More than 40 percent of Earth’s ecosystems are arid, an amount that is expected to increase significantly over the course of the 21st century. Some of these areas, such as those in Africa and Australia may be savannah or desert, where sparse rainfall has long been the norm. Within these biomes, vegetation and wildlife have adapted to making use of their scant water resources, but they are also extraordinarily vulnerable to climate change.

Using extensive imagery from satellites that monitor Earth every day, researchers from the University of Copenhagen’s Department of Geosciences and Natural Resources Management have studied the evolution of vegetation in arid regions. Their conclusion is unequivocal:

“We observe a clear trend of arid areas developing in a negative direction in the most economically challenged countries. Here, it is apparent that the growth of vegetation has become increasingly decoupled from the water resources available and that there is simply less vegetation in relation to the amount of rainfall. The opposite is the case in the wealthiest countries,” explains Professor Rasmus Fensholt of the Department of Geosciences and Natural Resource Management.

Worse in Asia and Africa, better in South America and Australia

The researchers analyzed 15 years worth of satellite imagery of vegetation and rainfall — from 2000 to 2015. To compare the evolution of vegetation in arid regions of the world, the researchers removed precipitation totals from the equation. In other words, they produced a calculation that accounts for the fact that some regions received more rain in past decades, while other regions received less.

This provides a more accurate picture of ecosystem health, as human influences become easier to identify: In other words, whether resource use is balanced or whether an ecosystem’s resources have been overexploited, with potentially fatal consequences — as imbalanced systems may be irreparable.

“Here, our results demonstrate that in arid regions, particularly those in Africa and Asia, less vegetation grows for the amount of rainwater that falls, while more vegetation grows in arid areas of South America and Australia,” says lead author Christin Abel, a postdoc at the Department of Geosciences and Natural Resource Management.

Infographic text: Areas with purple hues indicate where vegetation growth relative to rainfall is declining, while green hues reveal areas where vegetation has grown more than expected. White areas represent ecosystems where vegetation growth is in balance with the water resources available.

According to the researchers, there may be several explanations for why climate change and rising global temperatures are impacting vegetation in arid regions of the world’s poorest countries. Among the most obvious is rapid population growth, in Africa for example, where there is an increasing need to exploit land that is otherwise poorly suited for agriculture. Doing so produces lower yields and puts increasing amounts of livestock on too little grass in already fragile ecosystems.

Conversely, vegetation in arid areas of the world’s wealthier countries seems to be coping better with climate change. This is likely due to the intensification and expansion of larger farms, where more economic resources allow for, among other things, irrigation and fertilization.

Food crises and more climate refugees

As a result of climate change, future trends for the planet’s poorest areas only seem to be getting worse. Forecasts point to an expansion of today’s arid areas where they will make up a larger and larger share of our global ecosystems. This may result in more and more people being left without food and their needing to migrate.

“One consequence of declining vegetation in the world’s poorer arid regions areas may be an increase in climate refugees from various African countries. According to what we’ve seen in this study, there is no indication that the problem will diminish in the future,” explains Rasmus Fensholt.

For a number of years, satellite imagery has let researchers observe that, overall, it actually appears that the world’s arid regions have become greener. However, when researchers look at how much vegetation arid areas in developing countries get in relation to rainfall amounts, the picture looks different.

“We have been pleased to see that, for a number of years, vegetation has been on an upwards trend in arid regions. But if we dig only a tiny bit deeper and look at how successfully precipitation has translated into vegetation, then climate change seems to be hitting unevenly, which is troubling,” says Rasmus Fensholt.

The health harms associated with prolonged sitting can be offset by exceeding weekly recommended physical activity levels, says the World Health Organization (WHO) in new global guidelines on physical activity and sedentary behaviour, published in a special dedicated issue of the British Journal of Sports Medicine.

But all physical activity counts and is good for long term health, say the new guidelines.

It’s the first time that a recommendation of this kind has been made. It reflects a large and growing body of evidence linking extensive sedentary time to serious ill health and a heightened risk of early death.

New data also released today, and published in the same special issue, show that adults who clock up long hours of sedentary time every day can counter these risks by increasing the amount of physical activity they do.

The new research, involving more than 44,000 people wearing activity trackers from four countries, reveals that a high daily tally of sedentary time (defined in this study as 10 or more hours) is linked to a significantly heightened risk of death, particularly among people who are physically inactive.

