Science Alert

We may finally know what life on Earth breathed before there was oxygen

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By Carly Cassella
published

La Brava microbial mats.
La Brava microbial mats. (Image credit: Visscher et. al/Nature, 2020)

Billions of years ago, long before oxygen was readily available, the notorious poison arsenic could have been the compound that breathed new life into our planet.

In Chile's Atacama Desert, in a place called Laguna La Brava, scientists have been studying a purple ribbon of photosynthetic microbes living in a hypersaline lake that's permanently free of oxygen.

"I have been working with microbial mats for about 35 years or so," says geoscientist Pieter Visscher from the University of Connecticut.

"This is the only system on Earth where I could find a microbial mat that worked absolutely in the absence of oxygen."

Microbial mats, which fossilize into stromatolites, have been abundant on Earth for at least 3.5 billion years, and yet for the first billion years of their existence, there was no oxygen for photosynthesis.

How these life forms survived in such extreme conditions is still unknown, but examining stromatolites and extremophiles living today, researchers have figured out a handful of possibilities. 

While iron, sulphur, and hydrogen have long been proposed as possible replacements for oxygen, it wasn't until the discovery of 'arsenotrophy' in California's hypersaline Searles Lake and Mono Lake that arsenic also became a contender.

Since then, stromatolites from the Tumbiana Formation in Western Australia have revealed that trapping light and arsenic was once a valid mode of photosynthesis in the Precambrian. The same couldn't be said of iron or sulphur.

Just last year, researchers discovered an abundant life form in the Pacific Ocean that also breathes arsenic. 

Even the La Brava life forms closely resemble a purple sulphur bacterium called Ectothiorhodospira sp., which was recently found in an arsenic-rich lake in Nevada and which appears to photosynthesize by oxidising the compound arsenite into a different form -arsenate.

While more research needs to verify whether the La Brava microbes also metabolize arsenite, initial research found the rushing water surrounding these mats is heavily laden with hydrogen sulphide and arsenic.

If the authors are right and the La Brava microbes are indeed 'breathing' arsenic, these life forms would be the first to do so in a permanently and completely oxygen-free microbial mat, similar to what we would expect in Precambrian environments.

As such, its mats are a great model for understanding some of the possible earliest life forms on our planet. 

While genomic research suggests the La Brava mats have the tools to metabolize arsenic and sulphur, the authors say its arsenate reduction appears to be more effective than its sulfate reduction.

Regardless, they say there's strong evidence that both pathways exist, and these would have been enough to support extensive microbial mats in the early days of life on Earth.

If the team is right, then we might need to expand our search for life forms elsewhere.

"In looking for evidence of life on Mars, [scientists] will be looking at iron and probably they should be looking at arsenic also," says Visscher.

It really is so much more than just a poison.

The study was published in Communications Earth and Environment. 

This article was originally published by ScienceAlert. Read the original article here.

Carly Cassella
Carly Cassella
ScienceAlert

Carly Cassella is a journalist at ScienceAlert with a background in neuroscience. Carly cut her journalistic teeth at Farrago magazine while studying as an undergraduate at the University of Melbourne. Previously, she worked at the International Federation of Journalists in Brussels, where she gained the utmost respect for war correspondents. Since then, she has worked in award-winning podcast production, taught a class on science writing at the 2018 March for Science conference, and has written multiple YouTube scripts with millions of views. Carly currently lives in Seattle, where she enjoys clamming, oystering, fern-ing and pretending she knows how to identify birds and stars.

16 Comments Comment from the forums
  • Broadlands
    "Microbial mats, which fossilize into stromatolites, have been abundant on Earth for at least 3.5 billion years, and yet for the first billion years of their existence, there was no oxygen for photosynthesis."

    That's not true. The fossil record of cyanobacteria now goes back to over four billion years. These visible light-requiring microbes live in the shallow photic zone where the enhanced UV radiation from the young Sun would have impacted them and the evolution taking place before them. This means that there must have been at least some moderate protection from stratospheric ozone derived from oxygen. This oxygen was produced by the photodissociation of water vapor and the loss of light hydrogen to space. Arsenic is irrelevant where dissolved ferrous iron in the primitive ocean was being oxidized and precipitating.
    Reply
  • Kurt Risser
    This statement is not correct: "Microbial mats, which fossilize into stromatolites".
    Microbial mat trace fossils and stromatolites have distinctly different morphologies.
    Additionally, it is likely that there were low levels of oxygen present on Archean Earth.
    Reply
  • Broadlands
    Yes, there is a big difference between low levels (1-2% of present levels) and none at all. None at all (or close to zero) precludes the necessary ozone screen protection for photosynthesis in shallow, often desiccated microbial mats. The same would apply to Mars or any location where solar energy is involved in life.

