If higher levels of atmospheric oxygen can lead to larger animals, can it also lead to larger plants?

If higher levels of atmospheric oxygen can lead to larger animals, can it also lead to larger plants?

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According to some information, in the ancient past (at least some) animals grew larger due to a higher levels of oxygen in the atmosphere. So for example there is this study regarding dragonflies.

Can higher levels of atmospheric oxygen also lead to larger plants? For example, would it be possible to grow gigantic strawberries in a hermetically sealed greenhouse that maintains a higher atmospheric oxygen concentration?

Oxygen is good for animals because our basic metabolism is this:

High energy carbon molecules + O2 → energy + H2O + CO2

Plants do that too at night, but during the day, they mostly do this:

High energy photons + H2O + CO2 → High energy carbon molecules + O2

Rubisco, one of the most important enzymes in photosynthesis, can bind to O2, leading to less efficient photorespiration, instead of photosyntheis. Rubisco is often a bottleneck in photosynthesis, in large part for that reason. So high oxygen isn't helpful to plants. In fact, C4 plants and CAM plants have evolved more complicated physiology to minimize the effect of O2.

The already given answers are correct, but I think explaining basic metabolism does not answer the problem of how big organisms can grow.

The argument about metazoan body size and the growing athmospheric O2 levels is that this latter process makes the diffusion of oxygen through animal tissue faster (because of the growing partial pressure of O2). This, in turn, made it possible for metazoans, which at the time had no elaborate body structures and no developed respiratory apparatuses, to actually grow larger, since it was possible for cells further away from the surface of an organism to respirate as well. Or, to put it differently, cell masses could grow bigger without risking anoxy in deeper parts of the tissue.

The problem with plants is not a very easy one to tackle, and I don't think there's a clear answer to this based on current knowledge. One must experiment with this and see what happens.

My guess is that the response of a plant lineage to grown O2 levels would be very dependent on the exact habitat the plant lives in, since for an evolutionary change to happen fast, there must be a clear evolutionary-ecological adaptive value it provides, and that is very dependent on environmental givens. Also, in recent (current) plants, respiration is a solved problem, so I'm not sure a plant would benefit in any way from elevated O2 levels. It would surely affect the working of RuBisCO as mentioned. Maybe an immediate response would be the spread of C4 metabolism within plants.

Dealing with Dead Zones: Hypoxia in the Ocean

At 2,116 square miles , the 2020 hypoxic zone in the Gulf of Mexico is the 3rd smallest ever measured in the 34-year record, measured from July 25 to August 1. Red area denotes 2 milligrams per liter of oxygen or lower, the level which is considered hypoxic, at the bottom of the seafloor. (Bottom panel) Long-term measured size of the hypoxic zone (green bars) measured during the ship surveys since 1985, including the target goal established by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force and the 5-year average measured size (black dashed lines). In 2020, Hurricane Hanna passed through the central and western Gulf days prior to the research cruise and mixed the water column, disrupting the hypoxic zone which forms in the coastal ocean west of the Mississippi River delta. While the size of the hypoxic zone fluctuates naturally throughout the summer, it usually forms again within days or weeks after the passage of storms. Due to the close proximity of the storm to the survey cruise, the hypoxia area was only able to partially reform before the end of the monitoring cruise, resulting in a patchy distribution across the Gulf. Graphic credit: Louisiana Universities Marine Consortium


HOST: This is the NOAA Ocean podcast. I’m Troy Kitch.

In 2010, scientists discovered multicellular animals that don’t require oxygen to survive buried deep in the sediment at the bottom of the Mediterranean. Aside from some types of very simple bacteria and single-celled organisms, these are the only other known lifeforms on our planet that can survive in a zero oxygen environment.

As with life on land, practically all ocean life is dependent on oxygen to survive. It’s the key ingredient that makes life in the ocean work. The diversity and productivity of ocean life, and the complex biochemical cycles that keep ocean life in balance all depend on oxygen. Now here’s the problem. The ocean isn’t getting enough of it. And this lack of oxygen is leading to a chronic condition called hypoxia. Areas in the ocean that experience these hypoxic conditions over long periods of time are often referred to as “dead zones,” for reasons that will become very clear later in this episode.

So what’s causing this problem? Why and how is it getting worse? And can we do anything about it?

We’re joined in this episode by NOAA scientist Alan Lewitus to get some answers.

Alan directs the Competitive Research Program for the National Centers for Coastal Ocean Science, part of the National Ocean Service. His job is to oversee NOAA grants awarded to researchers around the nation who study topics like hypoxia — research that targets improving the health of our coastal ecosystems.

ALAN LEWITUS: Hypoxia refers to water conditions where the concentration of oxygen is so low that it is detrimental to organisms and very few organisms can survive in those conditions. Scientists refer to hypoxic waters as those waters where oxygen concentrations are below two milligrams per liter. Now, organisms that can swim away from those conditions do, they flee, and so they avoid hypoxic waters. But not always. Sometimes they’re trapped in bayments and other areas, so you see many cases where hypoxia events are associated with large-scale fish kills. In larger systems, they can flee, but you have other problems. Hypoxia can affect the habitat of fish. There’s a loss of bottom fauna, which are important food sources. Other organisms that can’t move such as shellfish and worms and so forth, are trapped and often suffocate and die.

