A HYDROTHERMALLY PRECIPITATED CATALYTIC IRON SULFIDE MEMBRANE AS A FIRST STEP TOWARD LIFE

We propose that life emerged from growing aggregates of iron sulphide bubbles containing alkaline and highly reduced hydrothermal solution. These bubbles were inflated hydrostatically at sulphidic submarine hot springs sited some distance from oceanic spreading centers four billi on years ago. The membrane enclosing the bubbles was precipitated in r esponse to contact between the spring waters and the mildly oxidized, acidic and iron-bearing Hadean ocean water. As the gelatinous sulphide bubbles aged and were inflated beyond their strength they budded, pro ducing contiguous daughter bubbles by the precipitation of new membran e. [Fe2S2](+/0) or [Fe4S4](2+/+) clusters, possibly bonded by hydrothe rmal thiolate ligands as proferredoxins, could have catalyzed oxidatio n of thiolates to disulphides, thereby modifying membrane properties. We envisage the earliest iron sulphide bubbles (probotryoids) first gr owing by hydrostatic inflation with hydrothermal fluid, but evolving t o grow mainly by osmosis (the protocellular stage), driven by (1) cata bolism of hydrothermal abiogenic organics trapped on the inner walls o f the membrane, catalyzed by the iron sulphide clusters; and (2) cleav age of hydrophobic compounds dissolved in the membrane to hydrophilic moieties which were translocated, by the proton motive force inherent in the acidic Hadean ocean, to the alkaline interior of the protocell. The organics were generated first within the hydrothermal convective system feeding the hot springs operating in the oceanic crust and late r in the pyritizing mound developing on the sea floor, as a consequenc e of the reduction of CO, CO2, and formaldehyde by Fe2+- and S2--beari ng minerals. We imagine the physicochemical interactions in and on the membrane to have been sufficiently complex to have engendered auto- a nd cross-catalytic replication. The membrane may have been constructed in such a way that a ''successful'' parent could have ''informed'' th e daughters of membrane characteristics functional for the then-curren t level of evolution.