Deep Ocean Chemistry

Hydrothermal vents, located deep below the ocean surface, are among the most extreme environments on the planet. At these seafloor geysers, superheated water full of toxic chemicals seeps out of tall rocky structures known as “chimneys.” Water temperatures at vents can exceed 572°F (300°C), while the surrounding bottom water is a cool 36°F (2°C). No sunlight reaches this depth, where the pressure is almost 300 times greater than at sea level.

This harsh environment provides scientists with clues on what early Earth might have been like.

Chemosynthesis based on sulfide

On land and at the ocean surface, food webs are largely driven by photosynthesis — a process by which plants convert sunlight into food energy. In the dark world of hydrothermal vents, however, sunlight is not available. Organisms must therefore rely on another process to produce food. That process is called chemosynthesis.

Hydrogen sulfide is just one of the many different toxic chemicals spewing out of hydrothermal vents. Through chemosynthesis, however, bacteria convert hydrogen sulfide, oxygen, and carbon dioxide into organic molecules such as sugar, which vent-dwelling organisms can consume. Hydrogen sulfide smells like rotten eggs; imagine using that to make food!

Something in Common

Hydrothermal vent tubeworms and humans have one thing in common: hemoglobin. Humans use the substance in their blood to help transport oxygen throughout the body; tubeworms use it to carry vent water chemicals like hydrogen sulfide and oxygen to bacteria located in their trophosomes (a softball sized “blob” sort of like “guts”).

The reaction of hydrogen sulfide and oxygen reduces carbon dioxide to organic matter (e.g., sugars, amino acids, etc.) on which the tubeworms live.


Different preferences

The two main species of tubeworms found at hydrothermal vents of the East Pacific Rise are Riftia pachyptila and Tevnia jerichonana. While both benefit from bacterial chemosynthesis, each does better under different environmental conditions.

Tevnia, for example, is capable of withstanding higher hydrogen sulfide levels, which tend to result when fresh lava rises through cracks on the seafloor. Because of this tolerance, Tevnia tends to be one of the first animals to appear at hydrothermal vents. Riftia eventually settle into the area once sulfide levels decrease and more oxygen is available.

Proof that size isn’t everything, Tevnia (less than 1 foot long) can survive higher levels of sulfide, hotter temperatures, and lower oxygen conditions than Riftia, which reaches greater than 6 feet in length.

Reduced iron oxidation

Similar to humans, the bacteria and tiny plants living in the ocean need iron for energy and growth. But their situation is quite different from ours — for one, they can’t exactly turn to natural iron sources like leafy greens or red meat for a pick-me-up.

So where does their iron come from? University of Delaware researchers point to a source on the seafloor: minute particles (called nanoparticles) of pyrite, or fool’s gold, from hydrothermal vents at the bottom of the ocean.

The scientists showed that the vents emit a significant amount of pyrite as nanoparticles, which have a diameter that is one thousand times smaller than that of a human hair. Because the nanoparticles are so small, they are widely dispersed into the ocean rather than falling to the bottom—forming a potentially important food source for life in the deep sea.

George Luther, one of the UD researchers, explained the importance of the pyrite’s lengthy residence times, or how long they exist in their current form. He said the pyrite, which consists of iron and sulfur as iron disulfide, does not rapidly react with oxygen in the seawater to form oxidized iron, or “rust,” allowing it to stay intact and move throughout the ocean better than other forms of iron.

“As pyrite travels from the vents to the ocean interior and toward the surface ocean, it oxidizes gradually to release iron, which becomes available in areas where iron is depleted so that organisms can assimilate it, then grow,” Luther said. “It’s an ongoing iron supplement for the ocean much as Geritol or multivitamins are for humans.”

Growth of the bacteria and tiny plants, known as phytoplankton, can affect atmospheric oxygen and carbon dioxide levels.