Lost City of the Atlantic –“The Birthplace of DNA and Earth’s First Living Organisms?” (Today’s Most Popular)

 

 

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This image above from the floor of the Atlantic Ocean shows a collection of limestone towers known as the "Lost City," a submarine hot-spring system (like no other yet seen within the world's oceans). Alkaline hydrothermal vents of this type are suggested to be the birthplace of the first living organisms on the ancient Earth. Scientists are interested in understanding early life on Earth because if we ever hope to find life on other worlds – especially icy worlds with subsurface oceans such as Jupiter's moon Europa and Saturn's Enceladus – we need to know what chemical signatures to look for.


The image above is a "space shot" of our own planet: NOAA's ROV Hercules (image below) approaches a ghostly, white, carbonate spire in the Lost City Hydrothermal Field, about 2500 feet below the surface of the Atlantic Ocean. The image was taken during the 10-day Lost City 2005 Expedition to conduct around-the-clock exploration of the Lost City Hydrothermal Field (LCHF) and the surrounding region.

How did life on Earth get started? Three papers co-authored in 2013 by Mike Russell, a research scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., strengthened the case that Earth's first life began at alkaline hydrothermal vents at the bottom of oceans. Scientists are interested in understanding early life on Earth because if we ever hope to find life on other worlds — especially icy worlds with subsurface oceans such as Jupiter's moon Europa and Saturn's Enceladus — we need to know what chemical signatures to look for.

 

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Two papers published in the journal Philosophical Transactions of the Royal Society B provided more detail on the chemical and precursor metabolic reactions that have to take place to pave the pathway for life. Russell and his co-authors described how the interactions between the earliest oceans and alkaline hydrothermal fluids likely produced acetate (comparable to vinegar).

The acetate is a product of methane and hydrogen from the alkaline hydrothermal vents and carbon dioxide dissolved in the surrounding ocean. Once this early chemical pathway was forged, acetate could become the basis of other biological molecules. They also describe how two kinds of "nano-engines" that create organic carbon and polymers — energy currency of the first cells — could have been assembled from inorganic minerals.

 

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A paper published in the journal Biochimica et Biophysica Acta analyzed the structural similarity between the most ancient enzymes of life and minerals precipitated at these alkaline vents, an indication that the first life didn't have to invent its first catalysts and engines.

"Our work on alkaline hot springs on the ocean floor makes what we believe is the most plausible case for the origin of the life's building blocks and its energy supply," Russell said. "Our hypothesis is testable, has the right assortment of ingredients and obeys the laws of thermodynamics."

But, it remains an unsolved mystery how life might have first arisen. The main building blocks of life now are DNA, which can store genetic data, and proteins, which include enzymes that can direct chemical reactions. However, DNA requires proteins in order to form, and proteins need DNA to form, raising the chicken-and-egg question of how protein and DNA could have formed without each other.

To resolve this conundrum, scientists have suggested that life may have first primarily depended on compounds known as RNA. These molecules can store genetic data like DNA, serve as enzymes like proteins, and help create both DNA and proteins. Later DNA and proteins replaced this “RNA world” because they are more efficient at their respective functions, although RNA still exists and serves vital roles in biology.

However, it remains uncertain how RNA might have arisen from simpler precursors in the primordial soup that existed on Earth before life originated. Like DNA, RNA is complex and made of helix-shaped chains of smaller molecules known as nucleotides.

One way that RNA might have first formed is with the help of minerals, such as metal hydrides. These minerals can serve as catalysts, helping create small organic compounds from inorganic building blocks. Such minerals are found at alkaline hydrothermal vents on the seafloor.

Alkaline hydrothermal vents are also home to large chimney-like structures rich in iron and sulfur. Prior studies suggested that ancient counterparts of these chimneys might have isolated and concentrated organic molecules together, spurring the origin of life on Earth.

To see how well these chimneys support the formation of strings of RNA, researchers synthesized chimneys by slowly injecting solutions containing iron, sulfur and silicon into glass jars. Depending on the concentrations of the different chemicals used to grow these structures, the chimneys were either mounds with single hollow centers or, more often, spires and “chemical gardens” with multiple hollow tubes.

“Being able to perform our experiments in chimney structures that looked like something one might encounter in the darker regions of Tolkien’s Middle Earth gave these studies a geologic context that sparked the imagination,” said study co-author Linda McGown, an analytical chemist and astrobiologist at Rensselaer Polytechnic Institute in Troy, N.Y.

The chimneys were grown in liquids and gases resembling the oceans and atmosphere of early Earth. The liquids were acidic and enriched with iron, while the gases were rich in nitrogen and had no oxygen. The scientists then poked syringes up the chimneys to pump alkaline solutions containing a variety of chemicals into the model oceans. This simulated ancient vent fluid seeping into primordial seas.

Sometimes the researchers added montmorillonite clay to their glass jars. Clays are produced by interactions between water and rock, and would likely have been common on the early Earth, McGown said.

The kind of nucleotides making up RNA are known as ribonucleotides, since they are made with the sugar ribose. The scientists found that unmodified ribonuclotides could form strings of two nucleotides. In addition, ribonucleotides “activated” with a compound known as imidazole — a molecule created during chemical reactions that synthesize nucleotides — could form RNA strings or polymers up to four ribonucleotides long.

“In order to observe significant RNA polymerization on the time scale of laboratory experiments, it is generally necessary to activate the nucleotides,” McGown said. “Imidazole is commonly used for nucleotide activation in these types of experiments.”

The scientists found that not only was the chemical composition of the chimneys important when it came to forming RNA, but the physical structure of the chimneys was key too. When the researchers mixed iron, sulfur and silicon solutions into their simulated oceans, instead of slowly injecting them into the seawater to form chimneys, the resulting blend could not trigger RNA formation.

“The chimneys, and not just their constituents, are responsible for the polymerization,” McGown said.

These experiments for the first time demonstrate that RNAs can form in alkaline hydrothermal chimneys, albeit synthetic ones.

“Our goal from the start of our RNA polymerization research has been to place the RNA polymerization experiments as closely as possible in the context of the most likely early Earth environments,” McGown said. “Most previous RNA polymerization research has focused on surface environments, and the exploration of deep-ocean hydrothermal vents could yield exciting new possibilities for the emergence of an RNA world.”

One concern about these findings is that the experiments were performed at room temperature. Hydrothermal vents are much hotter, and such temperatures could destroy RNA.

“Keep in mind, however, that hydrothermal vents are dynamic systems with gradients of chemical and physical conditions, including temperature,” McGown said.

In principle, cooler sections of hydrothermal vents might have nurtured RNA and its precursor molecules, she said.

In the future, McGown and her colleagues will perform experiments investigating what effects variables such as pressure, temperature and mineralogy might have on the formation of RNA molecules, focusing primarily on conditions mimicking deep-ocean environments on an early Earth and those that may also have existed on Mars and elsewhere, McGown said.

The scientists detailed their findings in the July 22 issue of the journal Astrobiology.

The Daily Galaxy via NASA/JPL – See more at: http://www.astrobio.net and oceanexplorer.noaa.gov

Image courtesy D. Kelley and M. Elend/University of Washington, IFE, URI-IAO, UW, Lost City science party, NOAA, National Geographic.

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