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Posted June 18, 2018 4:26 p.m. EDT

The Twilight of the Baobabs, Africa’s ‘Wooden Elephants’

On January 7, 2016, a group of tourists set out to visit Chapman’s Baobab, one of the oldest and mightiest trees in Africa.

Visible from miles away, it had long served as a landmark for travelers and explorers, including David Livingstone. The cavity inside its trunks — with an outer circumference of more than 80 feet — reportedly served as one of the continent’s first post offices. Botswana considered the tree a national monument and promoted it as a sightseeing attraction.

As the visitors neared that day, they heard a cracking boom like thunder. A cloud of dust obscured the site: Chapman’s Baobab had collapsed.

At around 1,400 years old, the tree’s loss sparked mournful news stories, even a commemorative poem. But it was not an isolated event.

Across Africa, the oldest and largest baobabs have begun to fall and die, according to new research in the journal Nature Plants. Scientists believe that prolonged droughts and increasing temperatures may have parched the trees, leaving them unable to support the weight of their massive trunks.

“The largest and oldest trees are more sensitive to changing climatic conditions because of their large dimensions,” said Adrian Patrut, a chemist at Babes-Bolyai University in Romania and lead author of the new study.

After Chapman’s Baobab collapsed, for example, Patrut found that the tree’s water content was just 40 percent, compared with 79 percent for healthy baobabs.

Patrut and his colleagues did not set out to document the death of Africa’s “wooden elephants,” as the species are sometimes called. Instead, they wanted to date them.

“There were some fairy tales and folklore that these trees could be as old as 6,000 years,” said Karl von Reden, an oceanographer at the Woods Hole Oceanographic Institution and co-author of the new paper.

Baobabs do not regularly produce tree rings, so the team turned to radiocarbon dating. The scientists compared carbon-14 levels from small samples taken in the oldest parts of the trees to samples from other tree species whose age had been determined by counting their rings.

In 2005, the researchers began collecting samples from more than 60 of the largest African baobabs — those with trunk circumferences of at least 65 feet. The oldest trees, they found, were around 2,500 years old.

While younger baobabs so far seem to have been spared, the largest trees often host rich communities of animal life, including bats and bees that nest inside their cavities and birds adapted to building nests in the tree’s branches.

— RACHEL NUWER

Using Harpoon-Like Appendages, Bacteria ‘Fish’ for New DNA

Two bacteria are sitting near free-floating DNA. Suddenly, one bacterium shoots out a long appendage, latches onto a DNA fragment and reels in its catch. It happens fast, but it’s clear: this organism had just gone fishing.

Biologists at Indiana University recently captured this maneuver on camera for the first time.

Their findings, published June 11 in the journal Nature Microbiology, verify the existence of a harpoon-like mechanism that scientists have been piecing together for decades.

The work also advances understanding of how bacteria take up DNA from their surroundings, which is called natural transformation. That process is key to the spread of antibiotic resistance, which has made bacterial illnesses increasingly difficult to treat with conventional drugs. Each year an estimated 2 million Americans become infected with antibiotic-resistant bacteria.

Researchers knew that bacteria rely on fibers called pili to capture foreign DNA. But the exact details have remained elusive because pili — more than 10,000 times thinner than human hair — are so hard to observe, said Lori Burrows, a professor of biochemistry and biomedical sciences at McMaster University in Ontario who was not involved in the study.

“It’s cool to actually see this in action,” she said.

We typically think of genes as passed down vertically, from parent to offspring. But there are also processes called horizontal gene transfer, in which DNA moves laterally between organisms that are not parent and child.

Natural transformation is one example, and it’s an important way in which bacteria, which typically reproduce asexually, introduce variation and new traits into their genetic code, said Dr. Ankur Dalia, an assistant professor of biology and an author of the new paper.

When bacteria die, their DNA becomes up for grabs by other microbes. Through natural transformation, bacteria can gain the ability to degrade compounds like pesticides, become better at infecting hosts and evolve antibiotic resistance.

In their study, Dalia and his colleagues used a custom fluorescent dyeing process created by Courtney Ellison, a graduate student, and Yves Brun, a biology professor, to visualize natural transformation in Vibrio cholera, the bacterium that causes cholera.

