Wednesday, November 3, 2010


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Some bacteria react to the cold by subtly changing the chemistry of their outer wall so that it remains pliable as temperatures drop. Scientists identified a key protein in this response mechanism a few years ago, but the question of how bacteria sense cold in the first place remained a mystery. Based on a study by scientists at Rice University and Argentina's National University of Rosario, the answer is: They use a measuring stick.

The study, published in the September issue of Current Biology, involved a series of intricate experiments on the bacteria Bacillus subtilis. The researchers found a specialized protein that protrudes through the bacteria's outer cell wall acts as a measuring stick that's tuned to give a signal when temperatures outside the cell drop.

Scientists have long known that cells use specialized proteins called "transmembrane" proteins to sense and react to the outside world. Transmembrane proteins protrude through the cell's outer wall, or membrane.

"All living cells have the ability to respond to external stimuli, but in most cases that we are aware of, signal recognition -- the event that triggers the response -- occurs when a transmembrane protein binds physically to another chemical outside the cell," said study co-author Ariel Fernandez, research professor at Rice.

Fernandez said the Bacillus subtilis study is one of the first to determine how a transmembrane protein can respond indirectly to a physical stimulus outside the cell. The research was highlighted in review articles in both Current Biology and Nature Reviews Microbiology.

Fernandez and colleagues examined a transmembrane protein called DesK (pronounced des-KAY). In previous studies, scientists had found that DesK responded to cold temperatures by causing the cell to make a special compound that keeps the membrane pliable. Without the compound, the fatty acids inside the cell wall become more rigid as temperatures fall.

Fernandez and colleagues found that the part of the DesK protein that protrudes outside the cell contains a sensitized tip. As long as the tip remains in contact with water molecules outside the cell, DesK remains switched off. As temperatures fall and the cell membrane becomes more rigid, the membrane also becomes thicker. As it thickens, it engulfs the sensitized end of the temperature probe, cutting off contact with water molecules outside the cell. This, in turn, activates DesK and sends the signal to release the cold-protecting chemicals. This mechanism, which Fernandez named the buried buoy trigger, was proposed by Fernandez and probed experimentally by the Argentinean team.

The molecular biology and experimental probes were conducted in the laboratory of Diego de Mendoza at the National University of Rosario in Rosario, Argentina. To confirm the findings, the group constructed versions of DesK proteins of varying lengths. Using these as longer or shorter measuring sticks, the researchers confirmed that the signaling mechanism was triggered based upon whether the tip of the transmembrane sensor remained in contact with water molecules outside the membrane.

Rice University


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Using new optics technology developed at the University of Arizona's Steward Observatory, an international team of astronomers has obtained images of a planet on a much closer orbit around its parent star than any other extrasolar planet previously found.

The discovery, published online in Astrophysical Journal Letters, is a result of an international collaboration among the Steward Observatory, the Swiss Federal Institute of Technology Zurich, the European Southern Observatory, Leiden University in the Netherlands and Germany's Max-Planck-Institute for Astronomy.

Installed on the European Southern Observatory's Very Large Telescope, or VLT, atop Paranal Mountain in Chile, the new technology enabled an international team of astronomers to confirm the existence and orbital movement of Beta Pictoris b, a planet about seven to 10 times the mass of Jupiter, around its parent star, Beta Pictoris, 63 light years away.

At the core of the system is a small piece of glass with a highly complex pattern inscribed into its surface. Called an Apodizing Phase Plate, or APP, the device blocks out the starlight in a very defined way, allowing planets to show up in the image whose signals were previously drowned out by the star's glare.

"This technique opens new doors in planet discovery," said Phil Hinz, director of the UA's Center for Astronomical Adaptive Optics at Steward Observatory. "Until now, we only were able to look at the outer planets in a solar system, in the range of Neptune's orbit and beyond. Now we can see planets on orbits much closer to their parent star."

In other words, if alien astronomers in another solar system were studying our solar system using the technology previously available for direct imaging detection, all they would see would be Uranus and Neptune. The inner planets, Mercury, Venus, Earth, Mars and Saturn, simply wouldn't show up in their telescope images.

To put the power of the new optics system in perspective: Neptune's mean distance from the sun is about 2.8 billion million miles, or 30 Astronomical Units, or AUs. One AU is defined as the mean distance between the sun and the Earth.

The newly imaged planet, Beta Pictoris b, orbits its star at about seven AUs, a distance where things get especially interesting, according to Hinz, "because that's where we believe the bulk of the planetary mass to be in most solar systems. Between five and 10 AUs."

While planet hunters have used a variety of indirect methods to detect the "footprints" of extrasolar planets – planets outside our solar system – for example the slight gravitational wobble an orbiting planet induces in its parent star, very few of them have been directly observed.

According to Hinz, the growing zoo of extrasolar planets discovered to date – mostly super-massive gas giants on wide orbits – represents a biased sample because their size and distance to their parent star makes them easier to detect.