But 30 to 40 daily minutes of moderate to vigorous intensity physical activity substantially weakens this risk, bringing it down to levels associated with very low amounts of sedentary time, indicate the findings, which broadly confirm the recommendations set out in the 2020 World Health Organization Global Guidelines on Physical Activity and Sedentary Behaviour.

There’s not enough evidence to recommend specific maximum thresholds for sedentary behaviour, say the guidelines. But everyone, irrespective of their age or abilities, should try to limit their daily sedentary time and replace it with physical activity of any intensity.

All physical activity counts. This could be anything from climbing the stairs instead of taking the lift, a walk around the block, a spot of gardening, or some household chores, to going for a run or bike ride, a high intensity interval training work-out, or team sport.

It all adds up to the weekly tally of 150-300 minutes of moderate intensity, or at least 75-100 minutes of vigorous intensity, physical activity, the WHO guidance recommends. But any amount of physical activity is better for health than none, it emphasises.

And those unable to meet these recommendations should start small and gradually build up the frequency, intensity, and duration of their physical activity over time, it says.

Boosting physical activity levels doesn’t just benefit mental and physical health, and help to stave off the risk of an early death, it’s also likely to benefit the global economy, through lower rates of presenteeism, higher productivity, and lower rates of working age sickness and death, indicates another study in this special issue.

Doing at least 150 minutes of moderate intensity physical activity every week, which is the lower end of the range recommended in the new WHO guidelines, would increase global gross domestic product (GDP) by between 0.15%-0.24% a year between now and 2050, estimate the researchers.

That’s worth up to US$314-446 billion a year and US$6.0-8.6 trillion cumulatively over the 30 years in 2019 prices.

The new guidelines, which aim to drive national policy and practice around the globe, involved more than 40 scientists from six continents. They provide a consensus on the latest science on the health impacts of physical activity and sedentary behaviour from early childhood through to older age, and update WHO global recommendations for physical health, published in 2010.

They highlight the importance of regularly undertaking both aerobic and muscle strengthening activities, and for the first time, make specific recommendations for important, but often neglected groups, including those who live with long term conditions or disabilities, pregnant women and new mothers.

Key recommendations for adults, including those living with long term conditions or disabilities at any age:

Aim to do 150-300 mins of moderate intensity, or 75-150 mins of vigorous-intensity physical activity, or some equivalent combination of moderate intensity and vigorous intensity aerobic physical activity every week.

Undertake muscle-strengthening activity (such as weights, core conditioning) at moderate or greater intensity on 2 or more days of the week.

Reduce sedentary behaviour and aim to exceed these weekly recommendations to offset health harms of prolonged sitting.

Older adults (65+) should do physical activity that emphasises functional balance and strength training at moderate or greater intensity on 3 or more days of the week, to enhance functional capacity and prevent falls.

Women should do regular physical activity throughout pregnancy and after the birth, to include various aerobic and muscle-strengthening activities. Gentle stretching may also be beneficial.

Light intensity physical activity doesn’t cause a substantial increase in heart rate or breathing and includes activities such as strolling.

Moderate intensity physical activity increases heart rate and induces a degree of breathlessness where it’s still possible to talk. Examples include brisk walking, dancing, or raking up leaves.

Vigorous intensity physical activity substantially increases heart rate and breathing rate. Examples include cycling, running/jogging, swimming, carrying heavy objects, walking up the stairs, digging the garden, playing tennis.

“Although the new guidelines reflect the best available science, there are still some gaps in our knowledge. We are still not clear, for example, where exactly the bar for ‘too much sitting’ is. But this is a fast paced field of research, and we will hopefully have answers in a few years’ time,” comments special issue co-editor Professor Emmanuel Stamatakis, of the University of Sydney.

He adds: “These guidelines are very timely, given that we are in the middle of a global pandemic, which has confined people indoors for long periods and encouraged an increase in sedentary behaviour.

“But people can still protect their health and offset the harmful effects of physical inactivity. As these guidelines emphasise, all physical activity counts and any amount of it is better than none.

“There are plenty of indoor options that don’t need a lot of space or equipment, such as climbing the stairs, active play with children or pets, dancing, or online yoga or Pilates classes.”

Co editor of the special issue, and development lead on the guidelines, Professor Fiona Bull of the WHO, adds: “The most recent global estimates show that one in four (27.5%) adults and more than three-quarters (81%) of teenagers don’t meet the recommendations for aerobic exercise, as outlined in the 2010 Global Recommendations. So there’s an urgent need for governments to prioritise and invest in national initiatives and health and community services that promote physical activity.

