Tag Archives: #oxygen

Oxygen And Multicellularity, A Complicated Relationship (Astronomy)

A study published in Nature Communications challenges the prevailing theory on the development of multicellular life forms on Earth, according to which the concentration of gas in the atmosphere played a crucial role in the evolution of large and complex organisms. The new findings instead highlight that oxygen would have behaved like a double-edged sword: providing significant metabolic benefits when abundant, but suppressing the evolution of large multicellular organisms in conditions of scarcity.

Scientists have long thought that at the basis of the transition from single-celled organisms to the first multicellular life forms was the increase in oxygen in the Earth’s atmosphere, which began 2.5 billion years ago with the so-called Great Oxygenation Event . This theory, called by the experts “ hypothesis of oxygen control”, Suggests that the transition from unicellular to multicellular life, in which single cells are able to cooperate with the typical mechanisms of more complex life forms, has strictly depended on the amount of oxygen available. Furthermore, the hypothesis predicts that with the increase in the concentration of oxygen in the atmosphere, the size of the multicellular organisms that populated the Earth also increased. According to a new study published this month in Nature Communications,  conducted by a team of researchers from the Georgia Institute of Technology in Atlanta (USA), this is not exactly the case.

Through laboratory experiments that used the unicellular yeast Saccharomyces cerevisiae as an animal model , and thanks to sophisticated evolutionary models, the researchers obtained important new information about the relationship between oxygenation of the early Earth and the emergence of large multicellular organisms. The results of the study suggest that the effect of oxygen on the evolution of multicellularity would not always have been positive, on the contrary: the initial oxygenation of the Earth’s atmosphere would have even severely limited the development of multicellular individuals, rather than selecting larger organisms and complex.

“The positive effect of oxygen on the evolution of multicellularity is dose-dependent: the first oxygenation of our planet would have strongly limited, and not promoted, the development of multicellular life forms”, says Ozan Gonensin Bozdag , researcher at the Georgia Institute of Technology and lead author of the study. “The positive effect of oxygen on the size of multicellular organisms was realized only when it reached high levels.”

Left, photograph obtained by confocal microscopy showing several multicellular clusters of yeast. On the right, the enlargement of a single cluster with the typical shape of a snowflake. Credits: Shane Jacobeen, Will Ratcliff, and Peter Yunker, Georgia Institute of Technology

As anticipated, the researchers used as a model the single-celled yeast Saccharomyces cerevisiae , a eukaryotic microorganism capable of obtaining energy both in the presence of oxygen through respiration and in its absence through fermentation – the chemical process that we have been using for centuries to produce bread. wine and beer. But the one used in the study is not the wild strain – or wild type , as they say in the jargon – but a mutant in the ability to divide and reproduce, and for this reason able to form a multicellular “individual” whose shape resembles the flakes of snow, hence the name snowflake yeastby which these cell clusters are called. After selecting around 800 generations of multicellular forms of this microorganism, the researchers examined their ability to evolve into larger multicellular aggregates by subjecting them to different concentrations of oxygen.

“Large sizes evolved easily when our yeasts lacked or had abundant oxygen, but not when oxygen was present at low levels,” explains Will Ratcliff , also a researcher at the Georgia Institute of Technology and co-founder. author of the study.

These results can be explained by a divergent oxygen-mediated selection mechanism that acts on the size of the organism. An outcome, also confirmed by mathematical models, of almost universal evolutionary and biophysical compromises.

“We have worked hard to show that this is actually a fairly predictable and understandable consequence of the fact that oxygen, when limited, acts as a resource if the cells that can use it get a huge metabolic benefit from it,” he adds. Ratcliff. “When oxygen is in short supply, it can’t spread much, so there’s an evolutionary incentive that leads multicellular organisms to be small in size, which allows most of their constituent cells to access oxygen. This limitation does not exist when oxygen is simply not present, or when there is enough to diffuse much deeper into the tissues [ in the internal cells of large multicellular clusters, ed. ] ».

This study, the researchers continue, not only challenges the oxygen control hypothesis, but helps us understand why the world of multicellular organisms evolved so little in the billion years after the Great Oxygenation Event. In this period – which geologists call the ” Boring Billion ” ( Boring Billion , in English), or the Middle Ages of the Earth – oxygen in the atmosphere was present, but its low levels, rather than selecting larger and more complex organisms , have exerted evolutionary pressure that has pushed multicellular organisms to remain relatively small and simple.

“In previous works, the relationship between oxygen and size of multicellular organisms has been studied mainly through the physical principles of gas diffusion,” emphasizes Bozdag. “This reasoning is fundamental, but when we study the origin of the complex multicellular life forms on our planet it is necessary to include the principles of Darwinian evolution as well,” concludes the researcher. Being able to grow microorganisms through numerous generations has made it possible to achieve this goal.

Featured image: Artistic illustration of the primeval Earth. Credits: Nasa


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Did Earth’s Early Rise in Oxygen Help Multicellular Life Evolve? (Earth Science)

A new study is taking the air out of a hypothesis linking early Earth’s oxygenation to larger, more complex organisms. Georgia Tech researchers report a more complex effect

Scientists have long thought that there was a direct connection between the rise in atmospheric oxygen, which started with the Great Oxygenation Event 2.5 billion years ago, and the rise of large, complex multicellular organisms.

