Scientists unravel mystery of photosynthesis

Sunlight and leaves (stock image).

Plants have been harnessing the sun's energy for hundreds of millions of years.

Algae and photosynthetic bacteria have been doing the same for even longer, all with remarkable efficiency and resiliency.

It's no wonder, then, that scientists have long sought to understand exactly how they do this, hoping to use this knowledge to improve human-made devices such as solar panels and sensors.

Scientists from the U.S. Department of Energy's (DOE) Argonne National Laboratory, working closely with collaborators at Washington University in St. Louis, recently solved a critical part of this age-old mystery, homing in on the initial, ultrafast events through which photosynthetic proteins capture light and use it to initiate a series of electron transfer reactions.

"In order to understand how biology fuels all of its engrained activities, you must understand electron transfer," said Argonne biophysicist Philip Laible. "The movement of electrons is crucial: it's how work is accomplished inside a cell."

In photosynthetic organisms, these processes begin with the absorption of a photon of light by pigments localized in proteins.

Each photon propels an electron across a membrane located inside specialized compartments within the cell.

"The separation of charge across a membrane -- and stabilization of it -- is critical as it generates energy that fuels cell growth," said Argonne biochemist Deborah Hanson.

The Argonne and Washington University research team has gained valuable insight on the initial steps in this process: the electron's journey.

Nearly 35 years ago, when the first structure of these types of complexes was unveiled, scientists were surprised to discover that after the absorption of light, the electron transfer processes faced a dilemma: there are two possible pathways for the electron to travel.

In nature, plants, algae and photosynthetic bacteria use just one of them -- and scientists had no idea why.

What they did know was that the propulsion of the electron across the membrane -- effectively harvesting the energy of the photon -- required multiple steps.

Argonne and Washington University scientists have managed to interfere with each one of them to change the electron's trajectory.

"We've been on this trail for more than three decades, and it is a great accomplishment that opens up many opportunities," said Dewey Holten, a chemist at Washington University.

The scientists' recent article, "Switching sides -- Reengineered primary charge separation in the bacterial photosynthetic reaction center," published in the Proceedings of the National Academy of Sciences, shows how they discovered an engineered version of this protein complex that switched the utilization of the pathways, enabling the one that was inactive while disabling the other.

"It is remarkable that we have managed to switch the direction of initial electron transfer," said Christine Kirmaier, Washington University chemist and project leader. "In nature, the electron chose one path 100 percent of the time. But through our efforts, we have been able to make the electron switch to an alternate path 90 percent of the time. These discoveries pose exciting questions for future research."

As a result of their efforts, the scientists are now closer than ever to being able to design electron transfer systems in which they can send an electron down a pathway of their choosing.

"This is important because we are gaining the ability to harness the flow of energy to understand design principles that will lead to new applications of abiotic systems," Laible said. "This would allow us to greatly improve the efficiency of many solar-powered devices, potentially making them far smaller. We have a tremendous opportunity here to open up completely new disciplines of light-driven biochemical reactions, ones that haven't been envisioned by nature. If we can do that, that's huge."

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Materials provided by DOE/Argonne National Laboratory. Original written by Jo Napolitano. Note: Content may be edited for style and length.

Unearthing the mystery of the meaning of Easter Island's Moai

Moai statues on Easter Island 

Rapa Nui (or Easter Island, as it is commonly known) is home to the enigmatic Moai, stone monoliths that have stood watch over the island landscape for hundreds of years. Their existence is a marvel of human ingenuity -- and their meaning a source of some mystery.

Ancient Rapanui carvers worked at the behest of the elite ruling class to carve nearly 1,000 Moai because they, and the community at large, believed the statues capable of producing agricultural fertility and thereby critical food supplies, according to a new study from Jo Anne Van Tilburg, director of the Easter Island Statue Project, recently published in Journal of Archaeological Science.

Van Tilburg and her team, working with geoarchaeologist and soils specialist Sarah Sherwood, believe they have found scientific evidence of that long-hypothesized meaning thanks to careful study of two particular Moai excavated over five years in the Rano Raraku quarry on the eastern side of the Polynesian island.

Van Tilburg's most recent analysis focused on two of the monoliths that stand within the inner region of the Rano Raraku quarry, which is the origin of 95 percent of the island's more than 1,000 Moai. Extensive laboratory testing of soil samples from the same area shows evidence of foods such as banana, taro and sweet potato.

