Iron-sulfur world theory

The iron-sulfur world theory is a hypothesis for the origin of life advanced by Günter Wächtershäuser, a Munich chemist and patent lawyer, involving forms of iron and sulfur. Wächtershäuser proposes that an early form of metabolism predated genetics. Metabolism here means a cycle of chemical reactions that produce energy in a form that can be harnessed by other processes. The idea is that once a primitive metabolic cycle was established, it began to produce ever more complex compounds.

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A key idea of the theory is that this early chemistry of life occurred not in bulk solution in the oceans, but on mineral surfaces (e.g. iron pyrites) near deep hydrothermal vents. This was an anaerobic, high-temperature (near 100°C), high-pressure environment. The first 'cells' would have been lipid bubbles on the mineral surfaces.

Wächtershäuser has hypothesized a special role for acetic acid, a simple combination of carbon, hydrogen, and oxygen found in vinegar. Acetic acid is part of the citric acid cycle that is fundamental to metabolism in cells.

A blueprint for life

Some of the fundamental ideas of the iron-sulfur theory can be summarized in the following brief recipe for life:

"Boil water. Stir in iron sulfide and nickel sulfide. Bubble in carbon monoxide and hydrogen sulfide gas. Wait for peptides to form."

More technically, Wächtershäuser hypothesized the following steps for producing proteins:

1. Produce acetic acid through metallic ion catalysis.
2. Add carbon to the acetic acid molecule to produce three-carbon pyruvic acid.
3. Add ammonia to form amino acids.
4. Produce peptides and then proteins.

Both acetic acid and pyruvic acid are key chemicals in the citric acid cycle.

In 1997, Wächtershäuser and Claudia Huber mixed carbon monoxide, hydrogen sulfide, nickel sulfide, and iron sulfide particles at 100°C and demonstrated that amino acids could form.^[1] The following year, using the same ingredients, they were able to produce peptides.^[2]

Proto-Ecological Systems

The idea fits closely with the discovery of hydrothermal black smoker ecologies associated with mid-ocean ridges and undersea volcanoes. In an abiotic world, a thermocline of temperatures and a chemocline in concentration is associated with the pre-biotic synthesis of organic molecules, hotter in proximity to the chemically rich vent, cooler but also less chemically rich at greater distances. The migration of synthesised compounds from areas of high concentration to areas of low concentration gives a directionality that provides both source and sink in a self-organising fashion, enabling a proto-metabolic process by which acetic acid production and its eventual oxidization can be spatially organised.

In this way many of the individual reactions, today found in glycolysis could originally have been found outside any developing cell membrane, where each smoker ecosystem is functionally equivalent to a single cell. Chemical communities having greater structural integrity and resilience to wildly fluctuating conditions are then selected for; their success would lead to local zones of depletion for important precursor chemicals. Progressive incorporation of these precursor components within a cell membrane would gradually increase metabolic complexity within the cell membrane, whilst leading to greater environmental simplicity in the external environment. An explosive chain reaction would result which would rapidly lead to the development of complex catalytic sets capable of self-maintenance.

Professor Russell at Glasgow University adds a significant factor to these ideas, by pointing out that semi-permeable mackinawite (an iron sulfide mineral) and silicate membranes could naturally develop under these conditions and electrochemically link reactions separated in space, if not in time. ^[3]

Despite this, it is not clear whether the proposed mechanism of abiogenetic life could actually work, or was how life did start. For example, see Geochemical Society Newsletter, main article. and also parent pages from the link.

References

1. ^ Huber, C. and Wächterhäuser, G. (July 1998). "Peptides by activation of amino acids with CO on (Ni, Fe)S surfaces: implications for the origin of life". Science 281: 670-672. doi:10.1126/science.281.5377.670.
2. ^ Günter Wächtershäuser (August 2000). "ORIGIN OF LIFE: Life as We Don't Know It". Science 289: 1307-1308. doi:10.1126/science.289.5483.1307.
3. ^ Michael Russell (2006), , vol. 94, American Scientist, pp. pp. 32-39

• Russell MJ, Daniel RM, Hall AJ, Sherringham JA (1994). "A Hydrothermally Precipitated Catalytic Iron Sulphide Membrane as a First Step Toward Life". J Mol Evol 39: 231-243.