That theory, the “Oxygen Control Hypothesis,” suggests that the size of these early multicellular organisms was limited by the depth to which oxygen could diffuse into their bodies. The hypothesis makes a simple prediction that has been highly influential within both evolutionary biology and geosciences: Greater atmospheric oxygen should always increase the size to which multicellular organisms can grow.

It’s a hypothesis that’s proven difficult to test in a lab. Yet a team of Georgia Tech researchers found a way — using directed evolution, synthetic biology, and mathematical modeling — all brought to bear on a simple multicellular lifeform called a ‘snowflake yeast’. The results? Significant new information on the correlations between oxygenation of the early Earth and the rise of large multicellular organisms — and it’s all about exactly how much O2 was available to some of our earliest multicellular ancestors.

“The positive effect of oxygen on the evolution of multicellularity is entirely dose-dependent — our planet’s first oxygenation would have strongly constrained, not promoted, the evolution of multicellular life,” explains G. Ozan Bozdag, research scientist in the School of Biological Sciences and the study’s lead author. “The positive effect of oxygen on multicellular size may only be realized when it reaches high levels.”

“Oxygen suppression of macroscopic multicellularity” is published in the May 14, 2021 edition of the journal Nature Communications. Bozdag’s co-authors on the paper include Georgia Tech researchers Will Ratcliff, associate professor in the School of Biological Sciences; Chris Reinhard, associate professor in the School of Earth and Atmospheric Sciences; Rozenn Pineau, Ph.D. student in the School of Biological Sciences and the Interdisciplinary Graduate Program in Quantitative Biosciences (QBioS); along with Eric Libby, assistant professor at Umea University in Sweden and the Santa Fe Institute in New Mexico.

Directing yeast to evolve in record time

“We show that the effect of oxygen is more complex than previously imagined. The early rise in global oxygen should in fact strongly constrain the evolution of macroscopic multicellularity, rather than selecting for larger and more complex organisms,” notes Ratcliff.

“People have long believed that the oxygenation of Earth’s surface was helpful — some going so far as to say it is a precondition — for the evolution of large, complex multicellular organisms,” he adds. “But nobody has ever tested this directly, because we haven’t had a model system that is both able to undergo lots of generations of evolution quickly, and able to grow over the full range of oxygen conditions,” from anaerobic conditions up to modern levels.

The researchers were able to do that, however, with snowflake yeast, simple multicellular organisms capable of rapid evolutionary change. By varying their growth environment, they evolved snowflake yeast for over 800 generations in the lab with selection for larger size.

The results surprised Bozdag. “I was astonished to see that multicellular yeast doubled their size very rapidly when they could not use oxygen, while populations that evolved in the moderately oxygenated environment showed no size increase at all,” he says. “This effect is robust — even over much longer timescales.”

Size — and oxygen levels — matter for multicellular growth

In the team’s research, “large size easily evolved either when our yeast had no oxygen or plenty of it, but not when oxygen was present at low levels,” Ratcliff says. “We did a lot more work to show that this is actually a totally predictable and understandable outcome of the fact that oxygen, when limiting, acts as a resource — if cells can access it, they get a big metabolic benefit. When oxygen is scarce, it can’t diffuse very far into organisms, so there is an evolutionary incentive for multicellular organisms to be small — allowing most of their cells access to oxygen — a constraint that is not there when oxygen simply isn’t present, or when there’s enough of it around to diffuse more deeply into tissues.”

Ratcliff says not only does his group’s work challenge the Oxygen Control Hypothesis, it also helps science understand why so little apparent evolutionary innovation was happening in the world of multicellular organisms in the billion years after the Great Oxygenation Event. Ratcliff explains that geologists call this period the “Boring Billion” in Earth’s history — also known as the Dullest Time in Earth’s History, and Earth’s Middle Ages — a period when oxygen was present in the atmosphere, but at low levels, and multicellular organisms stayed relatively small and simple.

Bozdag adds another insight into the unique nature of the study. “Previous work examined the interplay between oxygen and multicellular size mainly through the physical principles of gas diffusion,” he says. “While that reasoning is essential, we also need an inclusive consideration of principles of Darwinian evolution when studying the origin of complex multicellular life on our planet.” Finally being able to advance organisms through many generations of evolution helped the researchers accomplish just that, Bozdag adds.

This work was supported by National Science Foundation grant no. DEB-1845363 to W.C.R, NSF grant no. IOS-1656549 to W.C.R., NSF grant no. IOS-1656849 to E.L., and a Packard Foundation Fellowship for Science and Engineering to W.C.R. C.T.R. and W.C.R. acknowledge funding from the NASA Astrobiology Institute.

Featured image: Artist rendering of early Earth © NASA


Reference: Bozdag, G.O., Libby, E., Pineau, R. et al., “Oxygen suppression of macroscopic multicellularity.” (Nat Commun 12, 2838 2021). https://doi.org/10.1038/s41467-021-23104-0


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Looking For Life? Don’t Trust Oxygen (Planetary Science)

According to a recent study, conducted by a team of researchers using evolutionary computational models, rocky planets found in the habitable zones of Sun-like stars can contain oxygen of non-biological origin in their atmospheres, making its spectral detection a biological signature. not very reliable and certainly not sufficient to affirm the presence of life forms. All the details on Agu advances

The presence of oxygen in the atmosphere of an exoplanet could be an unreliable biological signature : this is what emerges from a new study conducted by teams of researchers led by the University of California at Santa Cruz (Ucsc).