HOST: As an example of how hypoxia can affect habitats, Alan pointed to the brown shrimp, a huge commercial fishery in the Gulf of Mexico. The area where hypoxia occurs today in the Gulf used to be the prime place for fisherman to harvest these shrimp.

ALAN LEWITUS: The habitat of the brown shrimp, the optimal habitat, was reduced by 25 percent. So you’re taking away 25 percent of the habitat. There’s other things, too. Hypoxia, by affecting the bottom fauna, you’re taking away a food source for fish and crustaceans and other things and that has ripple effects through the food chain. It’s a cascading effect.

HOST: And, he added, there are also sub-lethal effects on fish, which are becoming better understood as research progresses.

ALAN LEWITUS: Sub-lethal effects mean that the fish don’t need to be affected by death, they can be affected by exposure to hypoxia, which has certain physiological effects on fish function. A couple of common ones that are being found with Gulf of Mexico studies are that the reproductive potential and growth potential of certain fish, especially bottom-dwelling fish can be affected by even intermittent exposure to hypoxia. You could imagine, if these are bottom-dwelling fish that they’re probably going to hang around the edges of the hypoxic zone, but they still maintain some exposure through foraging activities as well as escape activities from predators. So they are exposed, in and out, in the hypoxic zone. And just this intermittent exposure can lead to serious reproductive impairments, changes in sex, and other bizarre things.

HOST: He added that scientists are now working on models that forecast how these cumulative sub-lethal effects from fish exposed off and on to hypoxia from year to year may be leading to long-term reductions in populations.

ALAN LEWITUS: It’s complicated because the more common effect of hypoxia on fish is not through fish kills. It more commonly affects them through these sub-lethal effects and indirect effects. Effects that sort of cascade through the food chain, as well as sub-lethal effects of hypoxia exposure on reproductive impairments and reductions in growth potential. These are hard to get at. You need sophisticated models to try to separate those adverse effects on fish health from other factors that sort of interact at the same time with hypoxia. But we’re making some headway with those models.

HOST: Talking about this complexity brought us back to the brown shrimp in the Gulf of Mexico and a study that came out in 2017.

ALAN LEWITUS: It found that, in years where hypoxia is large, there was an effect on the size of shrimp that were sold at market. There was an increase in the proportion of smaller size shrimp that were sold. It may be that these growth impairments of hypoxia on shrimp are at play and are causing a reduction in the growth rate. Another factor is that when hypoxia forms, fish and shrimp aggregate around the edges. They want to avoid the hypoxia, but they tend to stay around the edges and there’s a lot of reasons: there’s an accumulation of food sources there. But the fishermen know this, so they know where to go when hypoxia forms. They go around the edges and they can target the fish and shrimp in that way. So the other factor is that they think that some of the small shrimp are fished out and less make it to larger sizes. The bottom line is that when there are large hypoxia years, there’s an adverse effect on the economic profits of local fishermen. And brown shrimp is the largest commercial market in the Gulf of Mexico, so that’s a significant finding.

HOST: Alan said that hypoxic conditions can occur naturally under certain conditions. Records indicate that past events — say, earlier than 1970 — were episodic and generally small. But today, regions of the ocean experiencing hypoxia can be massive. Take the Gulf of Mexico, where scientists funded by NOAA map the size of a “dead zone” that appears every year. In 2017, it was measured at 8,776 square miles, about the size of New Jersey. It was the largest ever recorded.

Why are dead zones larger today and what’s causing this? It’s all about human activity. The culprit is runoff of polluted water that’s carrying tons of excess nutrients from agriculture and developed land from our interior waterways out to the ocean. But nutrients are good things, right?

ALAN LEWITUS: Nutrients are an essential element for plants and algae. So nitrogen and phosphorus are examples of nutrients that are needed by plants. And so they are a good thing from a standpoint that you have to have them to grow crops, for instance. But the problem is when they’re supplied in excess. They can become a bad thing. If you over-fertilize a field, the crops can’t take up all that fertilizer, so a lot of it leaks into water systems.

HOST: And these water systems carrying all this extra fertilizer ultimately flow to the ocean. For the Mississippi, this watershed is the third largest in the world and includes about 40 percent of the continental United States. Too much of the fertilized water we put on crops in the breadbasket of the U.S. eventually ends up in the Gulf.

ALAN LEWITUS: You have an immense amount of fertilizer application for the corn crops and so forth. A lot of it is leaked. Corn is actually a very inefficient plant in terms of using fertilizer, so a lot of it leaks out if not applied in a strategic way, and the nutrients are carried down the river into the Gulf of Mexico, where they stimulate algal blooms. Algae depend on nutrients and it’s good from the standpoint of providing the base of the food chain in aquatic systems, but when you have excess nutrients, you have excess algal growth. They can form blooms.

HOST: So these nutrients that were intended to be used by crops on land wash away to the sea, where they can lead to an explosive blooms of algae. It’s a process called nutrient loading. I asked Alan to connect the dots on how these blooms can lead to hypoxia.

ALAN LEWITUS: What happens is nutrients lead to excessive algal growth, which leads to algal blooms. And the algal blooms, at some point, start degrading and sinking to the bottom, and bacteria work on these algae — they decompose the algae. And as they do that, they consume the oxygen from the water. So that leads to low oxygen water, or hypoxia.

