Occasionally, journals not usually associated with mainstream geosciences publish something startling, but easily missed. Nature of 12 September 2013 alerted me to just such an oddity. It seems that the chemistry of sea-floor hydrothermal vents potentially can generate electrical power (Yamamoto, M. et al. 2013. Generation of electricity and illumination by an environmental fuel cell in deep-sea hydrothermal vents. Angewandte Chemie, online DOI: 10.1002/ange.201302704).
The team from the Japan Agency for Marine-Earth Science and Technology, the Riken Centre for Sustainable Resource Science and the University of Tokyo used a submersible ROV to suspend a fuel cell based on a platinum cathode and iridium anode in hydrothermal vents that emerge from the Okinawa Trough off southern Japan at a depth of over 1 km. It recorded a tiny, but significant power generation of a few milliwatts.
The fluids issuing from the vents are at over 300°C while seawater is around 4°C, creating a very high thermal gradient. More importantly, the fluid-seawater interface is also a boundary between geochemically very different conditions. The fluids are highly acidic (pH 4.8) compared with the slight alkalinity of seawater, and contain high concentrations of hydrogen and hydrogen sulfide but undetectable oxygen (sea water is slightly oxygenated).
The fuel cell was designed so that iridium in the anode speeds up the oxidation of H2S at the geochemical interface which yields the electrons necessary in electrical currents. The experiment neatly signified its success by lighting up three light-emitting diodes.
Does this herald entirely new means of renewable power generation? Perhaps, if the fuel cell is scaled-up enormously. Yet, the very basis of oxidation and reduction is expressed by the mnemonic OILRIG (Oxidation Is Loss Reduction Is Gain – of electrons) and any potential redox reaction in nature has potential, even plants can be electricity producers. In fact all fuel cells exploit oxidation reactions of one kind or another.
Palaeobiologists generally believe that without a significant boost to oxygen levels in the oceans macroscopic eukaryotes, animals in particular, could not have evolved. Although the first signs of a rise in atmospheric oxygen enter the stratigraphic record some 2.4 billion years ago and eukaryote microfossils appeared at around 2 Ga, traces of bulky creatures suddenly show up much later at ~610 Ma with possible fossil bilaterian embryos preserved in 630 Ma old sediments. An intriguing feature of this Ediacaran fauna is that it appeared shortly after one of the Neoproterozoic global glaciations, the Marinoan ‘Snowball’ event: a coincidence or was there some connection? It has looked very like happenstance because few if any signs of a tangible post-Marinoan rise in environmental oxygen have been detected. Perhaps the sluggish two billion-year accumulation of free oxygen simply passed the threshold needed for metazoan metabolism. But there are other, proxy means of assessing the oxidation-reduction balance, one of which depends on trace metals whose chemistry hinges on their variable valency. The balance between soluble iron-2 and iron-3 that readily forms insoluble compounds is a model, although iron itself is so common in sediments that its concentration is not much of a guide. Molybdenum, vanadium and uranium, being quite rare, are more likely to chart subtle changes in the redox conditions under which marine sediments were deposited.
Swapan Sahoo of the University of Nevada and colleagues from the USA, China and Canada detected a marked increase in the variability of Mo, V and U content of the basal black shales of the Doushantuo Formation of southern China, which contain the possible eukaryote embryos (Sahoo, S.K and 8 others 2012. Ocean oxygenation in the wake of the Marinoan glaciation. Nature, v. 489, p. 546-549). These rocks occur just above the last member of the Marinoan glacial to post-glacial sedimentary package and are around 632 Ma old. Since the black shales accumulated at depths well below those affected by surface waves that might have permitted local changes in the oxygen content of sea water the geochemistry of their formative environment ought not to have changed if global chemical conditions had been stable: the observed fluctuations may represent secular changes in global redox conditions. The earlier variability settles down to low levels towards the top of the analysed sequence, suggesting stabilised global chemistry.
What this might indicate is quite simple to work out. When the overall chemistry of the oceans is reducing Mo, V and U are more likely to enter sulfides in sediments, thereby forcing down their dissolved concentration in sea water. With a steady supply of those elements, probably by solution from basalt lavas at ocean ridges, sedimentary concentrations should stabilise at high levels in balance with low concentrations in solution. If seawater becomes more oxidising it holds more Mo, V and U in solution and sediment levels decline. So the high concentrations in sediments mark periods of global reducing conditions, whereas low values signal a more oxidising marine environment. Sahoo et al.’s observations suggest that marine geochemistry became unstable immediately after the Marinoan glaciation but settled to a fundamentally more oxidising state than it had been in earlier times, perhaps by tenfold increase in atmospheric oxygen content. So what might have caused this and the attendant potential for animals to get larger in the aftermath of the Snowball Earth event? One possibility is that the long period of glaciers’ grinding down continental crust added nutrients to the oceans. Once warmed and lit by the sun they hosted huge blooms of single-celled phytoplankton whose photosynthesis became an oxygen factory and whose burial in pervasive reducing conditions on the sea bed formed a permanent repository of organic carbon. The outcome an at-first hesitant oxygenation of the planet and then a permanent fixture opening a window of opportunity for the Ediacarans and ultimately life as we know it.
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…
Orphan, V.J. & Hoehler, T.M. 2011. Hydrogen for dinner. Nature, v. 476, p. 154-155.