The researchers based their discovery on a thermo-geochemical-climatic computational model that made it possible to study the evolution of the atmosphere of terrestrial planets by varying the initial inventory of the volatile chemical compounds present in it. The results obtained, published yesterday in the journal Agu Advances, show three different scenarios in which a rocky planet around a star similar to the Sun – after 4.5 billion years of evolution, corresponding to the age established for the Earth – can develop atmospheres rich in oxygen of abiotic origin, i.e. coming from non-biological activity, which would make any detection of the molecule’s spectral signature – if interpreted as a biosignature – a false positive.

The research, explains Joshua Krissansen-Totton , researcher at the University of Santa Cruz and first author of the publication, “shows that there are several ways to produce oxygen in the atmosphere without this indicating the presence of life, but there are other observations that they can do to help distinguish these false positives from the true signal ».

In the coming decades, perhaps as early as the late 1930s, astronomers will have telescopes capable of acquiring images and absorption spectra of molecules in potentially Earth-like planets around Sun-like stars. researchers, is to point telescopes towards these worlds where life may have emerged and characterize their atmospheres. But the false positive detections are, in fact, a problem.

“There has been a lot of discussion about considering the detection of ‘sufficient’ oxygen as a sign of life,” notes Jonathan Fortney , also from the University of Santa Cruz and co-author of the study. «This work highlights the need to know the context of the surveys. What molecules are found besides oxygen and what does this tell us about the evolution of the planet? ».

Oxygen can be produced in a planet’s upper atmosphere by high-energy ultraviolet light that splits water molecules into its components: hydrogen and, indeed, oxygen, the researchers say. The lighter hydrogen is lost in space, leaving behind oxygen, which thus accumulates. But there are also processes that can remove oxygen from the atmosphere: carbon monoxide and hydrogen released by outgassing from molten rock, for example, react with oxygen reducing its quantities. And the atmospheric agents that erode the rock also absorb the molecule. These are some of the processes the researchers incorporated into their model to study the evolution of a rocky planet.

The scenarios that these evolutionary models have returned, in which the atmospheres contained oxygen of abiotic origin, are three, we said. A fourth scenario, in which the soup of volatile gases inserted is what is thought to be the early Earth, has never produced oxygen of such an origin.

“If you run the model for the Earth, by inserting what we think is the initial inventory of volatile compounds, you get the same result every time: without life, no oxygen is produced in the atmosphere,” says Krissansen-Totton. “But we also found several scenarios in which it is possible to obtain oxygen without the presence of life.”

The first scenario that the authors describe in the publication is that in which the initial conditions of planetary formation are similar to those of the Earth but with a quantity of carbon dioxide greater than water (CO2 / H2O ratio greater than 1). This results in a constant and high greenhouse effect which, by making the planet very hot, prevents water vapor from condensing on the planet’s surface. The result is the constant atmospheric presence of water in the gaseous state available to the action of UV light which, as mentioned, splits it into hydrogen and oxygen.

“In this scenario, similar to that of Venus, all volatile molecules are in the atmosphere,” says Krissansen-Totton. “Few remain in the mantle to be degassed and prevent the formation of oxygen.”

The second and third scenarios also start from conditions similar to those of the Earth, but while in the first case the input is a quantity of water from 10 to 230 times greater than that present on our planet, in the second it is 0.3 times the quantity of water on Earth.

For water-rich worlds, the researchers explain, deep oceans exert tremendous pressure on the planetary crust. This effectively disrupts geological activity, including some processes such as melting or erosion of rocks that would remove oxygen from the atmosphere.

In the case of desert worlds, however, the surface magma of the initially molten planet can freeze rapidly, while water, albeit little, remains in the atmosphere, allowing, as it is split, the accumulation of oxygen and the escape of hydrogen. .

“The typical sequence is that the solidification of surface magma occurs simultaneously with the condensation of water in the surface oceans,” adds Krissansen-Totton. “On Earth, once the water condensed on the surface, the escape rates were low. But if water vapor still remains in the atmosphere after the molten surface has solidified, there is a window of about a million years in which oxygen can accumulate, because no molten surface can consume the oxygen produced by the leak. of hydrogen “.

The scenarios outlined in this study emphasize that no single observation, including the detection of oxygen on planets in habitable areas around Sun-like stars, can be an unambiguous sign of the presence of life, the researchers conclude. The evaluation of biosignatures should be a systemic approach, in which the biological origin of a molecule is judged by the presence of multiple lines of evidence.

Featured image: Artistic illustration showing the paths through which the atmospheres of planets orbiting stars similar to the Sun can be enriched with oxygen of non-biological origin. Credits: J. Krissansen-Totton.


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Rise of Oxygen on Earth: Initial Estimates Off by 100 Million Years (Earth Science)

Permanent oxygenation occurred much later than previously thought

New research shows the permanent rise of oxygen in our atmosphere, which set the stage for life as we know it, happened 100 million years later than previously thought.

A significant rise in oxygen occurred about 2.43 billion years ago, marking the start of the Great Oxidation Episode — a pivotal moment in Earth’s history. 

