Research Highlights

Our pick of the latest scientific literature

Materials science

Controlling gas with the flip of a switch 

A metal–organic framework can separate gas molecules that differ by just two hydrogen atoms.

Electric fields can be used to control the flow of gas through a membrane. 

Porous materials known as metal–organic frameworks (MOFs) can be used to separate gas molecules of different sizes. But their scaffold-like structures, composed of metals linked by organic struts, are often flexible, and struggle to separate molecules whose sizes differ by only a small amount. 

Jürgen Caro and Alexander Knebel at the Leibniz University of Hanover, Germany, and their colleagues investigated the effect of an electric field on a MOF called ZIF-8, made up of zinc ions and imidazolate (C3H3N2-) struts. They found that applying the field made a ZIF-8 membrane less permeable, allowing it to better sieve propane from propene — molecules that differ by just two hydrogen atoms.

X-ray studies suggested that this was due to the MOF’s crystal structure polarizing under the electric field and becoming more rigid, which barred the larger, propane molecules from passing through while permitting propene to pass. The reversible effect could be harnessed in a range of MOFs to create controllable materials for use in drug delivery and sensors, the authors suggest. 

This 28,000-year-old mandible from a sabre-toothed cat was recovered from the North Sea. Natural History Museum Rotterdam

Zoology

Sequencing reveals family history of sabre-toothed cats

Ancient beasts’ genomes suggest that two recognized species of Homotherium are actually one.

Sequencing of ancient DNA suggests that only one species of Homotherium, a sabre-toothed cat, roamed the Earth around 30,000 years ago.

Johanna Paijmans, at the University of Potsdam, Germany, and her colleagues sequenced the DNA of cell structures called mitochondria from three Homotherium specimens, two from over 50,000 and one from 28,000 years ago, and from the 11,000-year-old remains of a Smilodon, another sabre-toothed cat.

These partial genomes show major differences between the two lineages, and suggest that they split around 18 million years ago. However, the Homotherium samples showed little divergence, despite the two older ones being from North America and the other from Europe. 

Currently, separate species are thought to have existed in the two regions up to the late Pleistocene. But the team suggests that these animals should all be considered one species: Homotherium latidens.

Divers surveyed fish, invertebrates and other animal life at sites across the globe. Rick Stuart-Smith/Reef Life Survey

Ecology

Unpicking patterns of global reef biodiversity

Data from divers show what lives where at different latitudes.

The diversity of life in the tropics is abundantly clear. But why more species live there than in colder climates is debated.

Graham Edgar at the University of Tasmania in Australia and his colleagues undertook standardized surveys of animals found at 2,406 reef sites across the globe. Overall species richness is at its highest in the ‘Coral Triangle’ between Australia and Southeast Asia, and declines outwards from there, they found. Looking only at latitude, richness peaks at 15° north and 15° south of the Equator. But at the site level, many more large, mobile invertebrate species are observed at high latitudes than are seen near the Equator. Species richness is highly correlated with the amount of coral reef in a given region.

The authors suggest that fish predation limits invertebrate diversity in the tropics, and that local temperatures and nutrient levels control abundance.

Chemistry

Polymer chains grow one ‘hairball’ at a time

A magnetic-tracking technique revealed that molecules added to a polymer exhibit ‘wait-and-jump’ behaviour.

Chain-growth polymerization is widely used for synthesizing long molecules, through the addition of building blocks called monomers to the end of a molecular chain. 

A team led by Peng Chen, Geoffrey Coates and Fernando Escobedo of Cornell University in Ithaca, New York, has now observed the growth of an individual polymer as it gains monomers one at a time.

To follow this process, the researchers tethered magnetic particles to one end of polymers. They then used magnetic tweezers, which deploy magnetic fields to manipulate and image the particles, to track single monomers as they were inserted into the polymers. Chains, the team found, did not extend steadily, but instead jumped every few minutes by hundreds of nanometres — equivalent to the addition of thousands of monomers. 

Simulations indicate that this ‘wait-and-jump’ behaviour is due to newly appended monomers becoming tangled and held together by weak interactions, forming an ‘entangled hairball’ that suddenly unravels. The researchers used a gentle magnetic force to keep the polymer extended and aligned as it grew. In the absence of this influence, such hairballs might be even more prevalent in polymer growth, they say.