Van Tilburg said the analysis showed that in addition to serving as a quarry and a place for carving statues, Rano Raraku also was the site of a productive agricultural area.

"Our excavation broadens our perspective of the Moai and encourages us to realize that nothing, no matter how obvious, is ever exactly as it seems. I think our new analysis humanizes the production process of the Moai," Van Tilburg said.

Van Tilburg has been working on Rapa Nui for more than three decades. Her Easter Island Statue Project is supported in part by UCLA's Cotsen Institute of Archaeology. Tom Wake, a Cotsen Institute colleague, analyzes small-animal remains from the excavation site. Van Tilburg also serves as director of UCLA's Rock Art Archive.

Van Tilburg, in partnership with members of the local community, heads the first legally permitted excavations of Moai in Rano Raraku since 1955. Cristián Arévalo Pakarati, a noted Rapanui artist, is project co-director.

The soils in Rano Raraku are probably the richest on the island, certainly over the long term, Sherwood said. Coupled with a fresh-water source in the quarry, it appears the practice of quarrying itself helped boost soil fertility and food production in the immediate surroundings, she said. The soils in the quarry are rich in clay created by the weathering of lapilli tuff (the local bedrock) as the workers quarried into deeper rock and sculpted the Moai.

A professor of earth and environmental systems at the University of the South in Sewanee, Tenn., Sherwood joined the Easter Island Project after meeting another member of Van Tilburg's team at a geology conference.

She wasn't originally looking for soil fertility, but out of curiosity and research habit, she did some fine-scale testing of samples brought back from the quarry.

"When we got the chemistry results back, I did a double take," Sherwood said. "There were really high levels of things that I never would have thought would be there, such as calcium and phosphorus. The soil chemistry showed high levels of elements that are key to plant growth and essential for high yields. Everywhere else on the island the soil was being quickly worn out, eroding, being leeched of elements that feed plants, but in the quarry, with its constant new influx of small fragments of the bedrock generated by the quarrying process, there is a perfect feedback system of water, natural fertilizer and nutrients."

She said it also looks like the ancient indigenous people of Rapanui were very intuitive about what to grow -- planting multiple crops in the same area, which can help maintain soil fertility.

The Moai that Van Tilburg's team excavated were discovered upright in place, one on a pedestal and the other in a deep hole, indicating they were meant to remain there.

"This study radically alters the idea that all standing statues in Rano Raraku were simply awaiting transport out of the quarry," Van Tilburg said. "That is, these and probably other upright Moai in Rano Raraku were retained in place to ensure the sacred nature of the quarry itself. The Moai were central to the idea of fertility, and in Rapanui belief their presence here stimulated agricultural food production."

Van Tilburg and her team estimate the statues from the inner quarry were raised by or before A.D. 1510 to A.D.1645. Activity in this part of the quarry most likely began in A.D.1455. Most production of Moai had ceased in the early 1700s due to western contact.

The two statues Van Tilburg's team excavated had been almost completely buried by soils and rubble.

"We chose the statues for excavation based on careful scrutiny of historical photographs and mapped the entire Rano Raraku inner region before initiating excavations," she said.

Van Tilburg has worked hard to establish connections with the local community on Rapa Nui. The project's field and lab teams are made up of local workers, mentored by professional archeologists and geologists.

The result of their collective efforts is a massive detailed archive and comparative database that documents more than 1,000 sculptural objects on Rapa Nui, including the Moai, as well as similar records on more than 200 objects scattered in museums throughout the world. In 1995, UNESCO named Easter Island a World Heritage Site, with most of the island's sacred sites protected within Rapa Nui National Park.

This is the first definitive study to reveal the quarry as a complex landscape and to make a definitive statement that links soil fertility, agriculture, quarrying and the sacred nature of the Moai.

Van Tilburg and her team are working on another study that analyzes the rock art carvings that exist on only three of the Moai.

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Materials provided by University of California - Los Angeles. Original written by Jessica Wolf. Note: Content may be edited for style and length.

Mystery of how early animals survived ice age

Mystery of how early animals survived ice age

How did life survive the most severe ice age? A McGill University-led research team has found the first direct evidence that glacial meltwater provided a crucial lifeline to eukaryotes during Snowball Earth, when the oceans were cut off from life-giving oxygen, answering a question puzzling scientists for years.