HOST: So as the algae bloom dies off and sinks to the bottom, bacteria eat them up, consuming oxygen. What’s left behind is a low-oxygen dead zone on and near the seafloor. Now you might wonder why these conditions persist. After all, the ocean is always sloshing around and mixing, right? Alan said that’s because water layers of different temperature, salinity, and density don’t like to mix. So the fresher water coming in from, say, the Mississippi River, doesn’t mix well with the hypoxic water on the bottom. Alan said this layering of the water is called stratification.

ALAN LEWITUS: Stratification often occurs when fresh water is loaded into a system which creates a barrier for mixing, so the fresh water sits on top of the more saline water. So bottom waters are restricted from mixing from high-oxygenated surface waters. That combination of high stratification and high nutrient loading are the factors that, in combination, can lead to your most problematic hypoxic zones. The ones that are very large-scale as well as long lived, for a long period of time.

HOST: For the Gulf, dead zones start to form in the spring, because that’s when crops are getting fertilized heavily. Hypoxic conditions persist and peak sometime in the summer, because conditions are right to keep the water layers from mixing. Then the dead zone dissipates in the fall and winter when the flow of nutrients slow down and temperature and other conditions are more favorable for the water in the Gulf to more readily mix together. Then it starts all over again during the next spring.

But nutrients aren’t the only factor contributing to less oxygen in the ocean. There’s another big variable that complicates the hypoxia problem: climate change. I asked Alan how global warming factors in.

ALAN LEWITUS: There is a link between climate change and hypoxia and it all goes in the wrong direction (laughs). The factors that we think about in terms of climate change and the models are telling us that they’re all sort of leaning towards promoting more hypoxia in the future, if we don’t do anything about it. In the open ocean, you have global warming, which is causing a greater and greater rate of de-oxygenation of open ocean waters.

HOST: Alan said this is mainly due to three factors: oxygen is less soluble with higher temperatures so less of it dissolves into the ocean marine life consumes more oxygen because higher temperatures contribute to higher metabolic rates and higher temperatures lead to more stratification, meaning the more oxygenated surface water doesn’t mix well with more hypoxic bottom waters.

ALAN LEWITUS: So those are all working in the direction of reducing oxygen in the open ocean. In the coastal areas, those same factors are working in the same way. There’s not as great of an effect observed yet in the coastal areas, but models tell us that global warming is going to work in that general direction in terms of reducing oxygen levels and increasing hypoxia.

HOST: And, he said, climate change is also contributing to more nutrients entering our coastal waterways.

ALAN LEWITUS: The arrows are pointing to an increase of nutrient loading, things like greater frequency of storm events and greater precipitation in certain areas. And those lead to both higher nutrient loads off the land. In addition, they will lead to greater freshwater into coastal waters, which will increase the stratification. So these are the forecasts that our models are telling us right now.

HOST: And this leads back to what Alan says is the primary way we can help to reduce the growing problem of hypoxia: by reducing the amounts of nutrients flowing into our ocean.

ALAN LEWITUS: The best management strategy is to reduce nutrient loading from the watershed. It’s a huge challenge, especially in large watersheds. The classic example is the Gulf of Mexico hypoxic zone. Forty one states in the U.S. (contiguous) drain into the Gulf of Mexico, so you can imagine the management challenge. Now, there is an interagency Gulf hypoxia task force that has been around for a number of years. It’s composed of five federal agencies, twelve state agencies, and a tribal association. And they have, as their primary goal, to reduce the hypoxic zone in the Gulf of Mexico to a certain level by a certain year. In order to achieve that goal, they need to reduce nutrient loading in the watershed by a certain amount, and they have all this figured out quantitatively, actually through the models we develop to help inform them of that. However, it’s not an easy task. You have to get all the states, all the state agencies in agreement working in the same direction. A lot of effort, coordination, effort to do that. You have to have the resources, the money, to support different practices. So it’s a huge challenge. They’re making some progress, but it takes years and years and years to do that sort of thing.

HOST: While reducing the fertilizer and other nutrients that flow into the Gulf of Mexico is a work in progress, Alan said that there are dead zone reduction success stories. He called out Narragansett Bay, where the nutrient problem is mainly due to sewage and wastewater treatment plants.

ALAN LEWITUS: So that’s a much easier thing to regulate, and actually in response to a fish kill on the order of a decade ago, the state imposed regulations on sewage treatment plants to reduce nutrient loading by 50 percent. They achieved that goal and our studies show that hypoxia was reduced as a result. The Bay turned from a eutrophic Bay to an oligotrophic Bay — which means cleaner water, essentially, better water quality. They haven’t achieved the ultimate goal with respect to hypoxia yet. They might need to actually reduce nutrients a little more, but they’re going in a great direction. So that’s a real success story there. So hypoxia can be mitigated.

HOST: I wrapped up our talk by asking what drives Alan forward working on intractable coastal problems like hypoxia. He stressed that NOAA supports the research and provides information, tools, and training to coastal managers who make it happen. But he said he and his colleagues take some ownership when those successes occur.

ALAN LEWITUS: Hypoxia is a challenging field to work in. It’s a double-edged sword, because the pay-off is great. Having some influence on activities that will lead to reduction in such an important stressor and ultimately societal benefits from that, is what I’m working for. The other edge of the sword is it often is a long, long road with lots of fights along the way. It’s not like there are very frequent and numerous successes, but the successes when they do come are great, and that’s what keeps me going. And there have been some. The Gulf of Mexico is still ongoing, still a challenge, and we haven’t seen the benefits of that entirely yet, though we know we’re moving the needle.