“The publication of these new WHO global guidelines can support implementation of policy recommendations set out in the WHO’s Global Action Plan on Physical Activity 2018-2030 to achieve its ambitious target of 15% improvement by 2030.”

In particular, the study found that there was a strong association between diastolic blood pressure (the blood pressure between heart beats) before the age of 50 and brain damage in later life, even if the diastolic blood pressure was within what is normally considered to be a healthy range.

The findings come from a study of 37,041 participants enrolled in UK Biobank, a large group of people recruited from the general population aged between 40 and 69 years, and for whom medical information, including MRI brain scans was available.

The research, carried out by Dr Karolina Wartolowska, a clinical research fellow at the Centre for Prevention of Stroke and Dementia, University of Oxford, UK, looked for damage in the brain called “white matter hyperintensities” (WMH). These show up on MRI brain scans as brighter regions and they indicate damage to the small blood vessels in the brain that increases with age and blood pressure. WMH are associated with an increased risk of stroke, dementia, physical disabilities, depression and a decline in thinking abilities.

Dr Wartolowska said: “Not all people develop these changes as they age, but they are present in more than 50% of patients over the age of 65 and most people over the age of 80 even without high blood pressure, but it is more likely to develop with higher blood pressure and more likely to become severe.”

Information on the participants was collected when they enrolled in UK Biobank between March 2006 and October 2010, and follow-up data, including MRI scans, were acquired between August 2014 and October 2019. The researchers adjusted the information to take account of factors such as age, sex, risk factors such as smoking and diabetes, and diastolic as well as systolic blood pressure. Systolic blood pressure is the maximum blood pressure reached each time the heart beats and is the top number in blood pressure measurements.

“To compare the volume of white matter hyperintensities between people and to adjust the analysis for the fact that people’s brains vary slightly in size, we divided the volume of WMH by the total volume of white matter in the brain. In that way, we could analyse the WMH load, which is the proportion of the WMH volume to the total volume of white matter,” said Dr Wartolowska.

The researchers found that a higher load of WMH was strongly associated with current systolic blood pressure, but the strongest association was for past diastolic blood pressure, particularly when under the age of 50. Any increase in blood pressure, even below the usual treatment threshold of 140 mmHg for systolic and below 90 mmHg for diastolic, was linked to increased WMH, especially when people were taking medication to treat high blood pressure.*

For every 10mmHg increase in systolic blood pressure above the normal range, the proportion of WMH load increased by an average (median) of 1.126-fold and by 1.106-fold for every 5mmHg increase in diastolic blood pressure. Among the top 10% of people with the greatest WMH load, 24% of the load could be attributed to having a systolic blood pressure above 120mmHg, and 7% could be attributed to having diastolic blood pressure above 70mmHg, which reflects the fact that there is a greater incidence of elevated systolic rather than diastolic blood pressure in older patients.

Dr Wartolowska said: “We made two important findings. Firstly, the study showed that diastolic blood pressure in people in their 40s and 50s is associated with more extensive brain damage years later. This means that it is not just the systolic blood pressure, the first, higher number, but the diastolic blood pressure, the second, lower number, that is important to prevent brain tissue damage. Many people may think of hypertension and stroke as diseases of older people, but our results suggest that if we would like to keep a healthy brain well into our 60s and 70s, we may have to make sure our blood pressure, including the diastolic blood pressure, stays within a healthy range when we are in our 40s and 50s.

“The second important finding is that any increase in blood pressure beyond the normal range is associated with a higher amount of white matter hyperintensities. This suggests that even slightly elevated blood pressure before it meets the criteria for treating hypertension has a damaging effect on brain tissue.

“Our results suggest that to ensure the best prevention of white matter hyperintensities in later life, control of diastolic blood pressure, in particular, may be required in early midlife, even for diastolic blood pressure below 90mmHg, whilst control of systolic blood pressure may be more important in late life. The long time interval between the effects of blood pressure in midlife and the harms in late life emphasises how important it is to control blood pressure long-term, and that research has to adapt to consider the very long-term effects of often asymptomatic problems in midlife.”

