Just as people endlessly calculate how to upsize or downsize, bacteria continually adjust their volume (their stuff) to fit inside their membrane (their space). How they do this is not obvious: Petra Levin’s lab at Washington University in St. Louis has spent years trying to figure it out.

The lab had made considerable headway but still couldn’t entirely explain why some cells grew to two or three times the size of others. So they tried a new approach, looking at biosynthesis rather cell-cycle control (the coordination of division and growth), which had long been the focus of the cell-size field.

What fell out of this work was a stunningly simple revelation published online June 19 in Current Biology. Fat (lipids) limits how big bacterial cells can be. “If you prevent cells from making fat, they’re smaller, and if you give them extra fat or allow them to make more fat, they get bigger,” said Levin, professor of biology in Arts & Sciences. “Fat makes cells fat.”

“It makes sense when you think about what lipids do,” said Stephen Vadia, a postdoctoral research associate in the Levin lab and the lead author on the paper. “The membrane that defines the boundary between the inside and outside of the cell is made almost entirely of fat. So it’s not really surprising that fat synthesis would limit cell size.

“I thought, ‘Well, this seems pretty obvious, now that I see the data in front of me,'” he added, punctuating with a laugh.

Faking out cells

Scientists had been gnawing on this particular bone for many years. The basic question goes back to studies done in the 1950s, said Vadia. Scientists grew the bacteria Salmonella typhimurium in more than 20 different media with varying nutrient compositions. In the nutrient-poor media, the cells grew slowly and were small; in the nutrient-rich media, they grew faster and were larger.

“This is the basic observation that we’re trying to understand,” Vadia said. “Why do rapidly growing cells that are given a lot of nutrients end up a lot bigger than slow-growing cells in nutrient-poor environments?”

The cells in nutrient-poor media are about one-half to one-third the size of the fat cells in nutrient-rich media, he said. Previous work in the Levin lab had been able to account for 20 percent of that size difference but still didn’t understand what was responsible for the additional 80 percent shrinkage.

“Ultimately, what cells do with nutrients is take them apart and use them as building blocks to synthesize new molecules,” Vadia said. “So we wondered whether biosynthetic capacity had something to do with the size differences. And if biosynthesis was behind it, was it biosynthetic capacity in general? Or are particular biosynthetic pathways important for cell size?”

To find out, they added antibiotics to cultures of Escherichia coli and Bacillus subtilis that separately inhibited each of three major biosynthetic pathways. Inhibiting RNA synthesis or protein synthesis had a negligible effect on cell size. “But if we hit the cells with an antibiotic that targets fatty-acid synthesis, we really saw a significant drop in cell size,” Vadia said.

Next, they tried pushing things in the other direction by turning up FadR, a transcription factor that activates expression of the fatty-acid synthesis genes. When they used an antibiotic to turn down fatty-acid synthesis, the cells were smaller. But when they overproduced FadR to increase fatty-acid synthesis, the cells got bigger.

“It doesn’t seem to matter what the lipids are, really,” Levin said, “provided you have enough of them. We found we could give the cells oleic acid, a fat found in avocados and olive oil, to supplement diminished fatty-acid synthesis and as long as the added fatty acid got into the membrane, the cells could recover.”

Unlike many cellular processes, this one doesn’t appear to be regulated by an elaborate network of protein signals, she said. The membrane is literally just a bag, and how big that bag is determines how big the cell can grow.

“I was surprised that lipids were so dominant over proteins, because they’ve been kind of ignored,” she said.

“People don’t think about lipids,” Vadia said. “It probably goes back to the central dogma of biology that we’re all taught: DNA makes RNA makes protein, and proteins do everything. Lipids have been regarded as a sideshow.”

Raising the alarm

But the scientists weren’t done yet. “We knew that mutants that could not make a signaling molecule called guanosine tetraphosphate (ppGpp) were very sensitive to things that inhibited lipid synthesis, but nobody really understood why,” Levin said.

So next the lab ran experiments to track down the role of ppGpp in cell size. This molecule is an alarm signal, Vadia said. If E. coli encounters dire living conditions it starts to synthesize ppGpp, which shuts down DNA synthesis, protein synthesis, fatty-acid synthesis, and ribosome production, putting the cells in a quasi-dormant state (this is called “the stringent response”).

Under nutrient-rich conditions, ppGpp levels are extremely low, but as bacterial cells begin to starve they begin to rise.

To figure out more, Vadia exposed an E. coli mutant that cannot make ppGpp to an antibiotic that shuts down fatty-acid synthesis. Able to make more stuff but unable to make a bigger bag to put it in, the cells began to lyse, bursting at the seams.

“It’s like the Incredible Hulk,” Levin said. “Rising levels of ppGpp tell the cell, ‘OK, we’re not making enough lipids; you’ve got to turn down these other things you’re making or we’ll burst through our clothing.'”

Again, the scientists tried pushing things the other way as well. They grew cells that overproduced both a ppGpp synthase and FadR. So these cells had high ppGpp, even in nutrient-rich conditions, which would normally make them small, but they also had high FadR, which allowed them to take advantage of the available nutrients to increase lipid synthesis.