An international research team including a UC Riverside scientist analyzed rocks from South Africa formed during this event. Findings, published this week in the journal Nature, include the discovery that oxygen fluctuated dramatically after its early appearance until it became a permanent constituent of the atmosphere much later.

These fluctuations reinforce a direct link between atmospheric oxygen and concentrations of greenhouse gases such as methane, helping to explain some of the most extreme climate changes in the planet’s past.

During the same period, ancient Earth experienced four glaciations — periods when the whole planet was covered with ice and snow for millions of years. According to UC Riverside geologist Andrey Bekker, changes in atmospheric oxygen levels began and ended these events.

Research team leader sprays water on drill cores to see sedimentary rocks and select samples for this study. © Andrey Bekker/UCR

Scientists have often wondered how the planet could have emerged from the periods in which ice and snow covered everything, including the oceans. According to Bekker, increases in atmospheric oxygen levels resulted in low concentrations of greenhouse gases, such as methane and carbon dioxide. This ushered in global glaciations by maintaining surface conditions below the water-freezing temperature. 

Volcanoes also continued to erupt on the frozen planet, building required high levels of carbon dioxide in the atmosphere to exit from climatic catastrophe by warming the planet and melting the snow and ice.

“Before this work, we all wondered why the fourth glacial event happened if oxygen was already a steady component in the atmosphere,” Becker said. “We found it was not steady. The permanent rise of oxygen actually occurred after the fourth, final glaciation in the Paleoproterozoc Era, and not before it, and this solves what had previously been a major puzzle in our understanding.” 

The Great Oxidation Episode ushered in a 1.5 billion-year period of subsequent environmental stability, which lasted until a second major transitional period, marked by rising atmospheric oxygen and similar climatic changes at the end of the Precambrian time. 

“We thought once oxygen increased it wouldn’t ever return back to lower levels,” Bekker said. “Now we have learned it fluctuated to very low levels and this could have dramatic implications in terms of understanding extinction events and the evolution of life.”

Open questions include the reasons for these multiple fluctuations, and whether complex life could have evolved and then died out again in response to them, said Simon Poulton, a biogeochemist at Leeds University who led the research. 

“We cannot begin to understand the causes and consequences of atmospheric oxygenation, the most significant control on Earth’s habitability, if we do not know when permanent atmospheric oxygenation actually occurred,” he said. “Now at last we have that piece of the puzzle.”

Featured image: Changes in atmospheric oxygen levels began and ended four glaciations — periods when the whole planet was covered with ice and snow for millions of years. © Wikimedia


Reference: Poulton, S.W., Bekker, A., Cumming, V.M. et al. A 200-million-year delay in permanent atmospheric oxygenation. Nature (2021). https://www.nature.com/articles/s41586-021-03393-7 https://doi.org/10.1038/s41586-021-03393-7


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How Long Earth Oxygen Will Sustain? (Planetary Science)

Ozaki and Reinhard in their recent paper, examined the timescale of oxygen-rich atmosphere on Earth by using a combined biogeochemistry and climate model. They found that Earth will lose its oxygen-rich atmosphere in 1. 08 ± 0.14 billion years (which is appx. 1 billion years). Their paper recently appeared in Journal Nature Geoscience.

They showed that future deoxygenation is an inevitable consequence of increasing solar fluxes, whereas its precise timing is modulated by the exchange flux of reducing power between the mantle and the ocean-atmosphere-crust system.

Fig. 1. Schematic model structure. Boxes denote reservoirs, whereas arrows denote flux terms. The model tracks the major reservoirs and transfer fluxes within the surface carbon (C), sulphur (S), oxygen (O), and phosphorus (P) cycles, along with a comprehensive treatment of ocean biogeochemistry, and long-term transfers between the crust-ocean-atmosphere system and the mantle. DOA = degree of anoxia.

That is, around 1 billion years from now on, sun will grew hotter, releasing more energy, carbon dioxide levels in Earth’s atmosphere will begin to drop due to the gas absorbing the heat and breaking down. The ozone layer would also be burned away. Then, as carbon dioxide levels fall, plant life will begin to suffer, resulting in reduced production of oxygen.

Over a period of just 10,000, years, CO2 levels will drop so much that plant life would go extinct. Without plant life, land- and sea-dwelling creatures would soon go extinct, as well, due to the lack of a breathable atmosphere. Meanwhile, the model results also showed increasing levels of methane entering the atmosphere, speeding the demise of creatures needing oxygen to breathe. The result, according to the model, would be a planet without life, save for tiny anaerobic creatures such as bacteria—conditions very similar to Earth prior to the evolution of plants and animals.

But, it is also important to note that there are multiple biogeochemical and climate processes that are not considered in their model that may play a role in constraining the future lifespan of Earth’s biosphere and the timing/mode of transition to more reducing atmospheric conditions. In particular, “reverse weathering” (the formation of authigenic silicates in marine sediments, resulting in net CO2 release to the ocean-atmosphere system) could potentially extend the lifespan of oxygenated atmospheric conditions under certain scenarios by prolonging the timescale over which atmospheric CO2 is above the levels expected to result in CO2 limitation of the photosynthetic biosphere.

In addition, they also hypothesized that haze-induced climate cooling could potentially act as a brake on the overall magnitude of atmospheric deoxygenation, or result in the inception of oxygenation/deoxygenation cycles during Earth’s terminal habitability.