HOST: Thanks to Alan Lewitus for joining us on the program. Alan is director of the Competitive Research Program for the National Centers for Coastal Ocean Science.

And thank you for listening to the NOAA Ocean Podcast. Head to to learn about what we do.

From corals to coastal science, connect with ocean experts to explore questions about the ocean environment.

Rise of dinosaurs linked to increasing oxygen levels

IMAGE: Scientists have found that increasing oxygen levels are linked to the rise of North American dinosaurs around 215 M years ago. A new technique for measuring oxygen levels in ancient. view more

Credit: Pepper

Scientists have found that increasing oxygen levels are linked to the rise of North American dinosaurs around 215 M years ago. A new technique for measuring oxygen levels in ancient rocks shows that oxygen levels in North American rocks leapt by nearly a third in just a couple of million years, possibly setting the scene for a dinosaur expansion into the tropics of North America and elsewhere. This is presented in a Keynote talk at the Goldschmidt Geochemistry conference, in Barcelona.

The US-based scientists have developed a new technique for releasing tiny amounts of gas trapped inside ancient carbonate minerals. The gases are then channelled directly into a mass spectrometer, which measures their composition.

Lead researcher, Professor Morgan Schaller (Rensselaer Polytechnic Institute, New York) said: "We tested rocks from the Colorado Plateau and the Newark Basin that formed at the same time about 1000 km apart on the supercontinent of Pangea. Our results show that over a period of around 3 million years - which is very rapid in geological terms - the oxygen levels in the atmosphere jumped from around 15% to around 19%. For comparison, there is 21% oxygen in today's atmosphere. We really don't know what might have caused this increase, but we also see a drop in CO2 levels at that time."

"We expect that this change in oxygen concentration would have been global change, and in fact we found the change in samples which were 1000km apart. What is remarkable is that right at the oxygen peak we see the first dinosaurs appearing in the North American tropics, the Chindesaurus. The Sauropods followed soon afterwards. Again, we can't yet say if this was a global development, and the dinosaurs don't rise to ecological dominance in the tropics until after the End-Triassic extinction. What we can say is that this shows that the changing environment 215 M years ago was right for their evolutionary diversification, but of course oxygen levels may not have been the only factor".

Chindesaurus was an upright carnivorous dinosaur (around 2m long and nearly 1m high). Found extensively in North America, with origins in the North American Tropics, it was a characteristic late Triassic Dinosaur of the American Southwest. It was originally discovered in the Petrified Forest National Park. The Sauropods, which appeared soon after Chindesaurus, were the largest animals ever to live on land.

Commenting, Professor Mike Benton (University of Bristol) said: 'The first dinosaurs were quite small, but higher oxygen levels in the atmosphere are often associated with a trend to larger size. This new result is interesting as the timing of oxygen rise and dinosaur appearance is good, although dinosaurs had become abundant in South America rather earlier, about 232 million years ago.' Professor Benton was not involved in this work this is an independent comment.

At the time the gases were trapped, the Colorado Plateau and the Newark Basin were part of the giant supercontinent, Pangea. Both were located near the equator. The rocks containing the oxygen and carbon dioxide were dated by measuring the radioactive decay of Uranium which was found in the samples.

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.

Fossil fuel formation: Key to atmosphere’s oxygen?

Jon Husson points to a fossilized tree stump at Joggins Fossil Cliffs, Nova Scotia. These rocks contain large amounts of organic carbon, part of the carbon sequestration process studied by Husson and Shanan Peters. Courtesy of Jon Husson

For the development of animals, nothing — with the exception of DNA — may be more important than oxygen in the atmosphere.

Oxygen enables the chemical reactions that animals use to get energy from stored carbohydrates — from food. So it may be no coincidence that animals appeared and evolved during the “Cambrian explosion,” which coincided with a spike in atmospheric oxygen roughly 500 million years ago.

It was during the Cambrian explosion that most of the current animal designs appeared.

In green plants, photosynthesis separates carbon dioxide into molecular oxygen (which is released to the atmosphere), and carbon (which is stored in carbohydrates).

But photosynthesis had already been around for at least 2.5 billion years. So what accounted for the sudden spike in oxygen during the Cambrian?

A study now online in the February issue of Earth and Planetary Science Letters links the rise in oxygen to a rapid increase in the burial of sediment containing large amounts of carbon-rich organic matter. The key, says study co-author Shanan Peters, a professor of geoscience at the University of Wisconsin–Madison, is to recognize that sediment storage blocks the oxidation of carbon.

This black shale, formed 450 million years ago, contains fossils of trilobites and other organic material that helped support increases in oxygen in the atmosphere.

Without burial, this oxidation reaction causes dead plant material on Earth’s surface to burn. That causes the carbon it contains, which originated in the atmosphere, to bond with oxygen to form carbon dioxide. And for oxygen to build up in our atmosphere, plant organic matter must be protected from oxidation.

And that’s exactly what happens when organic matter — the raw material of coal, oil and natural gas — is buried through geologic processes.

To make this case, Peters and his postdoctoral fellow Jon Husson mined a unique data set called Macrostrat, an accumulation of geologic information on North America whose construction Peters has masterminded for 10 years.

