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Part I：Speaker
Which Came First: Low CO2 or an Ice Age?
【Rephrase 1】
[Dialog, 1:21]
MP3：
[Transcript hided]
What starts an Ice Age? Clues exist in the remains of coccolithophores, a type of marine algae with a shell. A study finds that some seven million years ago, the algae had to adapt to low levels of carbon dioxide in the atmosphere. They pulled CO2 from the surrounding seas for photosynthesis as well as bicarbonate—commonly known as baking soda. The study is in the journal Nature. [Clara T. Bolton and Heather M. Stoll, Late Miocene threshold response of marine algae to carbon dioxide limitation] At that same time, sea surface temperatures were dropping, plants that were more efficient at using CO2 came to predominate on land and vast glaciers began to expand on the continents—an Ice Age was underway. The low concentrations of greenhouse gases in the atmosphere were thus linked to the era’s cool climate. That situation is now reversed thanks mostly to fossil fuel burning. And the change is happening at least 30,000 times faster this time. In May, atmospheric concentrations of CO2 touched 400 parts per million for the first time in human existence. When they touch 500 ppm, the algae might no longer need the bicarbonate trick. [End]
https://www.scientificamerican.com/podcast/episode.cfm?id=which-came-first-low-co2-or-an-ice-13-09-01
Part II：Speed
Food-borne illnesses are not always home-grown
【Time 2】
Scottish cows have a bum rap. For decades, the local cattle have been prime suspects behind the country’s outbreaks of drug-resistant, food-borne illnesses. But research now suggests that humans and imported foods are the real culprits.
A team of researchers compared the genome sequences of nearly 400 samples of diarrhoea-causing Salmonella enterica collected from people and livestock in Scotland. They found that bacterial strains infecting humans were largely distinct from those found in local cattle, but had close ties to strains that had been isolated in other countries.
The results suggest that mass epidemics may spark from a complicated intermingling of bacteria between animals and humans and from exchanges between different countries, the authors say. Their findings are published today in Science1.
“There is a pervading wisdom that local animals are a predominant source of pathogens and resistance,” says study co-author Stuart Reid, a veterinary epidemiologist at the Royal Veterinary College in Hatfield, UK. But as his team's findings show, that may not always be the case. “It’s only if we can treat this as an international issue that we’re going to get to the bottom of it,” he says.
Reid and his colleagues focused on Scottish outbreaks because of the country’s ample collection of bacterial samples obtained from both humans and livestock. The collection was started 23 years ago, when global epidemics of drug-resistant salmonella infections began to arise.
Livestock was assumed to be the source of the epidemics because animals naturally harbour the bacteria. To find out whether this was really the case, the team used whole-genome sequencing to trace the tiny evolutionary steps of the collected bacterial strains. They analysed 142 samples isolated from Scottish patients and 120 from local animals, mostly cows, then compared them with 111 strains collected from people and animals in other countries.
(words:300)
【Time 3】
Foreign imports
The team found that strains infecting Scottish patients were different from those in local livestock. And they noted only a few instances in which strains isolated from local livestock had spread to humans. But they also found that strains could spread from humans to animals. “It’s occurring at a low frequency, but in both directions,” explains lead author Alison Mather, an epidemiologist at the Wellcome Trust Sanger Institute near Cambridge, UK.
When they looked at the strains' antimicrobial resistance, the researchers found that bacteria from humans had more diverse collections of resistance genes than those in local livestock. This indicates that local livestock cannot be the sole source of the resistance genes found in the strains found in humans.
The authors therefore suggest that local livestock are not the source of drug-resistant human salmonella outbreaks in Scotland. Rather, they say, foreign strains carried by other humans and in imported food probably entered the country and infected animals and humans separately, then continued to evolve and acquire resistance separately.
A global issue
The authors stress that the study does not imply that antimicrobial resistance developed on farms is less concerning than previously thought, including resistance stemming from the controversial practice of giving antibiotics in feed to promote animal growth.
"We're not saying it's not as bad, we're just saying that there are other sources that need to be considered," says Reid. Though local animals were not a main source of these pathogens, he explains that it does not eliminate the possibility that resistance genes from local farms and foreign farms played a role.
Mark Woolhouse, an epidemiologist at the University of Edinburgh, UK, says that the study clarifies how pathogens and drug-resistance genes spread. “It’s not just multi-bug, multi-drug,” he says, “but multi-country.”
Scotland imports most of its red meat, but the authors say that the country does not have adequate surveillance in place to determine whether imported food is a source of new pathogens. Both Woolhouse and the authors call for Scotland and other countries to boost the monitoring of their food supply.
Lance Price, a genomic epidemiologist at the George Washington University in Washington DC, says that it is not surprising that Scottish cattle are not the source of Scottish outbreaks, because the epidemics were international. He notes, however, that, to eliminate the possibility of a domestically derived outbreak, the authors should have analysed more strains from poultry and pigs, which also carry S. enterica.
