NASA Jet Propulsion Laboratory
Interest: The emergence of life and of oxygenic photosynthesis
The discovery of fossil hydrothermal chimneys in Ireland got us interested in the emergence of life, particularly in a hydrothermal source of energy and substrate.
1. We suggest that basic to life’s emergence was the formation of mineral compartments across which steep redox, pH, ionic and temperature gradients were poised. The pH gradient acting across the inorganic membrane (acid outside, alkaline inside) constituted a natural protonmotive force, and the redox gradient could augment this force by chemiosmosis. The energy focused across the barrier in this way is commensurate with that required by all life. Also the compartments solve the “concentration problem”.
2. The theory predicted the discovery of off-ridge alkaline hydrothermal springs of moderate temperature by Kelley et al. (2001, 2005).
3. As expected, these springs contain significant (i.e., 15 mmol/litre) H2, which could reduce the CO2 concentrated in the first oceans on this and other comparable planets. This process evolved from a geochemical feedback system to an autogenic metabolism.
4. We are impressed by the structural similarity between greigite (a Ni-Fe-sulfide) and the active centres of various essential proteins that must have existed within the first microbes (e.g., CODH/ACS & ferredoxins). Theoretically it is possible for moieties of this mineral structure to be sequestered by short achiral peptides and the whole to act as the first electron transfer agents, hydrogenases and synthetases.
5. Echoing early workers such as Fuchs, Wood and Edwards, it seems that metals and metal sulfides were first to do the biochemical work of CO2 fixation, and that the acetyl-coenzyme-A pathway developed first in an alkaline hydrothermal mound – a mound that acted as a flow reactor and affinity column. The generation of acetate and methane during serpentinization may be speeded up in the mound and eventually “quickened” through the onset of life at the same site. The different outcomes of reduction, to acetate- or methane-generating metabolists, may have gelled genetically to produce representatives of the two prokaryotic domains, viz., the acetogenic bacteria and the methanoarchaea
6. The continued coupling between life and convection may also explain the onset of oxygenic photosynthesis. A feature of this model is that a Ca-Mn-oxide entity in littoral manganiferous sediments obducted to the photic zone, was co-opted as the Oxygen-Evolving Complex by PS2 in the cyanobacterial ancestor. If so, it would appear that both chemosynthesis and oxygenic photosynthesis emerged within the confines of mineral constituents (Ni-Fe sulfides and Ca-Mn oxides respectively) — constituents that were then co-opted as catalysts by their prokaryotic hosts.
7. Experiments are underway to test the idea that a Hadean hydrothermal mound would act as a flow-through chemical reactor and affinity column to produce organic molecules from CO2 and H2.
NAI Project Collaborators
- Project collaborators as reported by the latest NAI Annual Report.