June 27, 2013
Manganese Oxidation Played Key Role In Formation Of Oxygen On Earth
April Flowers for redOrbit.com - Your Universe Online
Oxygen is a necessary component for the survival of most terrestrial life on Earth. The planet´s atmosphere, however, did not always contain this life-sustaining substance. How and when oxygenic photosynthesis — the process responsible for producing oxygen on Earth through the splitting of water molecules — first began has been considered one of the great science mysteries of our planet.A new study, led by geobiologists at the California Institute of Technology (Caltech), reveals evidence of a precursor photosystem involving manganese that predates cyanobacteria, which were the first organisms to release oxygen into the environment via photosynthesis.
The team´s findings support the idea that manganese oxidation provided an evolutionary stepping stone for the development of water-oxidizing photosynthesis in cyanobacteria. Despite the name, manganese oxidation is a chemical reaction that does not have to involve oxygen.
"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," explains Woodward Fischer, assistant professor of geobiology at Caltech. "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."
Plants and other organisms use energy from the sun to split water and carbon dioxide molecules, making carbohydrates and oxygen by the process of photosynthesis. For water splitting to work, manganese is required — leading the scientists to question what evolutionary steps may have led up to an oxygenated atmosphere on Earth. This questioning led them to look for evidence of manganese-oxidizing photosynthesis prior to cyanobacteria. They knew that because oxidation simply involves the transfer of electrons to increase the charge on an atom, which can be accomplished using light or O2, oxidation might have occurred before the rise of oxygen on the planet.
"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," says Jena Johnson, a graduate student in Fischer's laboratory at Caltech.
MANGANESE IN MARINE SEDIMENTS
The research team examined drill cores from 2.415 billion-year-old South African marine sedimentary rocks with large deposits of manganese to test the theory that manganese based photosynthesis happened prior to the evolution of oxygenic cyanobacteria. The core samples, and partial funding for the study, were obtained from the Agouron Institute.
Fisher explained manganese is soluble in seawater. In fact, if there are no strong oxidants to accept electrons from the manganese available, it will remain aqueous. The moment manganese is oxidized, or loses electrons, however, it precipitates and forms a solid that can become concentrated within seafloor sediments.
"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," Fischer says.
The researchers developed and used new techniques that allowed them to assess the abundance and oxidation state of manganese-bearing minerals at a very tiny scale of 2 microns to prove the manganese was part of the original South African rock and not from a later deposit by hydrothermal fluids.
"And it's warranted–these rocks are complicated at a micron scale!" Fischer says. "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 team demonstrated that the manganese was original to the rocks and first deposited as manganese oxides using these multi-scale approaches. Furthermore, they demonstrated the manganese oxidation occurred over a broad swath of the ancient marine basin during the entire timescale captured by the drill cores.
"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," says Samuel Webb, beam line scientist at the SLAC National Accelerator Laboratory at Stanford University, where many of the experiments for this study took place. "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."
With this proof in hand, the team tested to see if these manganese oxides were actually formed before water-splitting photosynthesis emerged, or if they formed after as a result of oxygen reactions. Two different techniques were used to determine whether oxygen was present. The results showed oxygen was not present, proving water-splitting photosynthesis had not yet evolved at that point in time and the manganese present in the deposits had been oxidized and deposited before the appearance of cyanobacteria.
The findings, published in the Proceedings of the National Academy of Sciences (PNAS), suggest manganese-oxidizing photosynthesis was a stepping-stone for oxygen-producing, water-splitting photosynthesis.
"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," Fischer says. "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 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 next research step for the team is to try and mutate cyanobacteria to “go backwards” and perform manganese oxidizing photosynthesis. The researchers also intend to investigate a set of rocks from Western Australia that are similar in age to the samples used in the current study and which may also contain beds of manganese. If their results are correct, the team says, they should be able to find evidence of the same processes in other parts of the world.
"Oxygen is the backdrop on which this story is playing out on, but really, this is a tale of the evolution of this very intense metabolism that happened once–an evolutionary singularity that transformed the planet," Fischer says. "We've provided insight into how the evolution of one of these remarkable molecular machines led up to the oxidation of our planet's atmosphere, and now we're going to follow up on all angles of our findings."