Insects such as these are less plentiful in Germany than they used to be.

Insects such as these are less plentiful in Germany than they used to be. Agencja Fotograficzna Caro/Alamy

Zoology

Flying insects are disappearing from German skies

The country has lost three-quarters of its aerial insects since 1989.

The number of flying insects in German skies has fallen sharply over the past three decades.

Caspar Hallmann of Radboud University in Nijmegen, the Netherlands, and his colleagues have been deploying traps to capture aerial insects at 63 nature reserves in Germany over a 27-year period. 

Most locations were sampled only once. In total, the team examined 96 separate combinations of year and site.

After correcting for seasonal changes and type of habitat, the researchers found a 6.1% annual decline in flying-insect biomass, and an overall decline of nearly 77% between 1989 and 2016. Declines were higher in summer months. Similar trends were seen in sites sampled more than once.

The authors say that this huge, unrecognized loss cannot be sufficiently explained by changes in weather, land use or habitat.

Researchers followed thousands of generations of E. coli as they evolved over decades.

Researchers followed thousands of generations of E. coli as they evolved over decades. NIAID (CC BY 2.0)

Evolution

Sixty thousand generations of bacterial evolution

Researchers have watched and sequenced the same 12 populations of E. coli since 1988.

The world’s longest-running bacterial-evolution experiment shows adaptation to environments is more complex, dynamic and random than previously thought.

In 1988, researchers left 12 identical populations of Escherichia coli to evolve in identical environments. Michael Desai at Harvard University in Cambridge, Massachusetts, and his colleagues sequenced the entire genomes of E. coli sampled from each population every 500 generations, eventually looking at more than 60,000 generations.

The team found that the rate of appearance of beneficial mutations — those that survive and spread through a population — has slowed over time, but periods of rapid adaptation continue to arise.

Multiple advantageous mutations sometimes appeared simultaneously and created groups that coexisted despite these competing mutations. Nine populations contained coexisting groups that had lasted more than 10,000 generations.

The team also find that genes targeted for natural selection change over time as the shifting genetic landscape opens up new opportunities for adaptation.

See also News & Views: No escape from the tangled bank.

Palaeontology

Ancient bird gland gives up its chemical secrets

Molecules used to preen feathers identified in 48-million-year-old fossil.

Organic molecules extracted from a 48-million-year-old fossilized bird are the remains of lipids produced by a gland involved in feather maintenance.

Previous studies have suggested that structures found in fossilized birds from Germany’s Messel Pit fossil site are preserved uropygial glands, which secrete oil for preening the feathers.

Shane O’Reilly at the Massachusetts Institute of Technology in Cambridge and his colleagues analysed one bird fossil using mass spectroscopy, and identified a range of alkenes, alkanes and complex lipid molecules in the purported gland. Comparisons with uropygial secretions from modern birds suggest that the preserved molecules are the chemically altered remains of wax esters used to maintain feathers. This shows that even lipid-rich soft tissues can be preserved over geological time scales, the authors say.

 

Medical research

Stretchy implants for growing bodies

Technology could one day reduce operations on children.

Implants that can stretch as a child grows could reduce the need for repeated operations.

Pedro del Nido and Jeffrey Karp at Harvard Medical School in Boston, Massachusetts, and their colleagues designed a surgical implant that adapts to growing tissues. They placed a biodegradable polymer core inside a braided sleeve, which is designed to stretch and thin out as the core degrades.

The group tested the device on the side of the left shin bone of three young rats and found that it guided normal growth. By contrast, fixed-size implants in three other rats restricted shin growth. The team also found that the device could support normal growth of heart valves in piglets, in a procedure mimicking surgery to repair leaking valves. Tweaking the braid design and the speed of the core’s degradation tunes the rate of elongation, the researchers say, and could lead to devices that minimize the need for repeated, risky surgery on growing children.

The Nilometer building in Cairo was used to measure the level of Nile in ancient times. Ivan Vdovin/Alamy

Climate sciences

Volcanoes stirred conflict in ancient Egypt

Disruption of Nile flooding, plus social stress, has been linked to revolts in the Ptolemaic dynasty.