Potential mechanisms for the development of WMH include damage to the delicate blood vessels in the brain through sustained elevated pressures over time that directly cause damage to the blood vessels; this leads to the lining of the vessels becoming leaky and results in WMH. Alternatively, diastolic pressure might cause large blood vessels to become stiffer with time, which increases pulsations of blood pressure to the brain; this causes high blood pressure with each heart beat, rapid changes in blood pressure, and blood flow that is too low between heart beats, resulting in damage to white matter.

As MRI scans were only available at one time point, the researchers could not quantify the progression of WMH directly. Other limitations include that further analysis is needed to identify differences in different regions of white matter, and that although the researchers showed associations with smoking and diabetes, the potential complex interaction between risk factors, which also include high cholesterol levels, obesity and kidney problems, require further investigation.

Notes:

* Patients with a ‘low normal’ blood pressure of 120/70mmHg were used as the ‘reference group’ with whom the researchers compared the other groups of patient in this analysis. Consistent with most guidelines, the researchers referred to people with blood pressure over 140/90 mmHg as ‘hypertensive’ and requiring treatment, and those between 140/90 and 130/80 mmHg as ‘pre-hypertensive’. People with blood pressure below the pre-hypertensive values were referred to as ‘high normotensive’ and those with values below 120/70 as ‘low normotensive’.

It is well known that rats will help each other out. What the researchers wanted to know was whether the rat’s odour during this behaviour had any effect on another rat’s helpfulness. They therefore carried out a series of tests to study the importance of the scent of a rat while making cooperative decisions. The rats being studied could choose to help another rat by pulling a platform containing a reward towards the other rat’s cage. This provided food for the other rat but did not have any immediate benefit for them personally. The researchers then provided the test rats either with the smell of a rat that was being helpful to another rat in a different room or with the smell of a rat that was not engaged in helpful behaviour. The researchers were surprised to find that just the scent of a rat engaged in helpful behavior was enough to illicit helpful behaviour in the other.

Dr Nina Gerber from the Wildlife Sciences at the University of Göttingen, who led the research, says: “Test rats increased their own helping behaviour when they were presented with the smell of a helpful rat. Remarkably, this holds true even though they did not experience this helpful behaviour themselves.” She goes on to say, “Furthermore, such a ‘smell of cooperation’ depends on the actual activity of helping and is not connected to an individual rat. There isn’t a “special smell” for certain nice rats: the same individual can release the scent of being helpful or not, depending solely on their behaviour.”

The researchers concluded that physical cues — such as smell — might be even more important for rats to encourage cooperation than actual experiences. Gerber adds, “Even though people do not seem to rely on communication through scent in the way rats do, some studies indicate that scent is key for finding partners, or that smelling certain chemicals can increase trust in others. Whether there is such a ‘smell of cooperation’ in humans, however, would be an interesting question for future studies.”

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This mechanism may provide alternative renewable resources of freshwater to volcanic islands worldwide. “Their evidence for separate freshwater lenses, stacked one above the other, near the Kona coast of Hawai’i, profoundly improves the prospects for sustainable development on volcanic islands,” said UH Manoa School of Ocean and Earth Science and Technology (SOEST) Dean Brian Taylor.

Paradigm shift

Through the use of marine controlled-source electromagnetic imaging, the study revealed the onshore-to-offshore movement of freshwater through a multilayer formation of basalts embedded between layers of ash and soil, diverging from previous groundwater models of this area. Conducted as a part of the National Science Foundation-supported ‘Ike Wai project, research affiliate faculty Eric Attias led the marine geophysics campaign.

“Our findings provide a paradigm shift from the conventional hydrologic conceptual models that have been vastly used by multiple studies and water organizations in Hawai’i and other volcanic islands to calculate sustainable yields and aquifer storage for the past 30 years,” said Attias. “We hope that our discovery will enhance future hydrologic models, and consequently, the availability of clean freshwater in volcanic islands.”

Co-author Steven Constable, a professor of geophysics at the Scripps Institution of Oceanography, who developed the controlled source electromagnetic system used in the project, said, “I have spent my entire career developing marine electromagnetic methods such as the one used here. It is really gratifying to see the equipment being used for such an impactful and important application. Electrical methods have long been used to study groundwater on land, and so it makes sense to extend the application offshore.”

Kerry Key, an associate professor at Columbia University who employs electromagnetic methods to image various oceanic Earth structures, who not involved in this study, said, “This new electromagnetic technique is a game changing tool for cost-effective reconnaissance surveys to identify regions containing freshwater aquifers, prior to more expensive drilling efforts to directly sample the pore waters. It can also be used to map the lateral extent of any aquifers already identified in isolated boreholes.”