“But it underscores that this is a global issue,” he says. “Meat sale and meat trade across borders is making it harder to control antibiotic-resistant pathogens at a local scale.”
(words:438)
http://www.nature.com/news/food-borne-illnesses-are-not-always-home-grown-1.13736
Genes for body symmetry may also control handedness
Left- or right-handedness may be determined by the genes that position people’s internal organs.
【Time 4】
About 10 percent of people prefer using their left hand. That ratio is found in every population in the world and scientists have long suspected that genetics controls hand preference. But finding the genes has been no simple task, says Chris McManus, a neuropsychologist at University College London who studies handedness but was not involved in the new research.
“There’s no single gene for the direction of handedness. That’s clear,” McManus says. Dozens of genes are probably involved, he says, which means that one person’s left-handedness might be caused by a variant in one gene, while another lefty might carry variants in an entirely different gene.
To find handedness genes, William Brandler, a geneticist at the University of Oxford, and colleagues conducted a statistical sweep of DNA from 3,394 people. Statistical searches such as this are known as genome-wide association studies; scientists often do such studies to uncover genes that contribute to complex diseases or traits such as diabetes and height. The people in this study had taken tests involving moving pegs on a board. The difference in the amount of time they took with one hand versus the other reflected how strongly left- or right-handed they were.
(words:212)
【Time 5】
A variant in a gene called PCSK6 was most tightly linked with strong hand preference, the researchers report in the Sept. 12 PLOS Genetics.. The gene has been implicated in handedness before, including in a 2011 study by the same research group. PCSK6 is involved in the asymmetrical positioning of internal organs in organisms from snails to vertebrates.
Brandler, who happens to be a lefty, knew the gene wasn’t the only cause of hand preference, so he and his colleagues looked at other genetic variants that didn’t quite cross the threshold of statistical significance. Many of the genes the team uncovered had previously been shown in studies of mice to be necessary for correctly placing organs such as the heart and liver. Four of the genes when disrupted in mice can cause cilia-related diseases. Cilia are hairlike appendages on cells that act a bit like GPS units and direct many aspects of development of a wide range of species, including humans.
One of the cilia genes, GLI3, also helps build the corpus callosum, a bundle of nerves that connects the two hemispheres of the brain. Some studies have suggested that the structure is bigger in left-handers.
It’s still a mystery how these genes direct handedness, says Larissa Arning, a human geneticist at Ruhr University Bochum in Germany. In addition to genes that direct body plans, she says, the study suggests that many more yet-to-be-discovered genes probably play a role in handedness.
Brandler hopes the study will also help remove some of the stigma of being left-handed. Left-handedness isn’t a character flaw or a sign of being sinister, he says: “It’s an outcome of genetic variation.”
(words:275)
http://www.sciencenews.org/view/generic/id/353240/description/Genes_for_body_symmetry_may_also_control_handedness
Grey wolves left out in the cold
【Time 6】
Central Kentucky is coyote country. But the 33-kilo¬gram animal shot by a hunter near Munfordville this spring was definitely not a coyote. Its huge paws, broad snout and massive build suggested that it was a grey wolf (Canis lupus) — the first to be shot in Kentucky in more than 150 years. DNA tests confirmed the animal’s identity in August.
The animal, a possible stray from hundreds of kilometres away in Michigan or Minnesota (although it cannot be ruled out that it was once captive), was also a player in a growing debate that mixes science, politics and passionate public opinion. From Kentucky to California, wolves are forcing biologists and policy-makers to re-examine the US Endangered Species Act (ESA) and the very definition of an ‘endangered’ species.
The act, introduced in 1973, was a landmark piece of legislation. Its purpose has been contentious ever since, but it is intended to save species “in danger of extinction throughout all or a significant portion” of their range. Although wolves have never been at risk of extinction in the United States as a whole, those in the 48 contiguous states were classified as endangered in 1978.
After decades of federal protection and re¬introduction programmes, the US Fish and Wildlife Service (FWS) undertook a comprehensive review, which found that wolf populations near the western Great Lakes and the northern Rocky Mountains had recovered sufficiently to warrant removing ESA protection. (There are now about 4,000 wolves in the Great Lakes area and nearly 1,700 in the northern Rockies.) Wolves in these areas were ‘delisted’ between May 2011 and August 2012.
But in June this year, the FWS proposed removing ESA protection from all US grey wolves, citing the earlier review as evidence of their recovery and arguing that the original listing had erroneously included regions outside the species’ historical range. The agency says that by delisting the rest of the US wolf population, it can concentrate its resources on ESA protection for the Mexican wolf (Canis lupus baileyi), a subspecies of the grey wolf.
(words:338)
http://www.nature.com/news/grey-wolves-left-out-in-the-cold-1.13716
Part III： Obstacle
Tropical Forest Carbon Absorption May Hinge On an Odd Couple
【Time 7】
Tropical forests thrive on natural nitrogen fertilizer pumped into the soil by trees in the legume family, a diverse group that includes beans and peas, the researchers report in the journal Nature. The researchers studied second-growth forests in Panama that had been used for agriculture five to 300 years ago. The presence of legume trees ensured rapid forest growth in the first 12 years of recovery and thus a substantial carbon "sink," or carbon-storage capacity. Tracts of land that were pasture only 12 years before had already accumulated as much as 40 percent of the carbon found in fully mature forests. Legumes contributed more than half of the nitrogen needed to make that happen, the researchers reported.
