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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Materials provided by Princeton University. Original written by Caitlin Sedwick. Note: Content may be edited for style and length.

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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Materials provided by Princeton University. Original written by Caitlin Sedwick. Note: Content may be edited for style and length.