In a new study published in the Proceedings of the National Academy of Sciences, researchers studied iron-rich rocks left behind by glacial deposits in Australia, Namibia, and California to get a window into the environmental conditions during the ice age. Using geological maps and clues from locals, they hiked to rock outcrops, navigating challenging trails to track down the rock formations.

By examining the chemistry of the iron formations in these rocks, the researchers were able to estimate the amount of oxygen in the oceans around 700 million years ago and better understand the effects this would have had on all oxygen-dependent marine life, including the earliest animals like simple sponges.

"The evidence suggests that although much of the oceans during the deep freeze would have been uninhabitable due to a lack of oxygen, in areas where the grounded ice sheet begins to float there was a critical supply of oxygenated meltwater. This trend can be explained by what we call a 'glacial oxygen pump'; air bubbles trapped in the glacial ice are released into the water as it melts, enriching it with oxygen," says Maxwell Lechte, a postdoctoral researcher in the Department of Earth and Planetary Sciences under the supervision of Galen Halverson at McGill University.

Around 700 million years ago, the Earth experienced the most severe ice age of its history, threatening the survival of much of the planet's life. Previous research has suggested that oxygen-dependent life may have been restricted to meltwater puddles on the surface of the ice, but this study provides new evidence of oxygenated marine environments.

"The fact that the global freeze occurred before the evolution of complex animals suggests a link between Snowball Earth and animal evolution. These harsh conditions could have stimulated their diversification into more complex forms," says Lechte, who is also the study's lead author.

Lechte points out that while the findings focus on the availability of oxygen, primitive eukaryotes would also have needed food to survive the harsh conditions of the ice age. Further research is needed to explore how these environments might have sustained a food web. A starting point might be modern ice environments that host complex ecosystems today.

"This study actually solves two mysteries about the Snowball Earth at once. It not only provides explanation for how early animals may have survived global glaciation, but also eloquently explains the return of iron deposits in the geological record after an absence of over a billion years," says Professor Galen Halverson.

About the study

"Subglacial meltwater supported aerobic marine habitats during Snowball Earth" by Maxwell Lechte, Malcolm Wallace, Ashleigh van Smeerdijk Hood, Weiqiang Li, Ganqing Jiang, Galen Halverson, Dan Asael, Stephanie McColl, and Noah Planavsky is published in Proceedings of the National Academy of Sciences of the United States of America. The paper is a collaboration between McGill University, University of Melbourne, Nanjing University, University of Nevada, Las Vegas, and Yale University.

This work received financial support from the Australian Government Research Training Program Scholarship, the Albert Shimmins Award, the Australian Research Council Discovery Grant, the NASA Astrobiology Postdoctoral Fellowship, the Puzey Fellowship, and the Australian Research Council.

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Materials provided by McGill University. Note: Content may be edited for style and length.

With Mars methane mystery unsolved, Curiosity serves scientists a new one: Oxygen

Curiosity serves scientists a new one: Oxygen

For the first time in the history of space exploration, scientists have measured the seasonal changes in the gases that fill the air directly above the surface of Gale Crater on Mars. As a result, they noticed something baffling: oxygen, the gas many Earth creatures use to breathe, behaves in a way that so far scientists cannot explain through any known chemical processes.

Over the course of three Mars years (or nearly six Earth years) an instrument in the Sample Analysis at Mars (SAM) portable chemistry lab inside the belly of NASA's Curiosity rover inhaled the air of Gale Crater and analyzed its composition. The results SAM spit out confirmed the makeup of the Martian atmosphere at the surface: 95% by volume of carbon dioxide (CO2), 2.6% molecular nitrogen (N2), 1.9% argon (Ar), 0.16% molecular oxygen (O2), and 0.06% carbon monoxide (CO). They also revealed how the molecules in the Martian air mix and circulate with the changes in air pressure throughout the year. These changes are caused when CO2 gas freezes over the poles in the winter, thereby lowering the air pressure across the planet following redistribution of air to maintain pressure equilibrium. When CO2 evaporates in the spring and summer and mixes across Mars, it raises the air pressure.

