Origin of Life -2001: Critical aspects

The Origin of Life at a submarine alkaline seepage
Michael J. Russell, Allan J. Hall, Laiq Rahman & Dugald Turner

I. Critical aspects

What does Life do?

It responds electrochemically to geochemical and photochemical tensions on Earth by attempting to resolve them, remaking itself in the process that it might create a greater overall disorder. Or in the words of Simon Black (2000) -"energy uses an organism as a mechanism for self-dissipation."

And that is why and how Life began.

Life resulted from attempts of alkaline reduced submarine seepage waters, bearing hydrogen and simple organic ("electron rich") molecules, to titrate into the carbonic acidulous ocean containing photolytic ferric iron, attempts frustrated by the spontaneous precipitation of impure and imperfect colloidal iron sulfide and hydroxide barriers (Fig. 1).

The ferric iron in the ocean was the ultimate electron acceptor. The presence of iron reducing bacteria in the lowest branches of the evolutionary tree supports this concept (Fig. 2) (Vargas et al. 1998; Reysenbach 2001).

The first barrier to chemical equilibration was the freshly precipitated mound of consolidating metal compounds, principally those embodying iron, which catalytically converted the simplest of molecules to the building components required for life's emergence.

A regulated metabolism - the response to the highly restricted electrochemical interactions achieved by the contrasting fluids through sparse natural pores and conductive iron monosulfide - processed these more complex molecules.

To resolve disequilibria and produce "waste" required specific cluster-catalysts so that the large activation energies, which inhibited reactions between primary building blocks, could be overcome (Russell and Hall 1997).

Initially waste was that material which failed to be involved or reinvolved in protometabolism.

It was carried away through tiny chimneys by the buoyant hydrothermal fluids.

The three geochemical stages in the emergence of protometabolism and chemosynthetic life are outlined in Table 1 below.

Note that stages 2 and 3 are rapidly integrated during earliest evolution.

Significant further evolution was required before particular prokaryotes could survive with only fundamental molecules such as hydrogen and carbon dioxide, i.e., autotrophism was not an original metabolic process.

Table 1

Stage 1: Hydrothermal convection (flow reactor)

Stage 2: Percolation through chemical sediment (flat bed reactor)

Stage 3: Seepage (photoelectrochemical reactor)

Chemically these three stages, the hydrothermal convection, the percolation through the recently precipitated chemical sediments and the seepage itself, may be considered in terms of a nexus of three reactors.

The main part of the system acts as a flow reactor (Fig. 1).

Here water and carbon oxides are reduced to the primary building components on reaction with native and ferrous iron over a period of a thousand years or so.

The modular components thus produced are then borne to the chemical sediments, which act as a flat bed reactor (Fig. 9b) where they concentrate and are oligermerised or otherwise converted to amino and short carboxylic acids, nucleic acid bases, and thiols in hours to days.

The final stage, counted in seconds and minutes, is the self-organised interaction of these molecules in the photoelectrochemical reactor (Fig. 4,6,7b,9c & 9d,11,12); semipermeable compartments comprised of iron sulfide and hydroxide.

This is where the redox and the acid-base catalysed reactions take place, in concert, that herald the onset of a regulated metabolism.

The hypothesis carries the implication that emergent life was chemolithotrophic - fed hydrothermally with simple C1 to C4 molecular components.

Only later did some organisms evolve "upstream" towards anaerobic autotrophy, as others move downstream to heterotrophy as well as phototrophy.

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Fig 1

Fig 2 Figure 2. Phylogenetic tree (Stetter 1996) modified from Woese et al. (1990), demonstrating the thermophilic root of the last universal common ancestor (in red) but also the putative mesophilic progenote (Forterre and Philippe 1999). The"primitive" nature of FeIII reducers (green) are also evident (Liu et al. 1997; Vargas et al. 1998).

Fig 4 Figure 4. Flow diagram illustrating the geochemical drive toward the onset of life. There are two pathways to consider - the provision of energy and the fixation of carbon.

Fig 6 FIGURE 6. Schematic illustration of the notional photoelectrochemical cell assumed to obtain on the Hadean Earth and ocean.(cf. Fig. 4)

Fig 7b Figure 7b. Pourbaix (Eh/pH) diagram comparing the electrochemical energy available to modern iron reducing bacteria. Positions of natural waters and prokaryotic bacterial cytoplasm are approximate (Bethke 1992,1996).

Fig 9b Photo of laminated sedimentary sulfides and sulfate and superposed seepage mound comprising fine pyrite chimneys and botryoids at Tynagh. This specimen is from the vicinity of the main hydrothermal conduit, but at approximately the same horizon as the iron formation (Banks 1985; and see Russell 1983).

Fig 9c Photo of polished cross-section through the top of the sulfide mound shown in Fig. 9b. FeS bubbles have been been oxidised to, and overgrown by, later pyrite (FeS2). Pristine FeS "membranes" are ~5mm (Fig. 9i) thick, two to three orders of magnitude thicker than biological membranes (Russell and Hall 1997).

Fig 9d Photograph of an FeS (nanocrystalline mackinawite) boundary to a plume of 10 millimolar Na2S solution injected into 10 millimoles of FeCl2 (visijar is 40 mm across). Addition of nickel to the lower pH mixes brings about the precipitation of violarite (see Fig. 17a). In cases where pH >7 green rust composes similar structures (see Fig. 10) (Russell et al. 1998).

Fig 11 Figure 11. Here we project the conditions employed by Hennet et al. (1992) in their synthesis of amino acids upon a putative hydrothermal mound which acts as a natural fluidised- or flat bed reactor (Russell et al. 1994; and see El-Kaissy and Homsy 1976; Couderc 1985).

Fig 12a