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A three-foot-long wreckfish swims by a portion of an 18-story (60 meter) chimney in the Lost City hydrothermal vent field. The white part of the edifice in the foreground is actively venting highly alkaline fluids rich in methane, hydrogen, and abiogenic hydrocarbons. The warm fluids support dense microbial communities that thrive on the chimney surface and interior. (Credit: D. Kelley/University of Washington, IFE, URI-IAO, Lost City science party, NOAA)

U. WASHINGTON (US)—On the marine microbial stage, there appears to be a vast group of understudies only too ready to step in when “star” microbes falter. At least that’s what happens at the Lost City hydrothermal vent field in the mid-Atlantic Ocean—the only one of its kind found thus far.

It’s offering scientists access to microorganisms living in vents that range in age from newly formed to tens of thousands of years old.

Researchers have uncovered the first evidence that microorganisms can remain rare for a long time before completely turning the tables to become dominant when ecosystems change, according to William Brazelton, a University of Washington postdoctoral researcher. Findings are published in the Proceedings of the National Academy of Sciences.

It seems logical, but until recently, scientists weren’t able to detect microorganisms at such low abundance, Brazelton says.

In 2006 scientists published the first paper saying microorganisms in the marine environment had been woefully undercounted. They used the latest DNA sequencing techniques and said marine microorganisms could be 10 to 100 times more diverse than previously thought.

They coined the term “rare biosphere” to describe a vast but unrecognized group of microorganisms—”rare” because each kind of microorganism appeared to be present in only very low numbers or abundance, so low that they were previously undetectable.

If the new way of determining microbial diversity was accurate, scientists are left to wonder why such a large collection of low-abundance organisms existed.

Fly to the top of the tallest chimney in the field, the 18-story (60 meter) structure called Poseidon, where vent fluids shimmer and filamentous bacteria thrive. (Credit: D. Kelley, M. Elend/University of Washington)

“A fundamental prediction of the ‘rare biosphere’ model is that when environmental conditions change, some of these rare, preadapted taxa can rapidly exploit the new conditions, increase in abundance and out-complete the once abundant organisms that were adapted to past conditions,” Brazelton and his coauthors write. Yet, they continue, “No studies have tested this prediction by examining a shift in species composition involving extremely rare taxa occurring during a known time interval.”

Until now.

Lost City was discovered during a National Science Foundation expedition in 2000 by UW oceanography professor and paper coauthor Deborah Kelley and others. The hot springs form in a very different way from the metal-rich, 700 degrees F black smoker vents scientists have known about since the 1970s.

Water venting at Lost City is generally 200 F or less, and the chimneys, vents, and other structures at Lost City are nearly pure carbonate, the same material as limestone in caves. They are formed by a process called serpentinization, a chemical reaction between seawater and mantle rocks that underlie the field.

The vent waters are highly alkaline and enriched in methane and hydrogen gases—important energy sources for the microbes that inhabit Lost City.

Lost City also differs from the magma-driven hydrothermal systems in that it is very long-lived. Whereas there have been numerous seasonal and short-term studies of microbial responses to environmental changes—lasting years at the most—the Lost City hydrothermal vent field provided a way to look at changes in vent ecosystems 1,000 years apart in age.

Analyses by Brazelton and colleagues revealed that DNA sequences that were rare in younger vents were abundant in older ones. Because it is likely that the older Lost City chimneys vented higher-temperature, more alkaline fluids when they were younger, scientists think that as the ecosystem changed some of the rare microorganisms came to the fore.

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