Within this environment, scientists found that nitrogen and argon follow a predictable seasonal pattern, waxing and waning in concentration in Gale Crater throughout the year relative to how much CO2 is in the air. They expected oxygen to do the same. But it didn't. Instead, the amount of the gas in the air rose throughout spring and summer by as much as 30%, and then dropped back to levels predicted by known chemistry in fall. This pattern repeated each spring, though the amount of oxygen added to the atmosphere varied, implying that something was producing it and then taking it away.

"The first time we saw that, it was just mind boggling," said Sushil Atreya, professor of climate and space sciences at the University of Michigan in Ann Arbor. Atreya is a co-author of a paper on this topic published on November 12 in the Journal of Geophysical Research: Planets.

As soon as scientists discovered the oxygen enigma, Mars experts set to work trying to explain it. They first double- and triple-checked the accuracy of the SAM instrument they used to measure the gases: the Quadrupole Mass Spectrometer. The instrument was fine. They considered the possibility that CO2 or water (H2O) molecules could have released oxygen when they broke apart in the atmosphere, leading to the short-lived rise. But it would take five times more water above Mars to produce the extra oxygen, and CO2 breaks up too slowly to generate it over such a short time. What about the oxygen decrease? Could solar radiation have broken up oxygen molecules into two atoms that blew away into space? No, scientists concluded, since it would take at least 10 years for the oxygen to disappear through this process.

"We're struggling to explain this," said Melissa Trainer, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland who led this research. "The fact that the oxygen behavior isn't perfectly repeatable every season makes us think that it's not an issue that has to do with atmospheric dynamics. It has to be some chemical source and sink that we can't yet account for."

To scientists who study Mars, the oxygen story is curiously similar to that of methane. Methane is constantly in the air inside Gale Crater in such small quantities (0.00000004% on average) that it's barely discernable even by the most sensitive instruments on Mars. Still, it's been measured by SAM's Tunable Laser Spectrometer. The instrument revealed that while methane rises and falls seasonally, it increases in abundance by about 60% in summer months for inexplicable reasons. (In fact, methane also spikes randomly and dramatically. Scientists are trying to figure out why.)

With the new oxygen findings in hand, Trainer's team is wondering if chemistry similar to what's driving methane's natural seasonal variations may also drive oxygen's. At least occasionally, the two gases appear to fluctuate in tandem.

"We're beginning to see this tantalizing correlation between methane and oxygen for a good part of the Mars year," Atreya said. "I think there's something to it. I just don't have the answers yet. Nobody does."

Oxygen and methane can be produced both biologically (from microbes, for instance) and abiotically (from chemistry related to water and rocks). Scientists are considering all options, although they don't have any convincing evidence of biological activity on Mars. Curiosity doesn't have instruments that can definitively say whether the source of the methane or oxygen on Mars is biological or geological. Scientists expect that non-biological explanations are more likely and are working diligently to fully understand them.

Trainer's team considered Martian soil as a source of the extra springtime oxygen. After all, it's known to be rich in the element, in the form of compounds such as hydrogen peroxide and perchlorates. One experiment on the Viking landers showed decades ago that heat and humidity could release oxygen from Martian soil. But that experiment took place in conditions quite different from the Martian spring environment, and it doesn't explain the oxygen drop, among other problems. Other possible explanations also don't quite add up for now. For example, high-energy radiation of the soil could produce extra O2 in the air, but it would take a million years to accumulate enough oxygen in the soil to account for the boost measured in only one spring, the researchers report in their paper.

"We have not been able to come up with one process yet that produces the amount of oxygen we need, but we think it has to be something in the surface soil that changes seasonally because there aren't enough available oxygen atoms in the atmosphere to create the behavior we see," said Timothy McConnochie, assistant research scientist at the University of Maryland in College Park and another co-author of the paper.

The only previous spacecraft with instruments capable of measuring the composition of the Martian air near the ground were NASA's twin Viking landers, which arrived on the planet in 1976. The Viking experiments covered only a few Martian days, though, so they couldn't reveal seasonal patterns of the different gases. The new SAM measurements are the first to do so. The SAM team will continue to measure atmospheric gases so scientists can gather more detailed data throughout each season. In the meantime, Trainer and her team hope that other Mars experts will work to solve the oxygen mystery.

"This is the first time where we're seeing this interesting behavior over multiple years. We don't totally understand it," Trainer said. "For me, this is an open call to all the smart people out there who are interested in this: See what you can come up with."

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Materials provided by NASA/Goddard Space Flight Center. Note: Content may be edited for style and length.