Volcanic eruptions may have triggered conflict in ancient Egypt, by disrupting annual flooding of the Nile.

Egyptians in the time of the Ptolemaic dynasty, which ruled from 305 to 30 bc, depended heavily on summer flooding for crop production.

Francis Ludlow of Trinity College Dublin, and his colleagues analysed ice-core data, which preserves evidence of ancient volcanic eruptions, as well as historic measurements of the Nile from the Nilometer building in Cairo and written records of the time. These data, and modelling based on climatic impacts of modern volcanism, suggest that ancient eruptions disrupted the annual Nile flood. This in turn both helped to trigger revolts against the empire and forced it to halt wars in which it was engaged, say the authors. They caution that other social factors were also important in these events, including high levels of taxation and ethnic tensions.

Physics

A new way to measure electron charge asymmetry

Watching particles wobble helps pin down shape of their electric field.

The electron is believed to be a pointlike particle. According to the standard model of particle physics, there should be virtually no asymmetry in an electron’s electric field — that is, its electric dipole moment should be close to zero. But some theories predict a larger charge imbalance, and hence a larger dipole moment.

By placing molecules in an electric field and seeing whether the electrons within them ‘wobble’ on their axes, like spinning tops, physicists hope to determine whether a non-zero dipole moment exists. Although they have not yet proved that it does, they have managed to place an upper bound on its size. The most precise limit so far has been determined using beams of neutral molecules.

Now, William Cairncross at the University of Colorado, Boulder, and his collaborators have introduced a second technique, based on trapping molecular ions. Their first results, using ionic hafnium fluoride, came within a factor of two of the best previous estimates and give an electric dipole moment of less than 1.3 × 10−28 e cm, meaning, in effect, that they found no dipole moment.

The loss of large herbivores, such as elephants, from modern ecosystems could mean the loss of a vital ecosystem service.

The loss of large herbivores, such as elephants, from modern ecosystems could mean the loss of a vital ecosystem service. Mike Fisher/EyeEm/Getty

Ecology

Rise of plant-eating dinosaurs spread nutrients across the globe

Coal deposits show the importance of large animals’ faeces.

The rise of huge herbivores around 300 million years ago significantly increased the availability of nutrients globally as the animals defecated waste far from where they ate.

Previous models have predicted this phenomenon, but empirical evidence has been lacking. Christopher Doughty of Northern Arizona University in Flagstaff looked at data on coal deposits from 323 to 299 million years ago, a period before large herbivores walked the planet. He compared these to coal from the Cretaceous period 145 to 66 million years ago, when the largest herbivores ever — the sauropods — roamed Earth.

Concentrations of nutrients vital to plants and animals were on average 136% higher in coal produced by vegetation from the Cretaceous than in coal laid down before the big herbivores appeared. These nutrients were also more widely distributed. But there was no significant difference between the two data sets in the distribution of aluminium, which is shifted around through the weathering of rocks over time and not by animals. 

This finding indicates that ongoing reductions in the populations of large herbivores, such as forest elephants, could also impoverish ecosystems.

Cancer

Cancer cells recycle ammonia waste to grow

Tumours dodge toxic effects by turning the by-product into amino acids.

Breast cancer cells can reuse their ammonia waste to grow, research suggests.

Toxic ammonia by-products from healthy cells are generally removed from the body as urea, through the liver. But many tumours lack the blood vessels needed to tap into the excretory system, so ammonia builds up in the surrounding area.

Now Marcia Haigis at Harvard Medical School in Boston, Massachusetts, and her colleagues report that ammonia accumulates around tumours and is used by their cells to synthesize amino acids. Breast cancer cells can efficiently metabolize up to 57% of nearby ammonia into useful glutamate, which is then used to build amino acids such as proline and aspartate, they found.

This recycling means that breast cancer cells avoid the toxic effects of ammonia by converting the compound into usable nitrogen, a necessary agent for rapid growth.

The New Zealand sea lion (Phocarctos hookeri) is an endangered species and is in decline. Tobias Bernhard/Getty

Conservation biology

Covering up the by-catch problem

‘Exclusion devices’ seem to mask the impact of trawling on marine mammals.