The parallel graphs of oxygen in the atmosphere and sediment burial, based on the formation of sedimentary rock, indicate a relationship between oxygen and sediment. Both graphs show a smaller peak at 2.3 billion years ago and a larger one about 500 million years ago.

“Why is there oxygen in the atmosphere? The high school explanation is ‘photosynthesis.’ But we’ve known for a long time … that building up oxygen requires the formation of rocks like black shale.”

Shanan Peters

“It’s a correlation, but our argument is that there are mechanistic connections between geology and the history of atmospheric oxygen,” Husson says. “When you store sediment, it contains organic matter that was formed by photosynthesis, which converted carbon dioxide into biomass and released oxygen into the atmosphere. Burial removes the carbon from Earth’s surface, preventing it from bonding molecular oxygen pulled from the atmosphere.”

Some of the surges in sediment burial that Husson and Peters identified coincided with the formation of vast fields of fossil fuel that are still mined today, including the oil-rich Permian Basin in Texas and the Pennsylvania coal fields of Appalachia.

“Burying the sediments that became fossil fuels was the key to advanced animal life on Earth,” Peters says, noting that multicellular life is largely a creation of the Cambrian.

Today, burning billions of tons of stored carbon in fossil fuels is removing large amounts of oxygen from the atmosphere, reversing the pattern that drove the rise in oxygen. And so the oxygen level in the atmosphere falls as the concentration of carbon dioxide rises.

The data about North America in Macrostrat reflects the work of thousands of geoscientists over more than a century. The current study only concerns North America, since comprehensive databases concerning the other 80 percent of Earth’s continental surface do not yet exist.

The ultimate geological cause for the accelerated sediment storage that promoted the two surges in oxygen remains murky. “There are many ideas to explain the different phases of oxygen concentration,” Husson concedes. “We suspect that deep-rooted changes in the movement of tectonic plates or conduction of heat or circulation in the mantle may be in play, but we don’t have an explanation at this point.”

Holding a chunk of trilobite-studded Ordovician shale that formed approximately 450 million years ago, Peters asks, “Why is there oxygen in the atmosphere? The high school explanation is ‘photosynthesis.’ But we’ve known for a long time, going all the way back to Wisconsin geologist (and University of Wisconsin president) Thomas Chrowder Chamberlin, that building up oxygen requires the formation of rocks like this black shale, which can be rich enough in carbon to actually burn. The organic carbon in this shale was fixed from the atmosphere by photosynthesis, and its burial and preservation in this rock liberated molecular oxygen.”

What’s new in the current study, Husson says, is the ability to document this relationship in a broad database that covers 20 percent of Earth’s land surface.

Continual burial of carbon is needed to keep the atmosphere pumped up with oxygen. Many pathways on Earth’s surface, Husson notes, like oxidation of iron — rust — consume free oxygen. “The secret to having oxygen in the atmosphere is to remove a tiny portion of the present biomass and sequester it in sedimentary deposits. That’s what happened when fossil fuels were deposited.”

Enough Oxygen for Life Found Millions of Years Too Early

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Earth's atmosphere contained enough oxygen for complex life to develop nearly 1.2 billion years ago -- 400 million years earlier than scientists previously believed.

The findings, reported in the Nov. 11 Nature, could lead scientists to reconsider the prerequisites for animal life, on Earth and other planets.

"It means that the conditions were in place for complex life to arise," said geologist John Parnell of the University of Aberdeen in Scotland, lead author of the new study. "There might be animals in that earlier window that we have not yet found."

Geological records show there was one major increase in the amount of oxygen in Earth's atmosphere around 2.3 billion years ago, and another around 800 million years ago.

That second spike in oxygen levels was thought to be connected to the Cambrian explosion, the swift development of most of the major animal groups that came around 550 million years ago.

Parnell's results suggest oxygen can't be the whole story.

"It may have been that something else gave evolution the kick-start which caused animals to evolve," he said. "Oxygen in the atmosphere was already there for quite a long time."

To figure out how much oxygen was in the early atmosphere, Parnell and his colleagues searched 1.2 billion-year-old rocks from what was once a lakebed in Scotland for the chemical signatures of ancient bacteria.

Before there was a useful amount of free oxygen around, these bacteria used to get energy by converting sulfate, a molecule with one sulfur atom and four oxygens, to sulfide, a sulfur atom that is missing two electrons.

Geologists can get a glimpse of how efficient the bacteria were by looking at two different sulfur isotopes, versions of the same element that have different atomic masses. Converting sulfate to sulfide leaves the rock with a lot more of the isotope sulfur-32 than would be there without the bacteria's help.

The geologists extracted pyrite, also known as fool's gold, from the rocks. They then pulled sulfur from the pyrite by chemical processing and by zapping the rocks with a laser. The amount of sulfur-32 was much higher than bacteria could have produced without oxygen.

Parnell suggests the bacteria were able to use oxygen in the atmosphere to convert between the two different forms of sulfur (sulfate and sulfide) many times.

"Their metabolism was becoming more complicated," he said. "The more cycles of that [reaction] that they caused, the more sulfur-32 you ended up with."

The team concluded that the amount of oxygen in the atmosphere 1.2 billion years ago approached the levels at the time of the Cambrian explosion, roughly 10 percent of current oxygen levels. Ten percent may be enough to start complex life, Parnell says.