"You could say we started out by looking at oddball solar systems out there. The technique we developed allows us to search for lower-mass gas giants about the size of Jupiter, which are more representative of what is out there."

He added: "For the first time, we can search around bright, nearby stars such as Alpha Centauri, to see if they have gas giants."

The breakthrough, which may allow observers to even block out starlight completely with further refinements, was made possible through highly complex mathematical modeling.

"Basically, we are canceling out the starlight halo that otherwise would drown out the light signal of the planet," said Johanan (John) Codona, a senior research scientist at the UA's Steward Observatory who developed the theory behind the technique, which he calls phase-apodization coronagraphy.

"If you're trying to find something that is thousands or a million times fainter than the star, dealing with the halo is a big challenge."

To detect the faint light signals from extrasolar planets, astronomers rely on coronagraphs to block out the bright disk of a star, much like the moon shielding the sun during an eclipse, allowing fainter, nearby objects to show up.

Using his own unconventional mathematical approach, Codona found a complex pattern of wavefront ripples, which, if present in the starlight entering the telescope, would cause the halo part to cancel out but leave the star image itself intact. The Steward Observatory team used a machined piece of infrared optical glass about the size and shape of a cough drop to introduce the ripples. Placed in the optical path of the telescope, the APP device steals a small portion of the starlight and diffracts it into the star's halo, canceling it out.

"It's a similar effect to what you would see if you were diving in the ocean and looked at the sun from below the surface," explained Sascha Quanz from the Swiss Federal Institute of Technology's Institute for Astronomy, the lead author of the study. "The waves on the surface bend the light rays and cause the sky and clouds to appear quite different. Our optic works in a similar way."

In order to block out glare from a star, conventional coronagraphs have to be precisely lined up and are highly susceptible to disturbance. A soft night breeze vibrating the telescope might be all it takes to ruin the image. The APP, on the other hand, requires no aiming and works equally well on any stars or locations in the image.

"Our system doesn't care about those kinds of disturbances," Codona said. "It makes observing dramatically easier and much more efficient."

In the development of APP, Codona was joined by Matt Kenworthy (now at Leiden Observatory in the Netherlands). Hinz, who is a member of the instrument upgrade team for the VLT, played a key role in the technique's implementation at the MMT Observatory on Mount Hopkins in southeastern Arizona.

Former UA astronomy professor Michael Meyer, now at the Swiss Federal Institute of Technology Zurich, where he led the group implementing the technology on the VLT, pointed out that APP is likely to advance areas of research in addition to the hunt for extrasolar planets.

"It will be exciting to see how astronomers will use the new technology on the VLT, since it lends itself to other faint structures around young stars and quasars, too."

(Photo: ESO)

University of Arizona


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The food chain — the number of organisms that feed on each other — in the world’s streams and rivers depends more upon the size of the stream and whether the waterways flood or run dry than the amount of available food resources, Yale University and Arizona State University (ASU) researchers report online in the Oct. 14 issue of the journal Science Express.

The findings suggest that large predators in river systems will be threatened by increased variability in water flow induced by climate change. The research also helps settle an old debate among ecologists about what determines the length of nature’s food chains, which sustain all life on earth.

“The food web is a regulatory network of ecosystems, and for nearly 100 years ecologists have debated the causes of variation in the length of the food chains, said David Post, professor of ecology and evolutionary biology at Yale and co-author of the study.

Researchers from Yale, ASU, the University of Minnesota, and the U.S. Geological Survey studied 36 North American streams and rivers. The researchers found that food chains – or the number of mouths that food passes through on the way to top predators – got longer as the size of the body of water increased. The findings are similar to another study conducted by Post a decade ago that found the key factor in food chain length was lake size, not the amount of food resources in a system, as many ecologists had believed.

A longer food chain supports more organisms and larger predators such as big fish but may also increase the concentration of contaminants in larger predators. However, the new study found that the more streams and rivers dried up or flooded, the shorter the food chain. This in turns puts pressure on the ecosystem’s ability to support organisms, particularly larger predators. In fact, when water is withdrawn from a stream or a river dries up it may be decades before the food chain recovers.

“Even very large rivers around the world are drying with increasing frequency and global climate models predict many rivers will experience more variable flows, both high and low,” said John L. Sabo, professor of ecology, evolution and environmental science at ASU and co-author of the study. “Our results suggest these changes to hydrology will simplify river food webs and increase the likelihood of loosing many top predator fish species from aquatic ecosystems.”

As climate variability increases, so will the number of disputes over water use, such as clash a decade ago between farmers and environmentalists over withdrawal of water from the Klamath River Basin, which flows from Oregon into northern California.

“Understanding what determines food chain length will help us make better policy decisions,” Post said.

(Photo: Yale U.)

Yale University




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