Follow the Sulfur, Part 1

January 22, 2014 (posted January 23, 2014)
by David Levin

At a hydrothermal vent, chemistry rules. The substances that make up vent fluid can form towering structures on the seafloor. They can feed microbes, which in turn feed tubeworms, mussels, and other organisms. In short, those chemicals drive the entire vent ecosystem, and make life possible in an extreme environment.

In our past dispatches, we’ve talked at length about the ways researchers are studying the chemistry and microbiology of vent sites—but what sorts of reactions actually take place inside a hydrothermal vent, and how do microbes take advantage of them to make a living?

“The line between chemistry and biology is thin at vents,” said geochemist Jeff Seewald. “You can’t understand how a vent system works by looking only at the chemistry, or only at the biology. It’s all connected.”

To get a sense of the complex relationship between chemistry and biology at a vent system, let’s take a deeper look at one of the most abundant elements at the vents: sulfur.

How to Build A Chimney

At a very basic level, life at the vent sites we’ve visited is made possible by three things: rocks, water, and heat.

On the seafloor of the East Pacific Rise, countless volcanic eruptions have occurred, leaving vast areas of craggy rock called basalt. These rocks are full of cracks and crevices that extend deep beneath the bottom, letting seawater trickle below. As it does, molten rock (magma) far below the seafloor heats the water to more than 400˚ Celsius (752˚ Fahrenheit). When the water temperature climbs, minerals that are embedded in the rock start to dissolve into the hot fluid. One such mineral is iron sulfide (Fe₂S). As it dissolves, hydrogen sulfide breaks apart to form a different type of iron (Fe²⁺) and hydrogen sulfide (H₂S), which smells like rotten eggs.

If the "plumbing" (the network of cracks) under the rocks opens directly to the ocean floor, the hot fluid will shoot upwards until it bursts out into the cold seawater. When that happens, the fluid instantly cools, and the chemicals inside it react with oxygen in the water. The formerly clear liquid turns dark and cloudy as chemicals that were dissolved in “precipitate,” or form solid minerals once again.

It’s a bit like making rock candy. If you dissolve a few tablespoons of sugar in a glass of hot water and then put the glass in a refrigerator, much of the sugar will eventually “un-dissolve,” or precipitate out, forming a hard crystal. At the vents, a similar process takes place, but instead of candy, the minerals that precipitate out form tall, cone-like structures like these black smokers.

Several kinds of minerals precipitate out of the fluid flowing through a cone, including pyrrhotite (FeS, a silvery, glittery substance) and anhydrite (CaSO₄, or anhydrous calcium sulfate). The fluid can also leave behind metals such as copper, zinc, and gold.

The fluid inside of a black smoker is so hot that no life can exist inside it. At other types of vents, though, temperatures aren't as extreme. There, microbes and other organisms thrive, changing the chemistry of the vents in the process. We will take a closer look at those vents in tomorrow’s dispatch.

Related link:
How to Build a Black Smoker Chimney, from WHOI's Oceanus Magazine

Get to know the crew

As a cook on the Atlantis, Brendon Todd is responsible for preparing many of our amazing daily meals. Brendon first started working on the ship in 2006 after graduating from culinary school, and one day plans to open his own restaurant in Providence, Rhode Island. Read the interview »

Graduate student Ashley Grosche took an unusual path into science. While studying hairdressing at a vocational high school, she realized she had a deep interest in biology, and later took the plunge into studying microbes. Today, she’s a Ph.D. student in Costa Vetriani’s lab at Rutgers University. This is her first research cruise, and so far, she’s loved every minute. Read the interview »

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