"It's only when you can start processing oxygen in a complex way that you can then start to produce different cells that do different things," Parnell said. "That's what gives rise to animals."

The evolution of large animals could have been triggered by changing geological conditions, like the end of a dramatic ice age about 600 million years ago, he says.

Parnell also hinted that the results could have implications for sulfur-eating bacteria on other planets like Mars, although because he has another paper in preparation, he didn’t want to go into very much detail.

"If there are microbes on Mars either today or in the past, this kind of metabolism is one which would be readily available to them," he said. "The stage of chemical reduction from sulfate to sulfide is completely feasible on Mars."

"I'm pretty thrilled by the paper," said geochemist Michael Russell of NASA's Jet Propulsion Lab, who was not involved in the new study. "Iɽ like to see this kind of thing done ever further back in time, so we can get a sense of just how much oxygen there was in the atmosphere."

Image: 1) The cave near Lochinver in the north-west Highlands of Scotland where Parnell and his colleagues collected sulfur-rich rocks. 2) Slivers of fool's gold that hold clues to Earth's early atmosphere. Credit: Stephen Bowden, University of Aberdeen

Bigger Than Coastal Dead Zones

Breitburg's research isn't focused just on coastal dead zones, such as the polluted runoff-fueled area in the Gulf of Mexico, but enormous stretches of deep water in the open ocean that can extend for thousands of miles.

These low-oxygen zones occur naturally, but have grown by more than 4.5 million square kilometers—an area roughly as large as the entire European Union—just since the mid-20th century. In part that's because of rising temperatures.

Warm water simply carries less oxygen. It also stokes the metabolism of both microbes and larger creatures, causing them to use more of whatever oxygen there is. Finally, as climate change warms the ocean from the surface down, making the surface layer more buoyant—warm water is lighter than cold water—it makes it harder for fresh oxygen from the air to mix down into the deep layers where the oxygen-poor zones are located.

Today, those low-oxygen zones are expanding toward the surface by as much as a meter a year. That includes major areas in the eastern Pacific and Baltic Sea. One area down deep off southern California has seen an oxygen decline of 30 percent in just a quarter century. A low-oxygen area of the Atlantic Ocean near the coast of Africa is broader than the continental United States, and has grown 15 percent since the 1960s

In fact, the world's oceans have lost about two percent of their oxygen in just 50 years, while the amount of water that's completely free of oxygen has increased fourfold, according to the new study. Scientists now can identify 500 sites along the coasts where oxygen is exceedingly low. Fewer than 10 percent of those were known before the mid-20th century.

Iron Record: Ancient Rocks Tell the Story of Oxygen, and Life

Beautiful striped rocks dating billions of years ago tell story of the dramatic risings and fallings of the cyanobacteria. Two billion years ago, the Earth had no plants and no animals. Single-celled organisms, like cyanobacteria, ruled the lakes and oceans.

Cyanobacteria make energy from sunlight by photosynthesis, creating oxygen as a waste product. As the cyanobacteria prospered, they made more and more oxygen. When oxygen concentrations reached a certain level, it poisoned the cyanobacteria, killing the cells. The oxygen concentration then decreased until it attained a level compatible with cyanobacteria growth. The populations then rose again, starting another cycle.

Evidence of the growth and death cycles is in the rocks formed at this time. During the periods of cyanobacteria growth, high concentrations of oxygen caused chemical changes in the water, including the transformation of iron into a form that sinks to the bottom, where rock is formed. During periods with little cyanobacteria, depleted oxygen in the water caused iron to stop falling to the ocean floor, and rock was formed without an iron component.

The rocks that formed at the bottom of these ancient oceans can be read like the rings of a tree trunk. Instead of recording years, the layers of rock record the cycles of ocean chemistry. These rocks show the concentration of iron gradually building as the cyanobacteria populations grew, then the iron band abruptly stops, suggesting that when the oxygen levels became too high, the cyanobacteria were quickly killed off.

Today’s waters still contain cyanobacteria, commonly known as blue-green algae. This name is a misnomer true algae belong to the domain eukaryotes, not bacteria. The larger and more complex true algae cells contain organelles that originated from cyanobacteria. In fact, all plants today owe their ability to make energy from the sun to the cyanobacteria.
Ancient polluters

Humans are not the first Earthlings to pollute the environment. Billions of years ago, chemical waste produced by a smaller Earth inhabitant changed the environment, causing many extinctions.

The earliest life on Earth enjoyed an atmosphere free of oxygen. For about 2,000 million years, life was anaerobic, meaning that it thrived without oxygen. Indeed, oxygen, produced as a waste product of energy production by photosynthesis, was poisonous to the cells that produced it as well as their neighbors.

The first photosynthesizing cells, the cyanobacteria commonly known as blue-green algae, started polluting the air with oxygen about 2.2 billion years ago. As life flourished, more and more oxygen was produced and released into the atmosphere. In 200 million years levels of oxygen in the air rose from 1 percent to 15 percent. This caused pronounced global changes.

How the rising oxygen levels affected the oceans is debated. While it’s clear that the pH of the ocean changed and that there were numerous repeated poisoning/extinctions then revivals for ocean life (see side bar), details about whether different parts of the ocean responded differently or the phases in which the change took place are not clear.

Scientists are acting as detectives, finding clues to reconstruct how the atmosphere and oceans on Earth changed so drastically billions of years ago. Many clues can be found in rocks that formed at this time. Because rock composition depends on the environment, oceans that contain more dissolved oxygen will create different rocks than oxygen-free waters.

