Lost City Ramblings
Who hath desired the Sea? -- the immense and contemptuous surges?
While exploring the Mid-Atlantic Ridge in 2000, a research submarine traveled 15 km west towards the dome-like Atlantis Massif. Here its crew encountered tall underwater structures that resembled glistening white cathedral spires. The area was dubbed the Lost City hydrothermal field. Some of the towers were 30 to 60 meters tall—taller than any of the celebrated black smokers—with as many as 30 of them in a single cluster. One of the people involved in their discovery, Jeff Karson commented: If this vent field was on land, it would be a national park. Considering the location, an international park would be more likely.
The top of a 100-foot-tall carbonate
pinnacle. Most chimneys display multiple
pinnacles. The porous structure of these
deposits and the nooks and crannies on
the outside surfaces provide wonderful
habitats for microorganisms and small
animals. Credit: University of Washington
Not only do the Lost City towers look different from the black smokers, but they have almost nothing in common with them in chemistry and mode of formation. Black smoker chimneys are made by the deposition of iron and sulfur containing minerals from hot (up to 365°C), acidic (pH ~2.8) fluids. Lost City fluid temperatures are a comparatively balmy 40-80°C, and the pH is high (9-11). The Lost City structures result from an exothermic reaction known to marine geologists as serpentinization. This is not the making of serpents, but rather is the formation of several minerals collectively known as serpentinite. Serpentinization happens when water comes in contact with rocks exposed from the earth’s mantle, called peridotites. What is particularly interesting about this encounter, from a microbe's point of view, is that the chemical reactions release hydrogen gas and methane. High pH (9-10) and the high Ca2+ content causes the precipitation of carbonates, which leads to the formation of large mounds, pinnacles, and chimneys.
Click here to view a video showing the Nature pinnacle from aboard the submarine. This 30 m tall tower is actively venting fluids at temperatures up to 62°C. The line on the summit marker is 1 m.
Lost City is quite old—especially when compared to the transient black smokers. It’s been active for at least 30,000 years and promises to be around for a considerably longer time.
You won't be surprised to learn that the Lost City's hydrogen- and methane-rich fluids harbor microbial communities that are different from those of the black smokers. Indeed, molecular surveys reveal Archaea that make methane from hydrogen anaerobically and bacteria that oxidize methane aerobically. The predominant bacteria are γ- and ε-proteobacteria and Firmicutes. And there are many others. The hottest surfaces close to the hydrogen vents are dominated by thick biofilms of an Archaea.
Click here to view a video showing strands of filamentous bacteria in 60°C to 70°C fluids near a tower summit. The red dots in the first portion of the video are lasers spaced 10 cm apart. The second portion shows the hospitable habitat provided when rising hydrothermal fluids pool beneath small overhanging carbonate ledges.
Zoologically speaking, Lost City is not as rich as the black smokers, but some animals are there all right, mussels being a common invertebrate. Do these mussels house dual endosymbiotic microbes, just like similar mussels in the black smokers and elsewhere? (The “endo-“ in endosymbiotic is justified because these organisms reside within specialized cells in the gills of the host, the bacteriocytes.) Indeed they do. Their symbionts include both a methanotroph and a chemoautotroph; in other words, some utilize the abundant methane as their carbon source while others make organic compounds from carbon dioxide. What might these chemoautotrophs use for energy? Sulfides are used by the symbionts of the famous tube worms at the black smokers, but such compounds are not abundant at the Lost City vents. Most of what is known about these organisms comes from analysis of their 16S rRNA and thus we can only guess what their energy metabolism is really like.
The high temperature systems that spawn the black smokers are restricted to a narrow corridor along the axis of the mid-oceanic ridges where most of the Earth's output of magma is localized. In contrast, the geology that produced the Lost City vents is likely widespread; there are many areas of old crust along the Mid-Atlantic, Indian, and Arctic ridges similar to the Atlantis Massif. We expect to be adding many more spectacular towering “cities” to our tour schedule in the coming years. Welcome aboard.
Source of Movies: D. S. Kelley et al. A serpentinite-hosted ecosystem: The Lost City Hydrothermal Field (2005) Science 307: 1428-1434.