Two-times more water

Donald Thomas, a geochemist with the Hawai’i Institute of Geophysics and Planetology in SOEST who also worked on the study, said the findings confirm two-times the presence of much larger quantities of stored groundwater than previously thought.

“Understanding this new mechanism for groundwater…is important to better manage groundwater resources in Hawai’i,” said Thomas, who leads the Humuʻula Groundwater Research project, which found another large freshwater supply on Hawai’i Island several years ago.

Offshore freshwater systems similar to those flanking the Hualālai aquifer are suggested to be present for the island of O’ahu, where the electromagnetic imaging technique has not yet been applied, but, if demonstrated, could provide an overall new concept to manage freshwater resources.

The study proposes that this newly discovered transport mechanism may be the governing mechanism in other volcanic islands. With offshore reservoirs considered more resilient to climate change-driven droughts, volcanic islands worldwide can potentially consider these resources in their water management strategies.

This project is supported by the National Science Foundation EPSCoR Program Award OIA #1557349.

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The CNO cycle is the dominant energy source powering stars heavier than the sun, but it had so far never been directly detected in any star, Pocar explains.

For much of their life, stars get energy by fusing hydrogen into helium, he adds. In stars like our sun or lighter, this mostly happens through the ‘proton-proton’ chains. However, many stars are heavier and hotter than our sun, and include elements heavier than helium in their composition, a quality known as metallicity. The prediction since the 1930’s is that the CNO-cycle will be dominant in heavy stars.

Neutrinos emitted as part of these processes provide a spectral signature allowing scientists to distinguish those from the ‘proton-proton chain’ from those from the ‘CNO-cycle.’ Pocar points out, “Confirmation of CNO burning in our sun, where it operates at only one percent, reinforces our confidence that we understand how stars work.”

Beyond this, CNO neutrinos can help resolve an important open question in stellar physics, he adds. That is, how the sun’s central metallicity, as can only be determined by the CNO neutrino rate from the core, is related to metallicity elsewhere in a star. Traditional models have run into a difficulty — surface metallicity measures by spectroscopy do not agree with the sub-surface metallicity measurements inferred from a different method, helioseismology observations.

Pocar says neutrinos are really the only direct probe science has for the core of stars, including the sun, but they are exceedingly difficult to measure. As many as 420 billion of them hit every square inch of the earth’s surface per second, yet virtually all pass through without interacting. Scientists can only detect them using very large detectors with exceptionally low background radiation levels.

The Borexino detector lies deep under the Apennine Mountains in central Italy at the INFN’s Laboratori Nazionali del Gran Sasso. It detects neutrinos as flashes of light produced when neutrinos collide with electrons in 300-tons of ultra-pure organic scintillator. Its great depth, size and purity make Borexino a unique detector for this type of science, alone in its class for low-background radiation, Pocar says. The project was initiated in the early 1990s by a group of physicists led by Gianpaolo Bellini at the University of Milan, Frank Calaprice at Princeton and the late Raju Raghavan at Bell Labs.

Until its latest detections, the Borexino collaboration had successfully measured components of the ‘proton-proton’ solar neutrino fluxes, helped refine neutrino flavor-oscillation parameters, and most impressively, even measured the first step in the cycle: the very low-energy ‘pp’ neutrinos, Pocar recalls.

Its researchers dreamed of expanding the science scope to also look for the CNO neutrinos — in a narrow spectral region with particularly low background — but that prize seemed out of reach. However, research groups at Princeton, Virginia Tech and UMass Amherst believed CNO neutrinos might yet be revealed using the additional purification steps and methods they had developed to realize the exquisite detector stability required.

Over the years and thanks to a sequence of moves to identify and stabilize the backgrounds, the U.S. scientists and the entire collaboration were successful. “Beyond revealing the CNO neutrinos which is the subject of this week’s Nature article, there is now even a potential to help resolve the metallicity problem as well,” Pocar says.

Before the CNO neutrino discovery, the lab had scheduled Borexino to end operations at the close of 2020. But because the data used in the analysis for the Nature paper was frozen, scientists have continued collecting data, as the central purity has continued to improve, making a new result focused on the metallicity a real possibility, Pocar says. Data collection could extend into 2021 since the logistics and permitting required, while underway, are non-trivial and time-consuming. “Every extra day helps,” he remarks.