Their results have important implications for the search for life on Earth-like planets beyond our solar system (e.g., habitable planets with abundant liquid water at the surface, exposed silicate crust, and a biosphere with oxygenic photosynthesis). According to authors, there is a need for robust atmospheric biosignatures applicable to weakly oxygenated and anoxic exoplanet atmospheres.


Reference: Ozaki, K., Reinhard, C.T. The future lifespan of Earth’s oxygenated atmosphere. Nat. Geosci. (2021). https://www.nature.com/articles/s41561-021-00693-5 https://doi.org/10.1038/s41561-021-00693-5


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Electric Cable Bacteria Breathe Oxygen With Unheard Efficiency (Biology)

Researchers document that a few cells operate with extremely high oxygen consumption while the rest of the cells process food and grow without oxygen; an outstanding way of life

Ten years ago, researchers at Aarhus University, Denmark, reported the discovery of centimeter-long cable bacteria, that live by conducting an electric current from one end to the other. Now the researchers document that a few cells operate with extremely high oxygen consumption while the rest of the cells process food and grow without oxygen. An outstanding way of life.

We humans need food and oxygen to live.

Now, imagine if oxygen was to be found only at the mountain top and food only in the valley. That’s how the world looks like for cable bacteria, which live in the bottom of seas and lakes. For them, oxygen is available only at the very surface of the bottom, whereas the food is buried centimeters down.

Biowires

“While other organisms try to solve the problem by moving oxygen and food up and down, cable bacteria have developed electric wires. When consuming food they produce electrons and send them through the ‘biowires’ to the surface for reduction of oxygen from the overlying water,” says Lars Peter Nielsen, head of Center for Electromicrobiology, Aarhus University, Denmark.

Microscope image of cable bacteria reaching one end out for oxygen. The deformed oxygen front is seen as a milky line consisting of smaller bacteria attracted to the interface with the lower oxygen free layer. © Photo: Stefano Scilipoti

A cable bacterium consists of many cells in line. It can grow centimeters long, the cells encased in a common coat wherein the wires stretch.

The researchers placed cable bacteria in a little, transparent chamber. In the middle, the bacteria had access to oxygen-free mud stuffed with food, while oxygen diffused in from the edges. Right where the intruding oxygen was depleted, numerous unicellular bacteria formed a distinct front. In that specific position they fought to capture food and oxygen from either side simultaneously.

“In the microscop, I watched how single cable bacteria placed themselves across the front with one end into the zone with oxygen,” explains Stefano Scilipoti, PhD-student at Center for Electromicrobiology, Aarhus University and the primary discoverer.

Less than 10% of the cells are “breathing”

Stefano Scilipoti watched how one single cable bacterium could distort the front made by unicellular, swimming bacteria. The cable bacterium respired so much oxygen that the unicellular bacteria had to move closer to the edge of the chamber to sustain the oxygen supply needed for their respiration. The cable bacterium could just dip a few cells in oxygen, and the magnitude of the distortion – in laboratory jargon called ‘bump’ – allowed to calculate how much oxygen was being consumed.

“Less than 10% of the cells of the cable bacterium consumed oxygen but they did it with a rate matching the highest rates known in biology. That only works because the cable bacterium runs an electric current between the cells being in contact with oxygen and the cells processing the food. The cells consuming oxygen can thus focus on this task only, while the other cells digest food and generate new cells,” says Stefano Scilipoti.

The cable bacterial machinery

The ancestors of cable bacteria lived without any oxygen. Anaerobic bacteria, as you call them. For these bacteria, oxygen is toxic and a prolonged exposure eventually lead them to death. With the evolution of electric connection to oxygen however, cable bacteria can explore the strength of breathing with oxygen without exposing many cells to oxygen stress, thus getting the best of oxygen (more energy) and avoiding the rest (damage to the cells).

At Center for Electromicrobiology, the chase to unravel the special mechanisms that enable this unique electric form of life continues.

Featured image: In the laboratory, cable bacteria were placed in a little, transparent chamber. In the middle, the bacteria had access to oxygen-free mud stuffed with food, while oxygen diffused in from the edges. © Photo: Maria Blach Nielsen


Reference: Stefano Scilipoti, Klaus Koren, Nils Risgaard-Petersen, Andreas Schramm and Lars Peter Nielsen, “Oxygen consumption of individual cable bacteria”, Science Advances 10 Feb 2021:
Vol. 7, no. 7, eabe1870 DOI: 10.1126/sciadv.abe1870 https://advances.sciencemag.org/content/7/7/eabe1870


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Using Earth’s History to Inform The Search for Life On Exoplanets (Planetary Science)

UC Riverside is leading one of the NASA Astrobiology Program’s eight new research teams tackling questions about the evolution and origins of life on Earth and the possibility of life beyond our solar system.

An Earth-like “exomoon” orbiting a gas giant planet in a star’s habitable zone. Credit: NASA/JPL-Caltech

The teams comprise the inaugural class of NASA’s Interdisciplinary Consortia for Astrobiology Research program. The UCR-led team is motivated by the fundamental question of how to detect planets that could host life and remain habitable despite tremendous change over time, which requires hunting for biological gases in the atmospheres of planets light years beyond our solar system.

