1. Pyruving the Origin of Life

    By simulating the conditions of hydrothermal vents in the lab, researchers at the Geophysical Laboratory of the Carnegie Institution of Washington (CIW) have created pyruvic acid, an organic chemical vital for cellular metabolism. CIW is one of eleven Lead Teams participating in the NASA Astrobiology Institute (NAI).

    Hydrothermal vents were first discovered in 1977. Similar to geysers like Yellowstone Park’s Old Faithful, these hot underwater vents spew out mixtures of chemicals and nutrients for bacteria while maintaining near-volcanic heat temperatures. With the sampling of these vents, scientists have also discovered that life thrives there, despite the elevated temperatures and extremely high pressure. Many have even suggested that life on Earth may have begun at hydrothermal vents billions of years ago.

    “We are not all wedded to the vent hypothesis,” says Dr. Robert Hazen, NAI member and one of the investigators in the study. “Indeed, a very plausible outcome of our research might be the absolute conclusion that life could NOT have arisen under such extreme conditions. That would be an exciting result.”

    If life did develop at hydrothermal vents, however, it may have gone something like this: volcanic gases and water around the hydrothermal vents combined with elements of the Earth’s crust to form acetic acid. With the addition of carbon, pyruvic acid formed. When pyruvic acid met with ammonia, it formed amino acids, which then linked up into proteins.

    In a series of experiments, the Carnegie researchers have replicated the harsh vent conditions to see if they foster some of the chemical steps thought necessary for life to emerge. Earlier experiments had resulted in the production of ammonia and the amino acid alanine.

    “We started all these experiments in the period 1998-1999,” says Hazen. “The pyruvate synthesis was actually something of a surprise in experiments that were designed primarily to study the formation of carbon bonds. Sometimes you’re just lucky!”

    This latest project to replicate primitive vent conditions used a recipe of iron sulfide (one of the ingredients of the earth’s crust), formic acid (detected even today in thermal vents), and alkyl thiol (a sulfurous compound similar to alcoholthat is produced by the combination of iron sulfide and carbon monoxide). These chemicals were enclosed in a small gold capsule, and then subjected to elevated temperatures and pressures similar to the conditions at hydrothermal vents. The researchers measured an increased yield of pyruvic acid with pressure. This finding was published in the August 25 edition of Science. Along with Robert Hazen, the investigators of this study include NAI members George D. Cody, Nabil Z. Boctor, Timothy R. Filley, James H. Scott, Anurag Sharma, and Hatten S. Yoder, Jr.

    Synthesis of pyruvic acid is necessary for a primitive anaerobic (non-oxygen-requiring) metabolism to develop. The scientists state that the synthesis of pyruvic acid is a critical step for the origin of life, and they conclude that the natural synthesis of such compounds would occur wherever hydrothermal fluids pass through iron sulfide-containing crust.

    “These results lend support to the theory that the …oceanic crust could have provided Earth’s most primitive life with a warm enclave continuously flooded by fluids rich in reduced carbon, [sulfur].. and potentially catalytic .. iron-sulfur clusters,” the scientists report.

    Replicating the complex conditions of hydrothermal vents is, of course, extremely difficult to achieve in the lab.

    “We have not attempted to duplicate a natural hydrothermal environment exactly,” says Hazen. “Natural environments clearly have complex gradients of temperature and fluid compositions flowing over a variety of minerals. That will take years to duplicate in a lab. However, I do think that we’ll be able to constrain the kind of environment in which organic synthesis, molecular selection and organization took place.”

    Future experiments planned by the Carnegie scientists include a survey of minerals to see which are most effective in reducing nitrogen to ammonia, and which are best at forming and stabilizing amino acids. The scientists are also studying the origins of biological homochirality (the uniform left- or right-handedness that occurs in all of life’s most important molecules), as well as investigating metal-based compounds that can act as catalysts in organic synthesis.

    Says Hazen, “The primary implication of our research so far is that organic synthesis relevant to prebiotic chemistry occurs under a wider range of conditions than previously thought, just as biologists are finding that life exists under a wider range of environments than was previously thought.”

    What Next?

    The discovery of life at hydrothermal vents on Earth has opened up the possibility of finding life elsewhere in our solar system. Mars and Jupiter’s moon Europa are two places in our solar system that may have geothermal forces great enough to generate volcanic vents. If liquid water is also found on these worlds, then life may already be thriving in the same hydrothermal vent conditions as on Earth. Missions to Mars continue to search for the presence of liquid water, and an upcoming mission to ice-covered Europa includes plans to search for a liquid water ocean.

    “The early Earth must have had several complementary sources of biological raw materials – comets and asteroids, synthesis near the interface of ocean and atmosphere, and hydrothermal processes,” says Hazen. “To the extent that other bodies such as Mars and Europa once had or still have similar sources, then similar organic synthesis can be expected. Whether these processes were followed by the very complex stages of molecular organization necessary to life is at present, however, pure speculation.”

    Reference: “Primordial Carbonylated Iron-Sulfur Compounds and the Synthesis of Pyruvate.” Science 289 (August 25): 1337-1340.