did G.sulfuraria absorb so much genetic material from other microbes? “This is the million-dollar question,” says Schönknecht. Maybe viruses, parasites, or partner bacteria are involved in smuggling genes from one algal cell into another? “We can only speculate.”

The fact that the alga is only one cell probably helps, says Andreas Weber, who led the study. “Once a gene has been incorporated into the genome, it is passed on to the progeny with each cell division,” he says. For a many-celled creature like an animal or plant to do this, the new genes would have to make their way into sperm or eggs.

Schönknecht now expects to find evidence of horizontal transfers in other single-celled eukaryotes. Some diatoms, which are commonly found among sea plankton, have more than 300 genes that came from bacteria, but no one really knows what those genes do. That’s the big thing about Schönknecht’s study: He showed that the borrowed genes are probably helping the alga to deal with some of life’s toughest challenges.

Reference: Schonknecht, Chen, Ternes, Barbier, Shrestha, Stanke, Brautigam, Baker, Banfield, Garavito, Carr, Wilkerson, Rensing, Gagneul, Dickenson, Oesterhelt, Lercher & Weber. 2013. Gene Transfer from Bacteria and Archaea Facilitated Evolution of an Extremophilic Eukaryote. Science

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