Devices designed to release endangered New Zealand sea lions when they are caught by fishing boats may be concealing the negative effects of trawling. 

Many fishing boats are equipped with such devices, which release unintentionally caught animals. Stefan Meyer at the University of Otago in Dunedin, New Zealand, and his colleagues looked at the frequency of contact between sea lions (Phocarctos hookeri) and squid trawl nets in waters around the Auckland Islands, and at the number of pups produced, between 1995 and 2015. 

Although the number of sea lions recorded as caught declined after 2001, when exclusion devices were installed, the birthrate of these animals did not increase. Modelling suggests that 35% of the annual variation in pup production was due to encounters between sea lions and fishing vessels. The team suggests that sea lions released by exclusion devices probably die shortly afterwards, or fail to reproduce, owing to injuries caused by the devices.

Chemistry

Rare drug made in major amounts

Total synthesis of complicated natural molecule could overcome supply problem.

A scarce but potent natural product that has shown promise as a medicine has been synthesized in the lab.

Bryostatin 1 is a candidate drug for treating HIV/AIDS, Alzheimer’s disease and cancer. But it is notoriously difficult to harvest from the wild. The US National Cancer Institute had an initial stock of only 18 grams, which was extracted from 13 metric tonnes of a marine organism called Bugula neritina.

Paul Wender at Stanford University in California and his colleagues used convergent synthesis to produce different sections of the complicated multi-ringed structure separately, which they then assembled to create more than 2 grams of the desired molecule. The longest production sequence for a single section consisted of 19 separate reactions, as part of an overall recipe of 29 steps in total. The average yield was more than 80% per step, for an overall yield of 4.8%. The authors note that the only previously reported synthesis of bryostatin 1 had 57 steps. 

Immunology

Immune cells go home to die

Neutrophil white blood cells can end up back in the bone marrow once their job is done.

Neutrophils are white blood cells produced in the bone marrow that can fight infection and inflammation by ingesting harmful microorganisms. But some evidence has suggested that if neutrophils do not die in a wound, they might leave and cause inflammation elsewhere in the body. 

Now researchers have found that the cells sometimes return to the bone marrow and self-destruct.

Paul Kubes at the University of Calgary in Canada and his colleagues used fluorescence imaging to track the activity of these cells in mice with damage to the liver. They watched as neutrophils infiltrated wounds and cleaned up DNA fragments, as expected. But 24 hours later, many of the cells had returned to the bone marrow and initiated their own deaths. This round trip could be essential for the resolution of inflammation, the authors say.

How the Sun’s corona can be millions of degrees hotter than its surface has been a mystery.

How the Sun’s corona can be millions of degrees hotter than its surface has been a mystery. NASA/SDO

Astronomy and astrophysics

Tiny, fleeting flares might heat Sun’s corona

Short-lived ‘nanoflares’ could explain why the Sun’s outer atmosphere can be several million degrees hotter than its surface.

Fleeting ‘nanoflares’ that are too small to see directly could explain why the Sun’s outer atmosphere, or corona, can be millions of degrees hotter than its surface. 

Giant eruptions of ionized gas occasionally release energy into the Sun’s outer layers, but this happens too rarely to explain the zone’s usual temperature of 1 million to 3 million kelvin.

Shin-nosuke Ishikawa at the Japan Aerospace Exploration Agency in Kanagawa and his colleagues looked for another potential source of heat: a plethora of small, short-lived explosions. The team compared X-ray observations of a region of the Sun made using a research rocket with data on the same area collected by the US Geostationary Operational Environmental Satellite. Although no distinct flares were indicated by the satellite data, the X-ray maps showed a faint emission, suggestive of a small amount of plasma at an extraordinary 10 million kelvin. The presence of nanoflares could explain this finding, say the authors. They add that the results will constrain models of how these flares could be distributed.

The huge South Pole-Aitken basin on the Moon was created by a massive impact around 4 billion years ago.

The huge South Pole-Aitken basin on the Moon was created by a massive impact around 4 billion years ago. NASA/GSFC/DLR/Arizona State Univ./Lunar Reconnaissance Orbiter

Planetary science

Moon mantle’s make-up revealed

Huge asteroid impact on far side ejected rocks from deep under the crust.