“To achieve this goal, our research focuses on the many diverse chapters of Earth’s history—or alterative Earths—that span billions of years and offer critical templates for examining exoplanets far beyond our solar system,” said UCR biogeochemist Timothy Lyons, the project leader.

Because of their immense distance from us, humans will likely never visit those planets, at least not soon, Lyons said. However, in the near future, scientists will be able to analyze the compositions of these planets’ atmospheres, looking for gases like oxygen and methane that could come from life.

Earth has undergone dramatic changes over the last 4.5 billion years, with major transitions occurring in plate tectonics, climate, ocean chemistry, the structure of our ecosystems, and composition of our atmosphere.

“These changes represent an opportunity,” Lyons said. “The different periods of Earth’s evolutionary history provide glimpses of many, largely alien worlds, some of which may be analogs for habitable planetary states that are very different from conditions on modern Earth.”

Exciting new research frontiers for Lyons’ team include studies of Earth’s first 500 million years, as well as predictions about our planet and its life billions of years in the future.

Studying biosignature gases in Earth’s past will allow the team to design telescopes and refine interpretative models for potential traces of life in distant exoplanet atmospheres, noted Georgia Tech biogeochemist Christopher Reinhard.

Once the researchers understand how Earth and its star—the sun—changed together to maintain liquid oceans teeming with life over billions of years, the team can predict how other planetary systems might also have developed and maintained life and better understand how to search for it.

“Such a ‘mission to early Earth’ must include broad interdisciplinarity within the team, impactful synergy within and across the Research Coordination Networks, or RCNs, of the NASA Astrobiology Program, and a commitment to deliverables that will help steer NASA science for decades to come,” said UCR astrobiologist Edward Schwieterman.

Success in this mission will require biological, chemical, geological, oceanographic, and astronomical expertise. Yale University biogeochemist Noah Planavsky said, “our team brings all that to the table.” Accordingly, the diverse expertise within the team includes astronomers, planetary scientists, geologists, geophysicists, oceanographers, biogeochemists, and geobiologists.

The team will collect ancient rock samples and modern sediments from around the world spanning billions of years and use the data they generate to drive wide-ranging computational models for Earth’s ancient and future oceans and atmospheres.

“The models will allow the team to evaluate whether different periods in Earth’s history were characterized by gases that would have been detectable from a distant vantage as products of life, much the way oxygen fingerprints life on our planet today,” said Purdue University Earth and exoplanetary scientist Stephanie Olson.

This work requires a multipronged view of the Earth as a complex system that has varied dramatically over time. Yet despite all the change, Earth has remained persistently habitable, with liquid water oceans teeming with life.

How Earth became and remained habitable and whether its life would have been detectable to a distant observer are the questions that will ultimately define and refine the search for life on exoplanets.

“In short,” said Lyons, “the exciting goal of our team is to provide a new and more holistic view of Earth’s evolutionary history in order to help guide NASA’s mission-specific search for life on distant worlds.”

The RCNs are the new face of astrobiology at NASA, following 20 years of exciting research under the umbrella of the NASA Astrobiology Institute, which supported the UCR-led team previously.

The $4.6 million new award from NASA will span five years and includes team members from Georgia Tech, Yale University, Purdue University, UCLA, NASA Ames Research Center and collaborators from around the world.

Provided by University of California – Riverside

Research Creates Hydrogen-producing Living Droplets, Paving Way For Alternative Future Energy Source (Chemistry)

Scientists have built tiny droplet-based microbial factories that produce hydrogen, instead of oxygen, when exposed to daylight in air.

Electron microscopy image of a densely packed droplet of hydrogen-producing algal cells. Scale bar, 10 micrometers. ©Prof Xin Huang, Harbin Institute of Technology

The findings of the international research team based at the University of Bristol and Harbin Institute of Technology in China, are published today in Nature Communications.

Normally, algal cells fix carbon dioxide and produce oxygen by photosynthesis. The study used sugary droplets packed with living algal cells to generate hydrogen, rather than oxygen, by photosynthesis.

Hydrogen is potentially a climate-neutral fuel, offering many possible uses as a future energy source. A major drawback is that making hydrogen involves using a lot of energy, so green alternatives are being sought and this discovery could provide an important step forward.

The team, comprising Professor Stephen Mann and Dr Mei Li from Bristol’s School of Chemistry together with Professor Xin Huang and colleagues at Harbin Institute of Technology in China, trapped ten thousand or so algal cells in each droplet, which were then crammed together by osmotic compression. By burying the cells deep inside the droplets, oxygen levels fell to a level that switched on special enzymes called hydrogenases that hijacked the normal photosynthetic pathway to produce hydrogen. In this way, around a quarter of a million microbial factories, typically only one-tenth of a millimetre in size, could be prepared in one millilitre of water.

To increase the level of hydrogen evolution, the team coated the living micro-reactors with a thin shell of bacteria, which were able to scavenge for oxygen and therefore increase the number of algal cells geared up for hydrogenase activity.

Although still at an early stage, the work provides a step towards photobiological green energy development under natural aerobic conditions.

Professor Stephen Mann, Co-Director of the Max Planck Bristol Centre for Minimal Biology at Bristol, said: “Using simple droplets as vectors for controlling algal cell organization and photosynthesis in synthetic micro-spaces offers a potentially environmentally benign approach to hydrogen production that we hope to develop in future work.”

