Having an interior that is dominated by reducing conditions and oxidising surface environments since free oxygen gradually permeated from its initial build up in the atmosphere to the ocean depths, the Earth has been likened to a massive self-charging battery. Electrons flow continually as a consequence of the nature of the linked oxidation-reduction: in terms of electrons, oxidation involves loss while reduction involves gain (the OILRIG mnemonic). Although there are natural electrical currents, most of the electron flow is in the form of reduced compounds rich in electrons that make their way through the flow of fluids from the deep Earth – effectively an anode – towards the surface  where the reduced compounds lose electrons to create the equivalent of a cathode. Reduction-oxidation (redox) is therefore a power source. Inorganic reactions, such as the precipitation on the sea floor of sulfides from hydrothermal fluids at ‘black smokers’ dissipate energy. Yet the power has considerable potential for organic life. Some bacteria oxidise hydrogen sulfide carried by hydrothermal fluids and others do the same to upwelling methane. In 1977 a teeming biome of worms, molluscs and higher animals was discovered in a totally dark environment around ocean-floor vents. It soon became clear that it could only subsist on chemical energy of this kind, rather than any form of photosynthesis. The key to some metazoans’ success had to be symbiosis with bacteria that could perform the chemical tricks possible in the cathode region of the Earth’s electron flow. There are several candidate compounds: H₂S, CH₄, NH₄, metal ions and even hydrogen gas.

As hydrothermal fluids cycle ocean water into the basaltic crust and underlying peridotite mantle, they not only hydrate the olivines and pyroxenes that dominate the oceanic lithosphere but trigger other reactions one of whose products is hydrogen. As well as a reaction being eyed by those keen on a cheap source of clean fuel, it generates more energy potential for biological metabolism in the guise of hydrogen than those which form other common compound in the returning fluids. Although the nature of hydrogen’s organic use has been elusive, it has now come to light in a surprising guise (Petersen, J.M. and 14 others 2011. Hydrogen is an energy source for hydrothermal vent symbioses. Nature, v. 476, p. 176-180).

One highly successful animal in ocean-floor hot spring systems is a mussel called Bathymodiolus. Genetic experiments by the German-French-US team revealed that a gene known as hupL is present in the mussels’ gill tissue; a gene found in bacteria that use either carbon monoxide or hydrogen as an electron donor. The hupL gene encodes for enzymes known as hydrogenases that are needed to set off the reaction H₂ = 2H⁺ + 2e^– that provides electrons needed in bacterial metabolism; a sort of living fuel cell. Hydrogen-using bacteria interact symbiotically with the mussels, which would otherwise be unable to live in the pitch black environment. Genomic sequencing of tube worms and shrimps that occur in the vent communities also contain the bacterial hupL gene. Hydrogenase enzymes are proteins with an iron-nickel core, and probably evolved far back in bacterial evolution around metal-rich hot springs. Interesting as the specific detail of hydrogen-based symbiosis is, the general concept of Earth’s redox systems’ having battery-like behaviour is very useful. On land groundwater sometimes comes into contact with sulfide ore bodies that are oxidised to yield hydrogen and sulfate ions ,while the groundwater is reduced: a battery comes into being with a cathode in the aerated groundwater and electrons flow from the unaltered orebody towards it. Such currents are useful in revealing hidden orebodies using the ‘self-potential’ or SP method. Indeed the downward change from oxidising to reducing groundwater, caused by the redox reactions involved in weathering and soil formation also result in weak negative and positive ‘electrodes’ with a sluggish flow of compounds that bacteria can exploit and thereby encourage metazoan life through symbiosis. In doing so, changes in redox conditions affect the inorganic load of the slowly moving groundwater so that reduced metal ions can be precipitated once they rise into the oxidising horizon. The general enrichment of the upper horizons of soils in iron oxides and hydroxides, and metal depletion in lower horizons probably stem from the ‘Earth battery’ produced by an interplay between inorganic and organic redox reactions. Be on the look-out for more on this topic as the quest for hydrogen fuels becomes more urgent. A former colleague, Gordon Stanger, investigating groundwater in the Semail ophiolite of the Oman for his PhD in the 1970s discovered to his surprise that in outcrops of the mantle sequence there were springs from which hydrogen bubbled freely: fortunately he was not a smoker…

Related articles

• Orphan, V.J. & Hoehler, T.M. 2011. Hydrogen for dinner. Nature, v. 476, p. 154-155.
• Hydrogen highway in the deep sea (eurekalert.org)