Oceanic lithosphere, Mg(II), Na(I), PPi and life's origin

Life may have started in connection with early plate tectonic processes coupled to alkaline hydrothermal activity. Pyrophosphate (PPi) and Na+ transport may have preceded ATP and H+ transport through primitive membranes in association with the dominant geochemistry of the Earth at the time of the origin and early evolution of life. A hydrothermal environment in which Na+ is abundant and H+ is rare exists in sediment-starved subduction zones, like the Mariana forearc in the W Pacific Ocean. It is considered to mimic the Archean Earth (Pons et al., 2011). There, the forearc pore fluids have a pH up to 12.6 and a Na+-concentration of 0.7 mol/kg seawater. PPi could have been formed during early subduction of oceanic lithosphere by dehydration of protonated orthophosphates. A key to PPi formation in these geological environments is a low local activity of water. Magnesium, on the other hand, is a common element on Earth, particularly in oceanic crust and the upper mantle (as well as on the other terrestrial planets) and plays a special role in biochemistry due to its ability to coordinate six oxygen atoms efficiently in its first coordination shell. Such oxygen atoms may be part of one or two charged oxyanions, which means that Mg2+can, for instance, tie together two different phosphate groups that are located at distance from each other in a macromolecule, and in this way be responsible for the folding of large molecules like RNA. This property of Mg2+ also helps the stabilization of diphosphate and triphosphate groups of nucleotides, as well as promoting the condensation of orthophosphate to oligophosphates, like PPi and trimetaphosphate. According to the ‘RNA World’ hypothesis the first enzymes – the ribozymes – consisted of RNA, which depended on Mg2+ for its self-cleavage (Gilbert, 1986). Mg2+ is also present in all DNA and RNA activation processes. The central role of Mg2+ in the function of ribozymes and its ‘archaic’ position in ribosomes (the protein making machinery in all cells), and the fact that magnesium generally has coordination properties different from other cations, suggests that the inorganic chemistry of magnesium, together with that of sodium and phosphate, had a key position in the first chemical processes leading to the origin and early evolution of life.