    "In Chile's Atacama Desert, in a place called Laguna La Brava, scientists have been studying a purple ribbon of photosynthetic microbes living in a hypersaline lake that's permanently free of oxygen." But protected from the DNA damaging UV radiation by a full ozone screen. Not a fair model for the early Archean.
    Reply
  • Chem721
    Broadlands said:
    This oxygen was produced by the photodissociation of water vapor and the loss of light hydrogen to space.

    This is quite probable, and could have resulted in substantial O2 in the atmosphere as there was a lot of water on the early earth (and there still is).

    It has been suggested that Venus in its early years had a geodynamo like earth's, producing a magnetic field which protected its surface from direct solar radiation. But the geodynamo was lost long ago, likely due to the cooling of a smaller iron-nickel mass than earth's. It is speculated that large volumes of water began to evaporate and photodissociate in the upper atmosphere. Venus, like Earth, likely had large bodies of water accumulated during its formation.

    The evidence for such an event lies in the water which still resides on Venus. Its deuterium: hydrogen ratio is about 100 times that of earth's water, suggesting some mechanism must have been at work in the enrichment. Photodissociation of water would permit more of the lighter hydrogen to escape into space at a faster rate, slowly enriching the deuterium.

    Most of the water is now gone, certainly at the surface of Venus at any rate. But it would seem to require the evaporation of large quantities of water and photodissociation to account for this significant variability in the D:H ratio. A similar mechanism could have occurred on an early earth, prior to photosynthesis. It is even possible that the evolution of photosynthesis supplied the required O2 to maintain an ozone layer, and limit the loss of earth's water.
    Reply
  • PSKresearch
    Although there may not have been much oxygen dissolved in ancient oceans, & the atmosphere, water itself, H2O, is a molecule of 1/3 oxygen that can be easily split with just a small amount of electric current, lightning, UV rays, or gamma rays. So plenty of oxygen was available anyway. Oxygen is a waste product of plants when they make food/energy (which is a type of hydrocarbon) for themselves from sunlight & some raw materials, like hydrogen & carbon. Oxygen or any other oxidizer isn't really required for the process.
    Reply
  • Broadlands
    Lots of things are possible but one thing is certain. Some UV radiation protection for life to begin on Earth (or elsewhere) was needed at the surface. Given the early primordial 'building blocks' (amino acids) the formation of the necessary initial peptide and polypeptide bonds requires the input of energy and removal of water. That can only be done at the surface. Stratospheric ozone is the only plausible source of that protection and oxygen is needed. This would have been a requirement all the way from the evolution of DNA/RNA up to the first cell and beyond... especially for any life experimenting with photo energy in the visible spectrum leading to the cyanobacteria. Cyanobacteria are not among the LUCAs, the last universal common ancestor candidates.
    Reply
  • TorbjornLarsson
    That prokaryotes switched over from the current main hypothesis sulfides to arsenic for photolitotrophy in stromatolites in the late Archean is plausible. Ironically the rather late divergent Chrenarchaeota (with a matching split date of ~ 2.5 Ga: Holly C. Betts et al, Integrated genomic and fossil evidence illuminates life's early evolution and eukaryote origin, Nature Ecology & Evolution (2018)) was initially thought to be sulphur-dependent (Sulfolobus solfataricus).
    Reply
  • TorbjornLarsson
    Broadlands said:
    "Microbial mats, which fossilize into stromatolites, have been abundant on Earth for at least 3.5 billion years, and yet for the first billion years of their existence, there was no oxygen for photosynthesis."
    That's not true.

    It is a well known fact, see the paper for references.

    Broadlands said:
    The fossil record of cyanobacteria now goes back to over four billion years.

    No, the microbial mats is concurrent with the generally accepted earliest fossils https://en.wikipedia.org/wiki/Fossil ]. While stem cyanobacteria split ~ 3 billion years ago, and the modern oxygenic photosynthesising evolved ~ 1.5 billion years ago (there is a gap) .