These fledgling woodlands had the capacity to store 50 metric tons of carbon per hectare (2.47 acres), which equates to roughly 185 tons of carbon dioxide, or the exhaust of some 21,285 gallons of gasoline. That much fuel would take the average car in the United States more than half a million miles. Though the legumes' nitrogen fertilizer output waned in later years, the species nonetheless took up carbon at rates that were up to nine times faster than non-legume trees.
The legumes' secret is a process known as nitrogen fixation, carried out in concert with infectious bacteria known as rhizobia, which dwell in little pods inside the tree's roots known as root nodules. As a nutrient, nitrogen is essential for plant growth, but tropical soil is short on nitrogen and surprisingly non-nutritious for trees. Legumes use secretions to invite rhizobia living in the soil to infect their roots, and the bacteria signal back to initiate nodule growth. The rhizobia move into the root cells of the host plant and -- in exchange for carbohydrates the tree produces by photosynthesis -- convert nitrogen in the air into the fertilizer form that plants need. Excess nitrogen from the legume eventually creates a nitrogen cycle that benefits neighboring trees.
By nurturing bigger, healthier trees that take up more carbon, legumes have a newly realized importance when it comes to influencing atmospheric carbon dioxide, said second author Lars Hedin, a Princeton professor of ecology and evolutionary biology and the Princeton Environmental Institute. Scientists have recently put numbers on how much carbon forests as a whole absorb, with a recent paper suggesting that the world's forests took up 2.4 quadrillion tons of carbon from 1990 to 2007.
"Tropical forests are a huge carbon sink. If trees could just grow and store carbon, you could have a rapid sink, but if they don't have enough nitrogen they don't take up carbon," said Hedin, adding that nitrogen-fixing trees are uncommon in temperate forests such as those in most of North America and Europe.
"Legumes are a group of plants that perform a valuable function, but no one knew how much they help with the carbon sink," Hedin said. "This work shows that they may be critical for the carbon sink, and that the level of biodiversity in a tropical forest may determine the size of the carbon sink."
First author Sarah Batterman, a postdoctoral research associate in Hedin's research group, said legumes, or nitrogen fixers, are especially important for forests recovering from agricultural use, logging, fire or other human activities. The researchers studied 16 forest plots that were formerly pasture and are maintained by the Smithsonian Tropical Research Institute (STRI).
Forest degradation, however, comes with a loss of biodiversity that can affect nitrogen fixers, too, even though legumes are not specifically coveted or threatened, Batterman said. If the numbers and diversity of nitrogen fixers plummet then the health of the surrounding forest would likely be affected for a very long time.
"This study is showing that there is an important place for nitrogen fixation in these disturbed areas," Batterman said. "Nitrogen fixers are a component of biodiversity and they're really important for the function of these forests, but we do not know enough about how this valuable group of trees influences forests. While some species may thrive on disturbance, others are in older forests where they may be sensitive to human activities."
The researchers found that the nine legume species they studied did not contribute nitrogen to surrounding trees at the same time. Certain species were more active in the youngest forests, others in middle-aged forests, and still other species went into action mainly in 300-year-old tracts, though not nearly to the same extent as legumes in younger plots. The researchers found that individual trees reduced their fixation as nitrogen accumulated in soils, with the number of legumes actively fixing nitrogen dropping from 71 to 23 percent between 12- and 80-year-old forests.
"In that way, the diversity of species that are present in the forest is really critical because it ensures that there can be fixation at all different time periods of forest recovery whenever it's necessary," Batterman said. "If you were to lose one of those species and it turned out to be essential for a specific time period, fixation might drop dramatically."
Such details can improve what scientists know about future climate change, Batterman said. Computer models that calculate the global balance of atmospheric carbon dioxide also must factor in sinks that offset carbon, such as tropical forests. And if forests take up carbon differently depending on the abundance and diversity of legumes, models should reflect that variation, she said. Batterman is currently working with Princeton Assistant Professor of Geosciences David Medvigy on a method for considering nitrogen fixation in models.
"This finding is really important because other researchers can now go and put this role of nitrogen fixation into their models and improve predictions about the carbon sink," Batterman said.
Batterman and Hedin worked with Michiel van Breugel, an STRI postdoctoral fellow; Johannes Ransijn, a University of Copenhagen doctoral student in geosciences and natural-resource management; Dylan Craven, a Yale University doctoral candidate in forestry and environmental studies; and Jefferson Hall, an STRI staff scientist and leader of the institute's Agua Salud Project that maintains and studies the plots the researchers examined.
(words:1005)
http://www.sciencedaily.com/releases/2013/09/130915134349.htm
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