Ellison recounted dashing into Dalia’s lab and blurting “you need to follow me right now” after she became the first person to directly witness the behavior.

These bacteria are actually acting “like fishermen,” Ellison said.

Eventually, the work could lead to new strategies for fighting antibiotic resistance.

— STEPH YIN

Aztec Turquoise Tiles May Solve a Mesoamerican Mystery

Turquoise was a highly prized gemstone to the ancient Aztecs and Mixtec in the region that stretches from central Mexico to Central America known as Mesoamerica. Because scientists have found little evidence of turquoise mining in Mesoamerica, some researchers have used the presence of turquoise artifacts in the area as evidence of a long-distance trade exchange with ancient civilizations thousands of miles away in the American Southwest, where turquoise mines have been found.

But a recent geochemical analysis of Aztec and Mixtec turquoise suggests that the mineral did not originate in the American Southwest, but rather in Mesoamerica. The finding, published June 13 in the journal Science Advances, also calls into question the idea that there was extensive contact between Mesoamerican and Southwest American cultures before the Spanish invasion in the 1500s.

Alyson Thibodeau, a geochemist at Dickinson College in Pennsylvania and lead author on the paper, was given a jar filled with turquoise tiles that were associated with Mesoamerican mosaics. Many had been excavated from offerings in the Templo Mayor, which was the main temple in the ancient Aztec city of Tenochtitlan, present day Mexico City. The pieces date mostly to the late 15th century. Some of the samples came from loose tiles associated with Mixteca-style turquoise mosaics held by the Smithsonian Institution in the collections of the National Museum of the American Indian.

After shaving off the tiles’ edges to remove adhesives, Thibodeau ground them up individually and dissolved them in acid. She then analyzed the samples for their isotopic fingerprints, which provided insight into their origins.

“Not only do they have isotopic signatures that are absolutely consistent with the geology of Mesoamerica,” she said, “but they are completely different from the isotopic signatures of the Southwestern turquoise deposits and artifacts that we have seen so far.”

Thibodeau said that even though archaeologists have not found remnants of turquoise mines in Mesoamerica, that does not mean they were never there.

Turquoise forms near the surface as a product of copper weathering, typically caused by rainwater or groundwater. Mesoamerica has plenty of copper mines, she said, which suggests that turquoise deposits may have once been present there, too. The isotopic signatures she found on the turquoise artifacts also matched the signatures of copper deposits found in Mesoamerica.

— NICHOLAS ST. FLEUR

Black Hole Drags Star to Dusty Death

Gulp. Burp.

And so it goes in the cruel and carnivorous universe according to Einstein.

Astronomers said this month that they had seen a giant black hole in a nearby galaxy rip apart an unfortunate wayward star and spread half of it into a messy blaze of light and heat swirling toward doom. The other half was spit outward, partly in a fiery high-energy jet at a quarter of the speed of light.

All this happened in the heart of a pair of colliding galaxies known collectively as Arp 299, about 150 million light years away. A team of astronomers, led by Dr. Seppo Mattila of the University of Turku in Finland and Miguel Perez-Torres of the Astrophysical Institute of Andalusia in Spain, teased out the story of what happened from observations of infrared, or heat, radiation and radio waves that can penetrate the dust and leak out to the rest of the universe. They published their report in the journal Science.

When Mattila and his colleagues first saw a bright burst of heat coming from the region of that black hole in January 2005, they thought they had discovered a supernova, a cataclysmic explosion in which a massive star ends its life.

When galaxies collide, as the two conglomerations of stars that make up Arp 299 are doing, Mattila explained in an email, large clouds of gas fall into the central regions. That triggers a burst of star formation and then a subsequent burst of supernova explosions as the most massive of these stars quickly burn out and die.

High-resolution radio measurements revealed that the object they observed coincided almost spot on with the black hole, but it was behaving in a way very unlike a supernova. Over the course of the next decade, it expanded into a long jet of radio energy whose head by now has traveled some 3 light years from its origin.

Supernovas don’t do this, but a star falling into a black hole could, in what is called a tidal disruption event.

Mattila and his colleagues concluded that a star passing too close to the black hole had been stretched and ripped by fierce tidal forces into a stream of hot gas.

Although the process has been studied in theoretical models and computer simulations, this destruction of a star has rarely been recorded.

— DENNIS OVERBYE