    Broadlands said:

    Lots of things are possible but one thing is certain. Some UV radiation protection for life to begin on Earth (or elsewhere) was needed at the surface.

    This is a biochemist inspired minority idea which is problematic due to all converging evidence for evolution from submarine hydrothermal vents.

    https://www.nature.com/articles/nmicrobiol2016116 ; https://www.liebertpub.com/doi/10.1089/AST.2019.2203 ].

    Although there is widespread speculation on the geological setting for the origin of life, the earliest geological and biological evidence for early life and the putative conditions on the early Archean or Hadean Earth provide some environmental constraints. Several theories suggest a hot, aqueous environment for the origin of life (e.g., Pace, 1991; Stetter, 2005), and submarine hydrothermal vents have been widely proposed as candidate environments for prebiotic chemistry (e.g., Orgel, 1998; Nisbet and Sleep, 2001; Copley et al.,2007; Martin et al.,2008; Russell et al.,2013). Indeed, while still debated, many phylogenetic tree reconstructions when using molecular analyses of 16S rRNA combined with metabolic studies suggest a hyperthermophilic last universal common ancestor (LUCA) (Woese et al.,1990; Pace, 1991; Di Giulio, 2001, 2003, 2007; Schwartzman and Lineweaver, 2004; Brack et al.,2010).

    (Note that the split between bacteria and our archaea ancestor is deep in the integrated best evidence, most likely at 4.5 billion years, so the LUCA is indicative of early evolution even if the above phylogeny method would get an erroneous result.)

    In any case, speaking of not fact, it is not 'certain'.
    Reply
  • PSKresearch
    Just a few inches of water block a sufficient amount of UV rays while still allowing enough light for photosynthesis under the water. Also perpetual shady areas would get very little UV... like the shadows on the northern side of mountains & cliffs in northern latitudes, southern sides of mountains & cliffs in the southern latitudes, & in deep valleys or valleys surrounded by highlands.
    Reply
  • TorbjornLarsson
    Chem721 said:
    It is even possible that the evolution of photosynthesis supplied the required O2 to maintain an ozone layer, and limit the loss of earth's water.

    Loss of hydrogen was important to lose the primordial atmoshere and set up a reductive, acidic ocean which seems to have been important for local biomolecule production evolving early life.

    Venus may have had water until 0.7 billion years ago when the surface was largely remodeled. In fact the latest generation of 3D models suggests it would still have water so now the wet greenhouse effect becomes more of a problem than a solution http://astrobiology.com/2020/09/nasa-scientists-explore-venus-habitable-climate-scenarios-at-nccs.html ].

    This visualization of Simulation 28 shows surface air temperature (Celsius) 2.9 billion years ago for a hypothetical Venus having a nitrogen (N2)-dominated atmosphere with .25-bar surface pressure and an Earth topography with a 310-meter deep ocean. It was the most Earth-like of the 45 simulations, even allowing for the possibility of snow at higher elevations. Figure from M.J. Way and A.D. Del Genio, J. Geophys. Res. Planets.

    I liked your analysis though, up to the above point.

    Modulo distance to Sun, planet mass, atmosphere scaling height, geodynamo all of Venus, Earth and Mars can, according to the probes, lose water at the same rate. The difference with our geodynamo is that the attrition mostly happens at the polar regions (and since it is transient phenomena I am not sure we have a handle on the integrated losses).
    And atmosphere oxygenation was late at ~ 2.5 billion years ago. There is now plausible meteorite evidence that the upper atmosphere could have been oxygen free https://advances.sciencemag.org/content/6/4/eaay4644 ].


    It was recently proposed that Archean, spherical, iron-rich (type I) micrometeorites could have been oxidized by modern levels of O2 in the upper atmosphere (10, 11). However, very low levels of O2 in the Archean atmosphere inferred from a large variety of proxies motivate considering oxidation by CO2 as an alternative (13). Thus, the micrometeorites could provide a new constraint on Archean CO2 levels.

    With a mean surface pressure of 0.23 ± 0.23 bar, our model predicts a CO2 partial pressure of >0.16 ± 0.16 bar (2σ). Provided some methane was present, such as 0.5% (30), this thin, CO2-rich atmosphere could provide enough greenhouse warming to sustain liquid water under a faint young Sun . Atmospheric methane would warm the surface during the Archean as a greenhouse gas but is not expected to interact with molten, Fe-rich micrometeorites (10) and thus should not alter their oxidation state.
    Reply