ScienceScientists Show Evidence of How Earth Got Its Oxygen
Jason Mick (Blog) - June 27, 2013 8:16 PM
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Researchers have solid evidence indicating that massive blooms of photosynthesizing algae in the primordial ocean used the power of the sun to "split" (oxidize) water, releasing its oxygen atoms as diatomic atmospheric oxygen gas, while merging the hydrogen with carbon dioxide to build up sugars -- life's primary form of chemical energy storage.
I. What Came Before Modern Photosynthesis?
Researchers even have a good idea of where algae got their photosynthesis genes. Describes geobiology Professor Woodward Fischer of the California Institute of Technology (CalTech), "Water-oxidizing or water-splitting photosynthesis was invented by cyanobacteria approximately 2.4 billion years ago and then borrowed by other groups of organisms thereafter. Algae borrowed this photosynthetic system from cyanobacteria, and plants are just a group of algae that took photosynthesis on land, so we think with this finding we're looking at the inception of the molecular machinery that would give rise to oxygen."
Cyanobacteria is a hardy phylum of bacteria (better known as "blue-green algae", a misleading name as it's not an algae at all) that's found in every continent. Some exobiologists even believe they may be descended from extraplanetary microorganisms that arrived aboard meteors hitting early Earth. Having survived billions of years, the cyanobacteria of today have photosynthetic systems similar to plants and true algae. But researchers believe it wasn't always that way -- there must have been an intermediate reaction that allowed the tiny sugar-factories to evolve towards full-blown synthesis.
Researchers have been hunting for the precursor to nature's primary modern energy storing process -- photosynthesis. [Image Source: Take Pride in Utah]
Professor Fischer and others have long hunted for this stepping stone which is thought to have preceded full-blown photosynthesis. Now he believes there may be an answer.
Maganese, a key component in photosynthesis involved in electron transfer, can also undergo alternative oxidation reactions (similar to the splitting of water) that transfer electrons to the manganese, changing its state without the presence of free oxygen. This reaction could drive the formation of energy-storing biomolecules. Prof. Fischer's graduate student Jena Johnson ponders, "Manganese plays an essential role in modern biological water splitting as a necessary catalyst in the process, so manganese-oxidizing photosynthesis makes sense as a potential transitional photosystem."
II. Manganese -- Can You Dig It?
But the researchers need evidence to support their hypothesis -- they need to document chemical and geological evidence of manganese oxidation being performed organically before traditional photosynthesis. The perfect place to look was in magnesium rich sediments deposited before the evolution of traditional photosynthesis.
Manganese is naturally soluble in seawater, with electrons in tow. But when exposed to strong oxidative agents (such as oxidative enzymes in a cyanobacteria), the manganese forms oxidated compounds which are insoluble and precipitate as solids, falling to the sea floor.
In an example of perfect timing the Agouron Institute (AGI) has just obtained drill cores of 2.415 billion-year-old South African marine sedimentary rocks. To the excitement of the CalTech team, these rocks showed a high concentration of manganese. Prof. Fisher comments, "Just the observation of these large enrichments -- 16 percent manganese in some samples -- provided a strong implication that the manganese had been oxidized, but this required confirmation."
Grad student Jena E. Johnson inspects an ancient African manganese-rich rock deposit.
[Image Source: CalTech]
The key question was first whether the manganese was truly oxidized and secondly when it arrived there. If it was truly dropped off 2.415 bya (billion years ago), then that would provide strong evidence of an organic origin and the researchers' hypothesis of how pre-photosynthetic bacteria formed oxygen. But ocean currents could easily have deposited the manganese at a later date, invalidating the deposits as solid evidence.
Thus the team had to examine the cores at a micron scale. To do that they developed new X-ray based techniques to examine the abundance of and oxidation states of manganese at a scale as small as 2 microns (2e-6 m, or 2000 nanometers).
Describes Prof. Fischer, "It's warranted -- these rocks are complicated at a micron scale! And yet, the rocks occupy hundreds of meters of stratigraphy across hundreds of square kilometers of ocean basin, so you need to be able to work between many scales -- very detailed ones, but also across the whole deposit to understand the ancient environmental processes at work."
The method was developed in collaboration with Stanford University's SLAC National Accelerator Laboratory. SLAC professor Samuel Webb describes, "It's really amazing to be able to use X-ray techniques to look back into the rock record and use the chemical observations on the microscale to shed light on some of the fundamental processes and mechanisms that occurred billions of years ago. Questions regarding the evolution of the photosynthetic pathway and the subsequent rise of oxygen in the atmosphere are critical for understanding not only the history of our own planet, but also the basics of how biology has perfected the process of photosynthesis."
The researchers next verified that the manganese was oxidized via an electron transfer and did not have attached oxygens. This was confirmed.
Together the evidence shows that something was widely oxidizing manganese across the ancient ocean, using a process that predated the oxygen-creating modern photosynthetic pathway. Where modern photosynthesis uses manganese to feed electrons on down a chain of reactions to drive water splitting, this earlier pathway stopped before these reactions, using the oxidation of manganese to directly drive processes, according to the supported hypothesis.
III. Can Researchers Recreate Ancient Bacteria?
Much work remains, though, in formulating what kinds of organic reactions the manganese oxidation may have directly driven. Comments Prof. Fisher:
I think that there will be a number of additional experiments that people will now attempt to try and reverse engineer a manganese photosynthetic photosystem or cell.
Once you know that this happened, it all of a sudden gives you reason to take more seriously an experimental program aimed at asking, 'Can we make a photosystem that's able to oxidize manganese but doesn't then go on to split water? How does it behave, and what is its chemistry?'
Even though we know what modern water splitting is and what it looks like, we still don't know exactly how it works. There is a still a major discovery to be made to find out exactly how the catalysis works, and now knowing where this machinery comes from may open new perspectives into its function -- an understanding that could help target technologies for energy production from artificial photosynthesis.
The team is moving on to a pair of new works to further support the hypothesis. The first involves examining similar core samples from ancient Australian deposits. If the samples show manganese, this would provide further evidence that manganese oxidizing pre-photosynthetic bacteria had spread around the globe.
Second, the team is looking to mutate a cyanobacteria to "go back", ditching the final pathways of photosynthesis to directly drive its biochemical pathways.
If the team can provide further evidence they could prove that bacteria first learned evolved a chain of enzymes to use sunlight to drive oxidation of manganese, before eventually acquiring more enzymes that allowed the splitting of water, improving the efficiency of the process, while allowing for the creation of an oxygen atmosphere and the high-density energy storage needed for multicellular life.
Researchers are working to "turn back the clock", engineering cyanobacteria that use manganese oxidation instead of water-splitting photosynthesis. [Image Source: CA.gov]
Currently the team has published the work on their hypotheses and inspection of the African rock samples in a paper [abstract] in the prestigious peer-reviewed journal PNAS. The work was funded by the Agouron Institute, The National Aeronautics and Space Administration's (NASA)'s Exobiology Branch, the David and Lucile Packard Foundation, and the National Science Foundation (NSF) Graduate Research Fellowship program.