Professor Xin Huang at Harbin Institute of Technology added: “Our methodology is facile and should be capable of scale-up without impairing the viability of the living cells. It also seems flexible; for example, we recently captured large numbers of yeast cells in the droplets and used the microbial reactors for ethanol production.”

References: Xu, Z., Wang, S., Zhao, C. et al. Photosynthetic hydrogen production by droplet-based microbial micro-reactors under aerobic conditions. Nat Commun 11, 5985 (2020). https://www.nature.com/articles/s41467-020-19823-5 https://doi.org/10.1038/s41467-020-19823-5

Provided by University of Bristol

Do You Know About The Eternal Young White Dwarfs? (Planetary Science)

White dwarfs are the most common fossil stars within the stellar graveyard. It is well known that more than 95% of all main-sequence stars will finish their lives as white dwarfs, earth-sized objects less massive than ~1.4 Mo—the Chandrasekhar limiting mass— supported by electron degeneracy. A remarkable property of the white-dwarf population is its mass distribution, which exhibits a main peak at ~0.6 Mo, a smaller peak at the tail of the distribution around ~0.82 Mo, and a low-mass excess near ~0.4 Mo. White dwarfs with masses lower than 1.05 Mo are expected to harbour carbon(C)-oxygen(O) cores, enveloped by a shell of helium which is surrounded by a layer of hydrogen. Traditionally, white dwarfs more massive than 1.05 Mo (ultra-massive white dwarfs) are thought to contain an oxygen-neon(Ne) core, and their formation is theoretically predicted as the end product of the isolated evolution of intermediate-mass stars with an initial mass larger than 6-9 Mo, depending on the metallicity and the treatment of convective boundaries during core hydrogen burning. Once the helium in the core has been exhausted, these stars evolve to the super asymptotic giant branch (SAGB) phase, where they reach temperatures high enough to start off-centre carbon ignition under partially degenerate conditions. A violent carbon ignition eventually leads to the formation of an ONe core, which is not hot enough to develop further nuclear burning. During the SAGB phase, the star loses most of its outer envelope by the action of stellar winds, ultimately becoming an ultra-massive ONe-core white dwarf star.

Artist impression of white dwarf ©gettyimages

On the other hand, the existence of a fraction of ultra-massive white dwarfs harbouring CO cores is supported by different piece of evidence. This population could be formed through binary evolution channels, involving the merger of two white dwarfs. Assuming that C is not ignited in the merger event, the merger of two CO-core white dwarfs with a combined mass below the Chandrasekhar limit is expected to lead to the formation of a single CO-core white dwarf substantially more massive than any CO-core white dwarf that can form from a single evolution ( 1.05 M ∼1.05 Mo). To the date, it has not been possible to distinguish a CO-core from a ONe-core ultra-massive white dwarf from their observed properties, although a promissory avenue to accomplish this is by means of white dwarf asteroseismology. Recent studies reveal that 10% -30% of all white dwarfs are expected to be formed as a result of merger events of any kind, and that this percentage raises up to 50 % for massive white dwarfs (M>0.9Mo). In particular, double white dwarf mergers contribute to the formation of massive white dwarfs in 20-30%. These results are in line with the existence of an excess of massive white dwarfs in the mass distribution. However, the existence of ultra-massive CO white dwarfs remains to be proven, and their exact percentage is still unclear.

The formation of white dwarfs as a result of stellar mergers is particularly important given the persistent historical interest in the study of the channels that lead to the occurrence of type Ia Supernovae, which are thought to be the violent explosion of a white dwarf exceeding the Chandrasekhar limiting mass. The main pathways to type Ia Supernovae involve binary evolution, namely the single-degenerate channel in which a white dwarf gains mass from a non-degenerate companion, or the double-degenerate channel involving the merger of two white dwarfs. Moreover, ultra-massive white dwarfs resulting from merger episodes are of utmost importance in connection with the formation of rapidly spinning neutron stars/magnetars. Mergers of white dwarfs have also been invoked as the most likely mechanism for the formation of Fast Radio Bursts, which are transient intense radio pulses with duration of milliseconds. Since they have been localized at redshifts z > 0.3, it is thought that Fast Radio Bursts could replace Supernovae of type Ia to probe the expansion of the universe to higher redshifts.

Recent observations provided by Gaia space mission, indicate that a fraction of the ultra-massive white dwarfs experience a strong delay in their cooling, which cannot be attributed only to the occurrence of crystallization, thus requiring an unknown energy source able to prolong their life for long periods of time.

In this study, Maria Camisassa and colleagues showed that these strong delays in the cooling times reported for a selected population of the ultra-massive white dwarfs are caused by the energy released by the sedimentation process of ²²Ne occurring in the interior of CO-core ultra-massive white dwarfs with high ²²Ne content, providing strong sustain to the formation of CO-core ultra-massive white dwarfs through merger events.

In order to demonstrate the possible existence of these eternal youth ultra-massive CO-core white dwarfs, they have analyzed the effect of ²²Ne sedimentation on the local white dwarf population revealed by Gaia observations by means of an up-to date population synthesis code. The code, based on Monte Carlo techniques, incorporates the different theoretical white dwarf cooling sequences under study, as well as an accurate modeling of the local white dwarf population and observational biases. They have performed a population synthesis analysis of the Galactic thin disk white dwarf population within 100 pc from the Sun under different input models. In order to minimize the selection effects, they chose the 100 pc sample, given that it represents the maximum size that the sample can be considered volume-limited and thus practically complete sample.

