Evidence for oldest microbes from Arctic Canada

Among the oldest known rocks are metamorphosed pillow basalts on Nuvvuagittuk Island in Quebec on the east side of Hudson Bay, Canada. They contain red and orange, iron-rich sediments probably formed by hydrothermal activity associated with sea water passing through hot basalts. The ironstones are made of silica in the form of jasper (SiO2) and carbonates that are coloured by hematite (Fe2O3). This rock sequence is cut by silica-rich intrusive igneous rocks dated between 3750 and 3775 Ma: a minimum, Eoarchaean age for the sequence. This is roughly the same as the age of the famous Isua supracrustal rocks of West Greenland, but dating of the basalts using the samarium–neodymium method suggested that they formed in the Hadean about 4300 Ma ago, which would make them by far the oldest known rocks. However, that date clashes with a zircon U-Pb age of 3780 Ma for associated metasedimentary mica schists: a still ‘live’ controversy. The ironstones have been suggested to contain signs of life, in the form of minute tubes and filaments similar to those formed in modern hydrothermal vents by iron-oxidising bacteria (see: Earliest hydrothermal vent and evidence for life, March 2017). If that can be proven this would push back the age of the earliest known life by at least 300 Ma and maybe far more if the Hadean Sm-Nd age is confirmed

The Nuvvuagittuk material has recently been re-examined by its original discoverers using a variety of advanced microscope techniques (Papineau, D. et al 2022. Metabolically diverse primordial microbial communities in Earth’s oldest seafloor-hydrothermal jasper. Science Advances, v. 8, article 2296; DOI: 10.1126/sciadv.abm2296.). The most revealing of these involve two very-high resolution imaging systems: X-ray micro-tomography and electron microscopy armed with a focused ion beam that repeatedly shaves away 200 nm of rock from a sample. Both build up highly detailed 3-D images of any minute structures within a sample. The techniques revealed details of twisted filaments, tubes, knob-like and branching structures up to a centimetre long. While the first three could possibly have some inorganic origin, a ‘comb-like’ branch, likened to a moth’s antenna, has never been known to have formed by chemical reactions alone.

An image of hematite tubes from microfossils discovered in hydrothermal vent precipitates in the Nuvvuagittuk ironstones, reconstructed from X-ray and ion-beam micro-tomography (credit: Matthew Dodd, UCL)

All the structures are formed from hematite within a silica or carbonate (mainly calcite CaCO3 and ankerite Ca(Fe,Mg,Mn)(CO3)2) matrix. Some of the hematite (dominated by Fe3+) contains significant amounts of reduced Fe2+. The structures also contain tiny grains of graphite (C), phosphate (apatite Ca5(PO4)3(F,Cl,OH)) and various metal (Mn, Co, Cu, Zn, Ni, Cd) sulfides. The presence of graphite obviously suggests – but does not prove – a biological origin. However, all Phanerozoic jaspers formed from hydrothermal fluids contain undisputed organic material and appear little different from these ancient examples. Filaments, tubes and comb-like structures are displayed by various iron-oxidising bacteria found living in modern sea-floor hydrothermal vent systems. The sulfur isotopes in metal sulfides suggest their formation in an environment with vanishingly low oxygen content. Carbon isotopes in graphite are more enriched in light 12C relative to 13C than those in associated carbonates, a feature produced by living organic processes today. Patterns in plots of rare-earth elements (REE) from the Nuvvuagittuk jaspers are similar to those from modern examples and suggest high-temperature interactions between sea water and basaltic igneous rocks.

It is clear from the paper just how comprehensively the team of authors have considered and tested various biotic and abiotic options for the origin of the features found in the Nuvvuagittuk jasper samples. They conclude that they probably do represent an ancient microbial ecosystem associated with sea-floor hydrothermal vents; a now widely supported scenario for the origin of life on Earth. But what metabolic processes did the Nuvvuagittuk microbes use? Their intimate association with Fe3+ oxides that contain some reduced Fe2+ suggests that they exploited chemical ‘energy’ from oxidation reactions that acted on Fe2+ dissolved in hydrothermal fluids. This would have been impossible by inorganic means because of the very low oxygen content of seawater shown by the sulfur isotopes in associated sulfide minerals. Iron oxidation and precipitation of iron oxide by organic processes must have involved dissociation of water to yield the necessary oxygen and loss of electrons from available Fe2+, a process used by modern deep-water bacteria that depends on the presence of nitrates. That can power the metabolism of inorganic carbon dissolved in water as, for instance, bicarbonate ions and water to yield cell-building carbohydrates: a form of autotrophy. There may have been other metabolic routes, such as reducing dissolved sulfate ions to sulfur, as suggested by the association of metal sulfides. If the sea floor was shallow enough to be lit CO2 and water may have been converted to carbohydrates by a form of photosynthesis that does not release oxygen, analogous to modern purple bacteria.

There may have been considerable biodiversity in the Nuvvuagittuk ecosystem. So despite its vast age – it may have been active only 300 Ma after the Earth formed, if the oldest date is verified – it has to be remembered that a great many earlier evolutionary steps, both inorganic and organic, must have been accomplished to have allowed these organisms to exist. The materials do not signify the origin of life, but life that was chemically extremely sophisticated: far more so than anything attempted so far in laboratories to figure out the tricks performed by natural inorganic systems. DNA and RNA alone are quite a challenge!

See also: Video by authors of the paper (YouTube) Diverse life forms may have evolved earlier than previously thought. ScienceDaily, 13

Electricity from ‘black smokers’

English: Black smoker at a mid-ocean ridge hyd...
Hydrothermal vent at the mid-Atlantic Ridge (credit: Wikipedia)

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.

Life at the battery terminal

Mussels of species Bathymodiolus childressi (B...
Hydrothermal-vent mussel Bathymodiolus. Image via Wikipedia

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…