Pocar has been with the project since his graduate school days at Princeton in the group led by Frank Calaprice, where he worked on the design, construction of the nylon vessel and the commissioning of the fluid handling system. He later worked with his students at UMass Amherst on data analysis and, most recently, on techniques to characterize the backgrounds for the CNO neutrino measurement.

This work was supported in the U.S. by the National Science Foundation. Borexino is an international collaboration also funded by the Italian National Institute for Nuclear Physics (INFN), and funding agencies in Germany, Russia and Poland.

To visualize a pyrenoid, think of a pomegranate. The pyrenoid contains kernels of Rubisco, the enzyme that carries out the molecular work of incorporating carbon dioxide into sugars. These kernels are embedded in a supportive flesh, or matrix, of other proteins, that is itself surrounded by an outer shell made of starch. The fruit is a bit worm-eaten; it is riddled with fingerlike channels — actually, tubules enclosed by membrane — that deliver concentrated carbon dioxide to the Rubisco kernels. The tubules are important to pyrenoid function because waterborne algae such as Chlamydomonas reinhardtii would otherwise struggle to get enough carbon dioxide to keep Rubisco operating at peak capacity.

The pyrenoid presents several enigmas for scientists. For example, how the proteins that make up the pyrenoid are routed there, and how they organize into such a complex arrangement, has been an enduring mystery. New work from the laboratory of Martin Jonikas, an Assistant Professor in the Department of Molecular Biology at Princeton, and collaborators, has now solved this riddle.

“The key initial discovery was made by chance,” says Jonikas.

Research Molecular Biologist Moritz Meyer and colleagues were trying to identify what proteins are present in the pyrenoid besides Rubisco. To do this, they used an antibody: a protein that, like a key, attaches to other proteins that possess a specific, matching lock. Meyer and colleagues planned to crack open algae and then add an antibody that binds a particular matrix protein to the resulting molecular soup. By pulling on the antibody, the scientists could drag that protein out. Any other proteins that bind to the antibody’s target protein would come along for the ride, and the scientists could then determine whether any of them were previously unknown pyrenoid components. But the experiment didn’t turn out as expected.

“We noticed that the antibody directly bound to several pyrenoid-localized proteins,” says Jonikas. In other words, they’d just discovered that all these proteins possess a lock matching their antibody’s key. Closer examination of the proteins revealed the existence of a sequence of amino acids, or motif, that is present in the antibody’s original target and also appears in all of the other proteins.

“We hypothesized that this motif may serve as a signal that targets the proteins to the pyrenoid, and the experiments we did support this hypothesis,” explains Jonikas. “Removing the motif from one of the motif-containing proteins caused it to no longer localize to the pyrenoid, while adding it to non-pyrenoid proteins caused them to localize to the pyrenoid.”

Meyer and colleagues found that the motif binds to Rubisco. This explains how the pyrenoid forms: its component proteins remain loose in the cell until they bump into Rubisco and become trapped.

“Several of the proteins do not simply localize to the pyrenoid matrix, but rather appear to localize to the interfaces between the matrix and the pyrenoid’s two other sub-compartments, the pyrenoid tubules and the starch sheath,” notes Jonikas. This may allow the proteins to self-organize into the complex pyrenoid structure.

“The study represents an exquisite example of investigative science,” says Dr. Howard Griffiths, Professor of Plant Science at Cambridge University in the United Kingdom. Dr. Griffiths has collaborated with Jonikas’s group on other studies, but he was not involved in this work.

“They used clever experimental manipulations to prove that a common motif could allow the specific linker to form the Rubisco matrix, and anchor other key elements both internally to the thylakoid tubules, and the starch sheath towards the periphery,” says Griffiths. “Overall, the report by Meyer and colleagues has made a significant contribution to our understanding of pyrenoid form and function, with relevance both for understanding aquatic primary productivity, and to underpin approaches seeking to incorporate such a mechanism to ‘turbocharge’ photosynthesis in terrestrial crop plants.”

This research was supported by grants to M.C.J. by NSF (IOS-1359682 and MCB-1935444), NIH (DP2-GM-119137), and the Simons Foundation and Howard Hughes Medical Institute (55108535); and to L.C.M.M. by the UK Biotechnology and Biological Sciences Research Council (BB/R001014/1).