Evidence of life in iron pyrite

Oxygen reacts with many metal ions, including iron. The formation of rocks from iron in ocean water and the absorption of iron from rock into ocean water is called iron cycling and is important to life, since cells need iron to live.

A team led by NASA Astrobiology Institute investigators hypothesized that the oxygen content in the ocean billions of years ago is recorded in iron pyrite rocks. They measured the iron isotope ratios in ancient rocks to reconstruct the history of oxygen in the Precambrian ocean, the time of oxygen pollution by cyanobacteria.

“Our most exciting discovery is that the rise of atmospheric oxygen had a direct affect on iron cycling in the ocean,” said lead author Oliver Rouxel of the Woods Hole Oceanographic Institution. This means that pyrite does contain a record of the oxygen in the ocean. The researchers analyzed the pyrite clues by using their knowledge of iron chemistry.

Oxygen prefers to form compounds with a particular form of iron called “ferric iron” or “iron ( III ).” In an ocean without much oxygen, ferric iron precipitates as iron oxide. This iron ( III ) oxide falls to the ground and becomes incorporated in rock while another form, ferrous iron or iron (II), mostly stays in the water. Some iron (II) will react with sulfur, forming iron pyrite, which gets incorporated into rock. In oxygenated water, all the iron ( III ) forms iron oxide and the iron (II) forms iron pyrite, leaving very little iron in the water.

In addition to coming in different forms, iron atoms can also have different weights. These isotopes generally act the same chemically, but, for reasons not well understood, heavier atoms of iron (II) are more likely to be transformed into iron ( III ) than light atoms. The iron (II) left over will be relatively lighter. Therefore, the pyrite formed in an oxygen-rich ocean will have lighter iron than pyrite formed in an ocean without oxygen. This separation of the isotopes is called isotope fractionation.

Rouxel’s study, published in the 18 February 2005 issue of Science, is the third leg of a stool, confirming previous results found through measuring carbon and sulfur, other elements that react with oxygen. Their results support the modern theory that describes a three-stage process of ocean evolution two billion years ago.

Rocks older than 2.3 billion years have low ratios of heavy to light iron isotopes, suggesting that there was little oxygen and a lot of iron in the water. In the second stage, between 2.3 and 1.8 billion years ago, the ratio increased, suggesting that while the atmosphere gained oxygen, the oceans remained mostly oxygen-free. Scientists are currently pondering the reason for the lagging ocean oxygenation. Rocks formed after about 1.6 billion years suggest conditions similar the modern oxygenated air and water environments.

Having found that iron isotope fractionation is a dependable measurement of the oxygen in oceans, Rouxel and his colleagues are now looking into why and how isotope fractionation occurs.

Towards finding life beyond Earth

NAI researcher Abby Kavner of the University of California at Los Angeles is also interested in isotope fractionation.

“Olivier’s paper is based on observations of natural systems—showing changes in iron isotope composition of oceans,” she said. “However, it does not address questions of the fundamental physical mechanisms by which fractionation may be occurring.”

Fractionation is thought to partially be due to chemical processes and partially due to reactions occurring within cells. Why life prefers a lighter form of iron is a mystery that grabbed Kavner’s interest.

The members of Kavner’s group have their eyes set beyond Earth’s oceans. They believe that understanding how life alters the environment on this planet could help people find evidence of life on other planets.

“I had a hypothesis, Electrochemical processes (or electron transfer processes) fractionate isotopes, and I tested it systematically in the lab,” said Kavner. “It turned out to be true.”

Kavner fractionated specific isotopes through electroplating, the same basic technique that coats costume jewelry with a thin layer of metal. This is the first time this has been done.

Kavner wondered if a similar process causes isotope fractionation by cells. She found that the small amount of electricity needed in her electroplating experiments is consistent with the amount produced in the cellular process of electron transfer, according to current electron transfer theories. It is possible that the cells prefer the lighter form of iron because of its particular electrochemical properties.

Iron’s high abundance in the solar system, its ability to take varying forms, and its high stability make it a good candidate when scientists look for biological signatures in rock from other planets. If scientists find a rock from Mars, for example, that has areas with a skewed ratio of iron isotopes, this could be evidence that living systems affected the rock’s formation.

While scientists are still far from being able to test for Martian life from the iron composition of a rock, the work of Rouxel and Kavner brings this goal closer.

After cyanobacteria “poisoning” its environment, life found ways to adapt and change, eventually evolving into the oxygen loving organisms we are familiar with.

The primitive anaerobic cells still exist in deep, dark places where there is little oxygen. They are both living fossils of a younger Earth and essential members of the modern global ecology.

Sign-up to get the latest in news, events, and opportunities from the NASA Astrobiology Program.

