Iron-sulfur world theory

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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 ^[1] . 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. 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.

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.

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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.^[2] The following year, using the same ingredients, they were able to produce peptides.^[3]

[edit] Proto-Ecological Systems

William Martin and Michael Russell reported a modified iron-sulfur-hypothesis in 2002.^[4] According to their scenario, the first cellular life forms may have evolved inside so-called black smokers at seafloor spreading zones in the deep sea. These structures consist of microscale caverns that are coated by thin membraneous metal sulfide walls. Therefore, these structures would solve several critical points of the "pure" Wächtershäuser systems at once:

1. the micro-caverns provide a means of concentrating newly synthesised molecules, thereby increasing the chance of forming oligomers;
2. the steep temperature gradients inside a black smoker allow for establishing "optimum zones" of partial reactions in different regions of the black smoker (e.g. monomer synthesis in the hotter, oligomerisation in the colder parts);
3. the flow of hydrothermal water through the structure provides a constant source of building blocks and energy (freshly precipitated metal sulfides);
4. the model allows for a succession of different steps of cellular evolution (prebiotic chemistry, monomer and oligomer synthesis, peptide and protein synthesis, RNA world, ribonucleoprotein assembly and DNA world) in a single structure, facilitating exchange between all developmental stages;
5. synthesis of lipids as a means of "closing" the cells against the environment is not necessary, until basically all cellular functions are developed.

This model locates the "last universal common ancestor" (LUCA) inside a black smoker, rather than assuming the existence of a free-living form of LUCA. The last evolutionary step would be the synthesis of a lipid membrane that finally allows the organisms to leave the microcavern system of the black smokers and start their independent lives. This postulated late acquisition of lipids is consistent with the presence of completely different types of membrane lipids in archaebacteria and eubacteria (plus eukaryotes) with highly similar cellular physiology of all life forms in most other aspects.

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.

Russell 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. ^[5]

Despite this, it is not clear whether the proposed mechanism of abiogenetic life could actually work, or was how life did start.^[6]

[edit] References