This makes it pretty
clear that manganese is far more important in a biological sense that had been
plausibly considered. It is certainly
heavily represented in sediments and related volcanics, something that I have
seen for myself.
Yet this was
unexpected. It may inform us a much
more.
A better understanding
of manganese biology is now clearly called for.
A Stepping-Stone for
Oxygen on Earth
06/26/2013
Caltech
researchers find evidence of an early manganese-oxidizing photosystem
For
most terrestrial life on Earth, oxygen is necessary for survival. But the
planet's atmosphere did not always contain this life-sustaining substance, and
one of science's greatest mysteries is how and when oxygenic photosynthesis—the
process responsible for producing oxygen on Earth through the splitting of
water molecules—first began. Now, a team led by geobiologists at the California
Institute of Technology (Caltech) has found evidence of a precursor
photosystem involving manganese that predates cyanobacteria, the first group of
organisms to release oxygen into the environment via photosynthesis.
The
findings, outlined in the June 24 early edition of the Proceedings of the
National Academy of Sciences (PNAS), strongly support the idea that manganese
oxidation—which, despite the name, is a chemical reaction that does
not have to involve oxygen—provided an evolutionary stepping-stone
for the development of water-oxidizing photosynthesis in cyanobacteria.
"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 and a coauthor of the study. "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."
Photosynthesis
is the process by which energy from the sun is used by plants and other
organisms to split water and carbon dioxide molecules to make carbohydrates and
oxygen. Manganese is required for water splitting to work, so when scientists
began to wonder what evolutionary steps may have led up to an oxygenated
atmosphere on Earth, they started to look for evidence of manganese-oxidizing
photosynthesis prior to cyanobacteria. Since oxidation simply involves the
transfer of electrons to increase the charge on an atom—and this can be
accomplished using light or O2—it could have occurred before the rise of oxygen
on this 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 and lead author of the study.
To
test the hypothesis that manganese-based photosynthesis occurred prior to the
evolution of oxygenic cyanobacteria, the researchers examined drill cores
(newly obtained by the Agouron Institute) from 2.415 billion-year-old South
African marine sedimentary rocks with large deposits of manganese.
Manganese
is soluble in seawater. Indeed, if there are no strong oxidants around to
accept electrons from the manganese, it will remain aqueous, Fischer explains,
but the second it is oxidized, or loses electrons, manganese precipitates,
forming 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," he says.
To
prove that the manganese was originally part of the South African rock and not
deposited there later by hydrothermal fluids or some other phenomena, Johnson
and colleagues developed and employed techniques that allowed the team to
assess the abundance and oxidation state of manganese-bearing minerals at a very
tiny scale of 2 microns.
"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."
Using
these multiscale approaches, Johnson and colleagues demonstrated that the
manganese was original to the rocks and first deposited in sediments as
manganese oxides, and that 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, coauthor on the paper and beam line
scientist at the SLAC National Accelerator Laboratory at Stanford University,
where many of the study's experiments 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."
Once
the team confirmed that the manganese had been deposited as an oxide phase when
the rock was first forming, they checked to see if these manganese oxides were
actually formed before water-splitting photosynthesis or if they formed after
as a result of reactions with oxygen. They used two different techniques to
check whether oxygen was present. It was not—proving that water-splitting photosynthesis
had not yet evolved at that point in time. The manganese in the deposits had
indeed been oxidized and deposited before the appearance of water-splitting
cyanobacteria. This implies, the researchers say, that 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. "
Next
up in Fischer's lab, Johnson plans to work with others to try and mutate a
cyanobacteria to "go backwards" and perform manganese-oxidizing
photosynthesis. The team also plans to investigate a set of rocks from western
Australia that are similar in age to the samples used in the current study and
may also contain beds of manganese. If their current study results are truly an
indication of manganese-oxidizing photosynthesis, they say, there should be
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."
Funding
for the research outlined in the PNAS paper, titled
"Manganese-oxidizing photosynthesis before the rise of
cyanobacteria," was provided by the Agouron Institute, NASA's Exobiology
Branch, the David and Lucile Packard Foundation, and the National Science
Foundation Graduate Research Fellowship program. Joseph Kirschvink, Nico and
Marilyn Van Wingen Professor of Geobiology at Caltech, also contributed to the
study along with Katherine Thomas and Shuhei Ono from the Massachusetts
Institute of Technology.
Written
by Katie Neith
This is gorgeous!
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