Why Bugs Are Not Huge

Dragonflies with hawk-sized wing spans and millipedes longer than a human leg lived more than 250 million years ago. Scientists have long wondered why sci-fi bugs don't exist today. The reason has to do with a bottleneck that occurs in insects' air pipes as they become humongous, new research shows. In the Paleozoic Era, insects were able to overcome the bottleneck due to a high-oxygen atmosphere. Unlike animals with backbones, like us, insects deliver oxygen to their tissues directly and bloodlessly through a network of dead-end tracheal tubes. In bigger insects, this mode of oxygen transport becomes less efficient, but no one has been exactly sure why. Alex Kaiser of Midwestern University and his colleagues at Argonne National Laboratory and Arizona State University delved deeper by shining X-rays on four living beetle species , ranging in body mass by a factor of 1,000. This allowed the team to measure the exact dimensions of the beetles' tracheal tubes. Kaiser found that bigger beetle species devote a larger portion of their bodies, proportionately, to airways than do smaller species. And the air passageways that lead from the body core to the legs turn out to be bottlenecks that limit how much oxygen can be delivered to the extremities, Kaiser said. The team also examined the passageways that lead from the body core to the head. "We were surprised to find that the effect is most pronounced in the orifices leading to the legs, where more and more of the space is taken up by tracheal tubes in larger species," he said. Kaiser and Argonne biologist Jake Socha also used the results to predict the largest size of currently living beetles. If data on the air passageways to the head were used as a limiting factor, they predicted a crazy-large, foot-long beetle, while the leg data predicted a beetle that matches the size of today's largest living beetle, Titaneus giganteus. The research is detailed in the Aug. 7 issue of the journal Proceedings of the National Academy of Sciences. "This study is the first step toward understanding what controls body size in insects," Socha said. "It's the legs that count in the beetles studied here, but what matters for the hundreds of thousands of beetle species and millions of insect species overall is still an open question. The research was funded by the National Science Foundation.

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Did Earth’s Early Rise in Oxygen Support The Evolution of Multicellular Life — or Suppress It?

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

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

&ldquoOxygen suppression of macroscopic multicellularity&rdquo is published in the May 14, 2021 edition of the journal Nature Communications. Bozdag&rsquos 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

&ldquoWe 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,&rdquo notes Ratcliff.

&ldquoPeople have long believed that the oxygenation of Earth's surface was helpful &mdash some going so far as to say it is a precondition &mdash for the evolution of large, complex multicellular organisms,&rdquo he adds. &ldquoBut 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,&rdquo 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. &ldquoI 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,&rdquo he says. &ldquoThis effect is robust &mdash even over much longer timescales.&rdquo

Size &mdash and oxygen levels &mdash matter for multicellular growth In the team&rsquos research, &ldquolarge size easily evolved either when our yeast had no oxygen or plenty of it, but not when oxygen was present at low levels,&rdquo Ratcliff says. &ldquoWe 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 &mdash 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 &mdash allowing most of their cells access to oxygen &mdash 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.&rdquo

Ratcliff says not only does his group&rsquos 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 &ldquoBoring Billion&rdquo in Earth&rsquos history &mdash also known as the Dullest Time in Earth's History, and Earth's Middle Ages &mdash 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. &ldquoPrevious work examined the interplay between oxygen and multicellular size mainly through the physical principles of gas diffusion,&rdquo he says. &ldquoWhile 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.&rdquo Finally being able to advance organisms through many generations of evolution helped the researchers accomplish just that, Bozdag adds.

Citation: Bozdag, G.O., Libby, E., Pineau, R. et al., &ldquoOxygen suppression of macroscopic multicellularity.&rdquo (Nat Commun 12, 2838 2021).

This work was supported by the National Science Foundation 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.

The Georgia Institute of Technology, or Georgia Tech, is a top 10 public research university developing leaders who advance technology and improve the human condition. The Institute offers business, computing, design, engineering, liberal arts, and sciences degrees. Its nearly 40,000 students, representing 50 states and 149 countries, study at the main campus in Atlanta, at campuses in France and China, and through distance and online learning. As a leading technological university, Georgia Tech is an engine of economic development for Georgia, the Southeast, and the nation, conducting more than $1 billion in research annually for government, industry, and society.


As a fringe benefit, if we can grow food in space, this is likely to lead to a happier crew. We aren't machines, and most human beings enjoy having plants around and growing plants.

First, there's the taste. Fresh food, lettuce leaves and tomatoes picked from the plant, and bread you bake yourself, from wheat you grew yourself tastes much better than food that is dried and reconstituted, which is all you'd have otherwise in a long duration journey.

Also most people enjoy having plants around and tending plants.

It's true that you can survive fine without plants. If you are a prisoner in solitary confinement, you have no choice, and may find that you adjust fine to your situation. And many hermits in the past, and even today, spend years on end in caves and other confined small places, without any plants or much of anything except blank walls, and come out of their retreats happy. It is the same also for the crew of rowing boats and such like on long distance voyages, for instance when rowing across the Pacific, they live in confined quarters for weeks or months on end, and are happy in those situations.

However, that's not for everyone. Having plants around in the spaceship, and fresh food that they have grown themselves seems likely to contribute to overall happiness and well being of the crew. This is often mentioned as a fringe benefit in the literature. And in a small way, this has already happened - in the ISS tending their small crop of plants has a calming effect on the astronauts and cosmonauts.

A happy crew will make better decisions, and are more likely to come up with inspired and creative solutions to problems, and so may be better at completing mission objectives. And in any case, all things being equal, surely it's better to go for a solution that is more enjoyable for the crew.

Especially on long duration missions, far from Earth, where their plants in their spaceship may be the only green thing there is for many light minutes, many millions of kilometers, in all directions. Even on the far side of the Moon, the green plants in their spaceship may be their one direct tangible link with the ecosystem of the Earth which they can no longer see in the sky.