GOURMET GEOCHEMICAL “PRIMORDIAL SOUPS” AT HYDROTHERMAL VENTS SUPPORT NOVEL THERMOPHILIC CHEMOLITHOTROPHS: IMPLICATIONS FOR THE EVOLUTION OF LIFE ON EARLY EARTH

Microorganisms thriving in high temperature terrestrial and deep-sea hydrothermal systems have stimulated new theories of life's origins. In these extreme environments, the microbial and geochemical interactions are tightly interwoven, providing many of the basic constituents for the primordial synthesis of organic molecules and for the evolution of fundamental metabolic processes. Given that the early Earth was a much warmer environment rich in compounds such as CO₂ and H₂, then perhaps life could have arisen in an environment analogous to present day hydrothermal systems. As the deep-rooted phylogenetic lineages within the small subunit rRNA tree of life are all thermophilic and many grow chemolithoautotrophically some of the earliest ancestors to all life might have been chemolithotrophic thermophiles. One such deeply rooted lineage, the Aquificales, occurs widely in both terrestrial and deep-sea hydrothermal system. A closely related lineage is actively involved in biomineralization (sulfur, iron, calcite) in thermal springs in Yellowstone National Park. This lineage was first identified using molecular phylogenetic approaches, however, we have recently isolated representatives from deep-sea vent hydrothermal deposits and terrestrial springs in the Azores. These organisms are metabolically plastic; they are able to use a range of inorganic electron donors and acceptors. Additionally, from comparisons of in situ CO₂ fixation rates, these chemolithotrophs are productive as their photosynthetic algal mat counterparts. If this novel group of microbes is a relative of the early chemolithotrophs, then chemolithotrophy could have contributed significantly to the biological productivity on early Earth and metabolic plasticity may have provided a competitive advantage over other more restricted metabolisms. The genome of one of isolates, "Persephonella marina", will soon be available and may provide additional insights into the evolutionary significance of the lineage. Whether life originated at vents in the Archaean and whether chemolithotrophic thermophiles are ancestors to all life is unresolved; however, hydrothermal environments probably did support the early evolution of chemolithoautotrophic thermophiles.