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The enzyme that performs the first step of the reaction to assimilate carbon dioxide into sugars is a bulky protein called Rubisco assembled from eight identical small subunits and eight identical large subunits arranged together symmetrically. All the parts of this assembly, which is called a holoenzyme, work in concert to perform Rubisco’s enzymatic duty. Rubisco’s rate of activity — and by extension, the rate at which plants and algae can grow — is limited by its access to carbon dioxide. Free carbon dioxide can be scarce in water, so aquatic algae such as Chlamydomonas reinhardtii sometimes struggle to keep Rubisco working at peak capacity. To counteract this, these algae evolved a special structure called the pyrenoid to supply concentrated carbon dioxide to Rubisco. The pyrenoid is so important that almost all algae on the planet have one. Different species of algae are thought to have evolved the structure independently.

“The defining feature of a pyrenoid is the matrix, a giant liquid-like condensate that contains nearly all of the cell’s Rubisco,” explains Jonikas, an Assistant Professor in the Department of Molecular Biology at Princeton.

Rubisco is the main component of the pyrenoid matrix, but not the only one; in 2016, Jonikas’s lab discovered another abundant protein in the pyrenoid called EPYC1. In their 2016 paper, Jonikas’s group showed that EPYC1 binds to Rubisco and helps concentrate Rubisco in the pyrenoid. The researchers theorized that EPYC1 works like a molecular glue to link together Rubisco holoenzymes. Postdoc Shan He, together with colleagues in Jonikas’s lab and collaborators from Germany, Singapore and England, set out to test this theory.

“In the present work, we demonstrate that this is indeed how it works,” says Jonikas, “by showing that EPYC1 has five binding sites for Rubisco, allowing it to ‘link’ together multiple Rubisco holoenzymes.”

EPYC1 is a loosely structured, extended protein, and its five Rubisco binding sites are evenly distributed across its length. The researchers also found that Rubisco has eight EPYC1 binding sites distributed evenly across its ball-like surface. Computer modeling showed that the loosely structured and flexible EPYC1 protein can make multiple contacts with a single Rubisco holoenzyme or bridge together neighboring ones. In this way, EPYC1 drives Rubisco to cluster in the pyrenoid matrix.

Although this offers a satisfying explanation for how the matrix is assembled, it poses something of a conundrum. Other proteins need to be able to access Rubisco to repair it when it breaks down. If the EPYC1-Rubisco network is rigid, it could block these proteins from accessing Rubisco. However, He and colleagues found that EPYC1’s interactions with Rubisco are fairly weak, so although the two proteins may form many contacts with each other, these contacts are exchanging rapidly.

“This allows EPYC1 and Rubisco to flow past each other while staying in a densely packed condensate, allowing other pyrenoid proteins to also access Rubisco,” notes Jonikas. “Our work solves the longstanding mystery of how Rubisco is held together in the pyrenoid matrix.”

Land plants don’t have pyrenoids, and scientists think that engineering a pyrenoid-like structure into crop plants could boost their growth rates. Understanding how the pyrenoid is assembled in algae represents a significant step toward such efforts.

“He and colleagues provide a very nice molecular study of the protein-protein interactions between the Rubisco small subunit and EPYC1,” says Dr. James Moroney, Professor of Biology at the Louisiana State University department of Biological Sciences, whose lab studies photosynthesis in plants and algae.

“This work is encouraging for researchers trying to introduce pyrenoid-like structures into plants to improve photosynthesis,” he adds.

In a world beset by hunger and disease, we can use all the boosts we can get.

Funding: The work described here was supported by grants to M.C.J. from the National Science Foundation (nos IOS-1359682 and MCB-1935444), National Institutes of Health (no. DP2-GM-119137), and Simons Foundation and Howard Hughes Medical Institute (no. 55108535); to B.D.E. by Deutsche Forschungsgemeinschaft (EN 1194/1-1 as part of FOR2092); to O.M.-C. by the Ministry of Education (MOE Singapore) Tier 2 (no. MOE2018-T2-2-059); to A.J.M. and N.A. by the UK Biotechnology and Biological Sciences Research Council (no. BB/ S015531/1) and Leverhulme Trust (no. RPG-2017-402); to F.M.H by NIH ( R01GM071574); to S.A. P. by Deutsche Forschungsgemeinschaft fellowship (no. PO2195/1-1); and to V.K.C. by a National Institute of General Medical Sciences of the Institutes of Health (no. T32GM007276) training grant.

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