Follow the Sulfur, Part 2

January 23, 2014 (posted January 24, 2014)
by David Levin and Cherie Winner

Yesterday we talked about how chimneys form at hydrothermal vents. Today we’re going to explore a different kind of venting—the kind that supports most of the exotic life forms in these unusual habitats.

Where the chemical action is

Chimneys form where hydrothermal fluid shoots out of the seafloor like a geyser. In other cases, the fluid seeps out over a wider area. This is called “diffuse flow.” It occurs in areas where the "plumbing" system between rocks below the seafloor doesn’t form a “pipe” that goes directly to the seafloor. Instead, ultra-hot vent fluid percolates in twisting, turning pathways. Cold seawater trickles down through cracks and pores and mixes with it below the seafloor. That does two things: It cools down the fluid, and it mixes in oxygen from seawater.

Once the fluid’s temperature drops below 130˚ Celsius (266˚ Fahrenheit), the highest known temperature in which life can survive, microbes begin to grow. These microbes live below the seafloor, far beyond the reach of sunlight. Many of them gain their energy from chemicals such as hydrogen sulfide that dissolved into the original vent fluid.

To extract energy using these molecules, the microbes need to carry out a special type of chemical reaction within their cells. It’s called a “redox” reaction, and it’s critical for the existence of life.

Redox reactions

The name “redox” comes from a combination of the two types of chemicals involved in the reactions. The first is a “reductant,” a compound that has plenty of extra electrons that it can give away. The second is an “oxidant,” a compound that desperately wants to have more electrons.

As electrons transfer from the reductant to the oxidant, energy is released. Some bacteria and archaea capture this energy for their own use. Redox reactions are the first stage of the process of chemosynthesis.

For many microbes living in hydrothermal vent fluids, hydrogen sulfide (H₂S) is the reductant. It gives electrons to oxygen (O₂), which is in the seawater that trickles down below the seafloor. In the process, microbes turn the hydrogen sulfide into sulfate (SO₄^2-) and release hydrogen ions (H⁺):

H₂S + 2O₂ —> SO₄^2-  + 2H⁺

You can see that this reaction uses a lot of oxygen. In places where little oxygen exists, such as under the seafloor, the bacteria can’t completely oxidize all the hydrogen sulfide into sulfate (SO₄^2-) because that would require more oxygen atoms than are available. Instead, the microbes turn some of it into elemental sulfur (S⁰), which requires much less oxygen.

2H₂S + O₂ —> S⁰ + H₂O

This sort of reaction is common at vent sites such as Crab Spa, where microbes living deep within the vent create snowy bits of sulfur that billow out of the vent as yellow-white flecks. (You can see them streaming out of the Crab Spa vent in this video.)

Nothing wasted

In most cases there is not even enough oxygen (or other types of oxidants) in diffuse vent fluid to convert all the available H₂S to S⁰. In that case, the microbes can’t use all of the available H₂S “fuel,” and any leftover H₂S in the vent fluid will flow into the ocean unchanged.

Think of it like trying to build a fire: If you have lots of sticks, but only a small amount of air, your fire will only burn for a short time before going out. If you blow on it to increase the air (oxygen) supply, the fire will grow quickly, burning up all the fuel within its reach. The fire can’t continue to burn without the right amount of air (oxygen) and fuel.

The same idea applies when microbes try to get energy from hydrogen sulfide at deep-sea vents. If they don’t have enough oxygen, they can only “burn” a small amount of the H₂S. The rest of the H₂S will remain dissolved in the fluid.

Leftover H₂S doesn’t go to waste, though. At the seafloor, it mixes with seawater, which has lots of oxygen dissolved in it. Microbes living in the water just above the seafloor use this oxygen in redox reactions with hydrogen sulfide in the vent fluid. So chemicals in the fluid at diffuse-flow vents provide energy for microbes both above and below the seafloor.

On up the (food) chain

Redox reactions don’t happen only in free-living microbes. They also occur inside larger organisms, such as tubeworms and mussels, that live near vents. Those animals absorb H₂S and O₂ from vent fluid and deliver them to bacteria living inside their bodies. These microbes combine the H₂S and O₂ to gain energy just like their relatives living in the fluid in and around the vents.

Other chemicals in vent fluids, such as hydrogen and nitrate, can also donate or receive electrons, so there is a variety of redox reactions that microbes at vents can use to gain energy. The microbes then use that energy to convert carbon dioxide (CO₂) into nutrients such as sugars. This is the second, or “synthesis” stage, of chemosynthesis.

The microbes use the nutrients they make for themselves. But if they live inside a tubeworm, mussel, or other animal, they also make the nutrients available to their hosts. Free-living microbes are eaten by protists and other small organisms, which digest them to produce energy and materials to build their own bodies. These small organisms, in turn, are eaten by larger animals. In these ways, microbes use energy from chemicals in vent fluids to produce food that supports the entire ecosystem.

Any chemicals or nutrients in the fluids that aren’t used at the vents float away into the open ocean, where still other microbes can use them. In that way, the chemistry of a hydrothermal vent can help support life not just on the ocean floor, but also in the water at great distances from the vents.

Signing off

And with that, our expedition ends. We said our good-byes the other day, so now I’ll just say thank you again for joining us!

Get to know the crew

Ileana Pérez-Rodríguez is a post-doctoral researcher at the Carnegie Institution of Washington. Although she didn’t set out to study microbes at first, she’s hooked on them today. Read the interview »

While other scientists on this cruise are bringing samples of vent fluid up from the seafloor for study, marine chemist Nadine Le Bris is taking a different approach. She’s leaving sensors on the bottom that can measure changes in vent chemistry over time. Ultimately, she says, these measurments might help explain how animals at the vents affect the environment around them. Read the interview »

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