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Gold in Them Thar’ Hot Springs

Posted by | March 17, 2010

Twenty-five years ago I was spending my summers beside sulfur-belching hot springs in northern California. The hot springs were not as big as Yellowstone’s. Most were just a few feet across, one or two about a dozen feet wide. None of them were truly boiling, but they were hot to the touch and gases bubbled vigorously out of them.

To add to the general ambiance of roasting sulfur, air temperatures in that part of California each July and August are in the 100-degree range, and in addition to sulfur, the hot springs carried a lot of mercury, arsenic and other toxic metals.

Any normal person would have fled the scene.

But I loved it all, because in addition to the hellish aspect of the hot springs, they carried trace amounts of gold in their waters. That meant they precipitated meaningful amounts of gold where the waters cooled.

That’s all pretty unusual in this world. In fact, it’s the only place I knew where gold makes it all the way to the surface of the Earth in such low temperature waters.

Memories and images of those summers crowded my mind recently as I was learning about something else that’s unique to certain hot springs, namely those that are more acid and higher temperature. Yellowstone has some like these, with pH values down to 1 and even 0 (amazing, but true). If you have been to Yellowstone, these are the “ugly” and stinky hot springs and mud pots to the north of the main lake.

It’s natural to think nothing could live in the conditions of such spring water. But the Yellowstone hot springs have a host of “archaeal” organisms living in them. That name means they are single cell creatures that, like bacteria, have no nucleus, but they are even more primitive than bacteria.

One kind of archaeal organism is Sulfolobus solfataricus. (Yes, I know microbiologists love names that can look intimidating at first. But just concentrate on the fact the ‘sulf’ and ‘solf’ syllables suggests a love of sulfur, and the ‘lob’ syllable suggests a lobe-like shape for the microbe when you look at it under the microscope.) Sulfolobus solfataricus live in Yellowstone’s hot springs and elsewhere in the world, too.

Professor Cynthia Haseltine of Washington State University and her graduate students study the creatures in question. And she’s trying to teach me a little about them, because of my old interest in hot springs. (And, I suspect, because she’s interested in “my” hot springs that can transport mercury and gold. But we’ll get to that.)

But already I’ve been in for big surprises.

In middle age, I thought I knew the basics of life. But it turns out I’m really woefully ignorant. Haseltine’s microscopic “bugs” can fuel themselves in two radically different ways. Like you and me, they can eat carbon and nutrients and live on that. But they can also make a living off carbon dioxide and sulfur. I knew little bugs could do the carbon dioxide and sulfur diet, but I didn’t know they could switch back and forth to “our” sort of eating, too, when it suits them.

“But that makes sense in evolutionary terms,” Haseltine says. “If a microbe is living in the hot spring with sulfur and carbon dioxide available, it can grow – but that’s slow growth. But then if leaves fall into the hot spring in the fall, Sulfolobus solfataricus can ‘switch’ and make a living and grow more quickly using the leaves as fuel.”

Back to me in California, crouching over the hot springs that carried gold. All the work we did in that project tried to explain gold transport without respect to living creatures.

But what excited both Professor Haseltine and this Rock Head in talking to each other is the possibility that some Sulfolobus organism is not just present in the spring – Haseltine is sure it is – but that it may be interacting with the trace amounts of gold in the waters in ways that bring them to the surface.

Imagine that: from eating sulfur to dead leaves to, just maybe, ingesting gold, too.

That’s quite a bug.