The surprising composition of the Moon’s upper mantle — the layer below the crust — is revealed by ancient asteroid-impact debris.

Planetary scientists have long searched for olivine on the Moon, because this mineral is commonly found in Earth’s upper mantle. They thought that huge asteroid impacts would have punched through to the lunar mantle, leaving olivine scattered around the surface.

Jay Melosh at Purdue University in West Lafayette, Indiana, and his colleagues used computer simulations to show that one such impact — which created the 2,500-kilometre-wide South Pole-Aitken basin — blasted into the Moon to a depth of at least 100 kilometres. Debris from this farside impact would have scattered across the surrounding highlands, the simulations show.

But that area is covered by a different mineral: low-calcium pyroxene. So the Moon’s mantle must be mostly made of low-calcium pyroxene, the team argues — not olivine, as long thought.

Ice formed six-leaved-clover patterns on hydrophobic surfaces, and sunflower-like arrangements on hydrophilic ones.

Ice formed six-leaved-clover patterns on hydrophobic surfaces, and sunflower-like arrangements on hydrophilic ones. J. Liu et al./PNAS

Physics

Watching how surface properties affect ice growth

Water-repellent surfaces trigger formation of crystals shaped like six-leaved-clover.

Ice crystals grow on solid surfaces in two distinctly different ways, depending on the degree to which the surface attracts or repels water. Jianjun Wang at the Chinese Academy of Sciences in Beijing and his colleagues triggered ice formation using silver iodide nanoparticles, and watched as crystals formed on hydrophilic and hydrophobic surfaces.

When ice formed on the former, it grew along the surface, producing sunflower-like patterns (bottom). On the latter, it minimized its contact with the substrate by growing away from the surface at an angle, and forming six-leaved-clover shapes (top). 

Although the influence of factors such as temperature and humidity on ice formation has been well studied, the influence of surface properties is less well understood. The new findings could inform the design of surfaces that need to repel ice, such as windscreens.

This ceramic gear is part of a pump that can move molten tin at temperatures of 1,400 °C.

This ceramic gear is part of a pump that can move molten tin at temperatures of 1,400 °C. Christopher Moore/Georgia Tech

Materials science

Ceramic pump is hot-tin proof 

Machine withstands molten metal at more than 1,000 °C.

A ceramic pump can transport liquid tin heated to 1,400 °C, the highest temperature any liquid has been pumped at so far.

Molten metals offer an attractive option for transporting thermal energy, but finding materials that can contain them at extreme temperatures is tricky.

Asegun Henry at the Georgia Institute of Technology in Atlanta and his team made a mechanical pump entirely from ceramics including Shapal — a mixture of aluminium and boron nitrides. Ceramics had been considered too brittle for such use, but the team was able to pump liquid tin through the machine for three days at temperatures averaging 1,200 °C. The carefully engineered ceramics held fast under mechanical pressures and did not react with the flowing metal, even when temperatures peaked at 1,400 °C. 

Ceramic pumps could be used to store and redistribute heat energy that is a waste product of many industrial activities, the authors say.

This artist’s impression shows the ring system of Haumea that was discovered earlier this year.

This artist’s impression shows the ring system of Haumea that was discovered earlier this year. IAA-CSIC/UHU

Astronomy and astrophysics

A ring far beyond Pluto

Dwarf planet Haumea joins select list of Solar System bodies with rings.

The dwarf planet Haumea, which orbits 1.9 billion kilometres beyond Neptune, has a ring — the first object beyond the orbit of Neptune known to have such a feature.

A team led by Jose Ortiz from the Institute of Astrophysics of Andalusia in Spain combined observations taken by multiple telescopes of changes in light intensity as Haumea passed in front of a star. Dips in the light just before and after the main bulk of Haumea obscured the distant star indicated the presence of a dense, narrow ring encircling the planet’s equator. The team estimate the ring’s radius at 2,287 kilometres and width at 70 kilometres. Only two other minor planets have been observed with rings: Chariklo and Chiron, which orbit the Sun between Jupiter and Neptune.

The observations also enabled the team to calculate that Haumea is about one-third as dense as Earth and shaped like a rugby ball.