First, they considered that all the ultra-massive white dwarfs in the simulated sample have ONe core composition. This first synthetic population is shown in the Gaia HR diagram in the upper left panel of Figure 1. The histogram of this synthetic population is shown in the upper right panel (black steps), together with the Gaia 100 pc white dwarf sample (red steps). The Q branch can easily be regarded as the main peak in the histogram of the Gaia 100 pc white dwarf sample, between 13.0 and 13.4. A first glance at these three histograms reveals that ultra-massive ONe white dwarfs fail to account for the pile-up in the Q branch, even though the ONe white dwarf sequences used include all the energy sources resulting from the crystallization process. A quantitative statistical reduced x²-test analysis of the synthetic population distribution in the Q branch reveals a value of 11.18 when compared to the observed distribution.

Figure 1: Left panels: Synthetic white dwarf populations (gray points) in the Gaia Hertzsprung-Russell diagram considering different prescriptions. The ultra-massive Q branch is delimited by solid red lines. Dashed red lines mark the region where they have counted white dwarfs to prepare the histograms. Right panels: synthetic (black line) and observed (red line) histograms for the 100pc white dwarf population. The observed Q branch can be regarded as the red peak between 13.0 and 13.4 in the 100 pc white dwarf sample observed by Gaia. Note that, in order to theoretically reproduce the peak in the observed population, it is necessary to assume a high merger fraction leading to ultra massive white dwarfs with CO core composition and a high ²²Ne abundance. ©Maria Camisassa et al.

The middle left panel of Figure 1 illustrates the HR diagram of a typical synthetic white dwarf population realization, considering that 20% of the ultra-massive white dwarfs come from merger events. That is, 20% of the ultra-massive white dwarfs harbour a CO-core. In this model they also have assumed white dwarfs with high ²²Ne abundance, X²²Ne=0.06. The histogram of this synthetic population, shown in black steps in the middle right panel, reveals that, although a mixed white dwarf population with both CO-core and ONe-core white dwarfs is in better agreement with the observations, the pile-up is still not fully reproduced. The reduced x² value of this synthetic population is 2.60.

Finally, they have also performed a population synthesis realization in which 50% of the ultra-massive white dwarfs have a merger origin and their core-chemical composition is CO. As in the previous model, the ²²Ne content of the white dwarf sequences with merger origin was set to X²²Ne=0.06. The results of this synthetic population are shown in the lower panels of Figure 1. They found that this simulation is in perfect agreement with the observed white dwarf sample, being its reduced x²-test value 1.36.

The better agreement with the observations revealed by Gaia of the synthetic populations that include CO-core ultra-massive white dwarf sequences is in line with the longer cooling times that characterize these stars due to ²²Ne sedimentation process. They have also simulated a synthetic population that considers a fraction of merger of 50% and a ²²Ne abundance of 0.02, finding a better agreement when compared to simulations computed with only ONe-core white dwarfs, but not as good as the agreement they found for a population with high ²²Ne content (The reduced x² value of this synthetic population is 4.86). They have also generated synthetic populations considering different ²²Ne abundances in the white dwarf models and found that the best fit models are obtained for a high ²²Ne abundance (X²²Ne=0.06), as the one shown in the middle and lower panels of Figure 1. Such a high ²²Ne abundance is not consistent with the isolated standard evolutionary history channel, because it would imply that these white dwarfs come from high-metallicity progenitors. However, merger events provide a possible scenario to create such a high ²²Ne abundance. If H were burnt in C-rich layers during the merger event, it would create a high amount of ¹⁴N that could later capture He ions, creating a high ²²Ne abundance before the ultra-massive white dwarf is born.

The analysis of the ultra-massive white dwarf population revealed by Gaia shows that ONe-core white dwarfs alone are not able to account for the pile-up in the ultra-massive Q branch. Indeed, energy sources as latent heat and phase separation process due to crystallization, and ²²Ne sedimentation can not prevent the fast cooling of these stars. Their study finds that CO-core ultra-massive white dwarfs with high ²²Ne content are long-standing living objects, that should stay on the Q branch for long periods of time. Indeed, their CO core composition, combined with a high ²²Ne abundance, provides a favorable scenario for ²²Ne sedimentation to effectively operate, producing strong delays in the cooling times due to the combination of three effects: crystallization, ²²Ne sedimentation and higher thermal content, and leading to an eternal youth source.

Their study indicates that the observed evidence of these delays from Gaia provides valuable sustain on their CO chemical composition, and their past history involving merger events, whilst ONe core white dwarfs are unable to predict these delays. Moreover, the high percentage of observed carbon-rich atmosphere stars (DQ white dwarfs) on the Q branch 20 supports the hypothesis that a large fraction of the white dwarfs on the Q branch would certainly have been formed through merger events.

References: María E. Camisassa, Leandro G. Althaus, Santiago Torres, Alejandro H. Córsico, Sihao Cheng, Alberto Rebassa-Mansergas, “Forever young white dwarfs: when stellar ageing stops”, ArXiv, pp. 1-27, 2020. https://arxiv.org/abs/2008.03028

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