Given enough lipids to grow a bigger bag, these cells bulked up.

“So lipids really dictate the size of everything else,” Levin said. “There’s a signaling pathway that says, ‘OK, we’re getting way too big for our britches, we have to shut down, stop eating so much, stop making so much.’

“This is true for the eukaryote Saccharomyces cerevisiae (yeast) as well as for E. coli and B. subtilis,” she said. “It’s probably true — I would be shocked if it’s not — across all organisms: that lipids limit the size of individual cells.”

Levin continued: “These experiments are the most fun thing we’ve ever done. So many ideas that had been ill-defined suddenly snapped into focus, which is what every research scientist hopes for.”

According to a new study, airborne particles and their accumulation on solar cells are cutting energy output by more than 25 percent in certain parts of the world. The regions hardest hit are also those investing the most in solar energy installations: China, India and the Arabian Peninsula.

The study appears online June 23 in Environmental Science & Technology Letters.

“My colleagues in India were showing off some of their rooftop solar installations, and I was blown away by how dirty the panels were,” said Michael Bergin, professor of civil and environmental engineering at Duke University and lead author of the study. “I thought the dirt had to affect their efficiencies, but there weren’t any studies out there estimating the losses. So we put together a comprehensive model to do just that.”

With colleagues at the Indian Institute of Technology-Gandhinagar and the University of Wisconsin at Madison, Bergin measured the decrease in solar energy gathered by the IITGN’s solar panels as they became dirtier over time. The data showed a 50-percent jump in efficiency each time the panels were cleaned after being left alone for several weeks.

The researchers also sampled the grime to analyze its composition, revealing that 92 percent was dust while the remaining fraction was composed of carbon and ion pollutants from human activity. While this may sound like a small amount, light is blocked more efficiently by smaller humanmade particles than by natural dust. As a result, the human contributions to energy loss are much greater than those from dust, making the two sources roughly equal antagonists in this case.

“The humanmade particles are also small and sticky, making them much more difficult to clean off,” said Bergin. “You might think you could just clean the solar panels more often, but the more you clean them, the higher your risk of damaging them.”

Having previously analyzed pollutants discoloring India’s Taj Mahal, Bergin already had a good idea of how these different particles react to sunlight. Using his earlier work as a base, he created an equation that accurately estimates the amount of sunlight blocked by different compositions of solar panel dust and pollution buildup.

But grimy buildup on solar panels isn’t the only thing blocking sunlight — the ambient particles in the air also have a screening effect.

For that half of the sun-blocking equation, Bergin turned to Drew Shindell, professor of climate sciences at Duke and an expert in using the NASA GISS Global Climate Model.

Because the climate model already accounts for the amount of the sun’s energy blocked by different types of airborne particles, it was not a stretch to estimate the particles’ effects on solar energy. The NASA model also estimates the amount of particulate matter deposited on surfaces worldwide, providing a basis for Bergin’s equation to calculate how much sunlight would be blocked by accumulated dust and pollution.

The resulting calculations estimate the total loss of solar energy production in every part of the world. While the United States has relatively little migratory dust, more arid regions such as the Arabian Peninsula, Northern India and Eastern China are looking at heavy losses — 17 to 25 percent or more, assuming monthly cleanings. If cleanings take place every two months, those numbers jump to 25 or 35 percent.

There are, of course, multiple variables that affect solar power production both on a local and regional level. For example, a large construction zone can cause a swift buildup of dust on a nearby solar array.

The Arabian Peninsula loses much more solar power to dust than it does humanmade pollutants, Bergin said. But the reverse is true for regions of China, and regions of India are not far behind.

“China is already looking at tens of billions of dollars being lost each year, with more than 80 percent of that coming from losses due to pollution,” said Bergin. “With the explosion of renewables taking place in China and their recent commitment to expanding their solar power capacity, that number is only going to go up.”

“We always knew these pollutants were bad for human health and climate change, but now we’ve shown how bad they are for solar energy as well,” continued Bergin. “It’s yet another reason for policymakers worldwide to adopt emissions controls.”

This work was supported by the US Agency for International Development and the Office of the Vice Provost for Research at Duke University.

Story Source:

Materials provided by Duke University. Original written by Ken Kingery. Note: Content may be edited for style and length.

The material — known as 1T’-WTe2 — bridges two flourishing fields of research: that of so-called 2-D materials, which include monolayer materials such as graphene that behave in different ways than their thicker forms; and topological materials, in which electrons can zip around in predictable ways with next to no resistance and regardless of defects that would ordinarily impede their movement.

At the edges of this material, the spin of electrons — a particle property that functions a bit like a compass needle pointing either north or south — and their momentum are closely tied and predictable.

This latest experimental evidence could elevate the material’s use as a test subject for next-gen applications, such as a new breed of electronic devices that manipulate its spin property to carry and store data more efficiently than present-day devices. These traits are fundamental to spintronics.

The material is called a topological insulator because its interior surface does not conduct electricity, and its electrical conductivity (the flow of electrons) is restricted to its edges.

“This material should be very useful for spintronics studies,” said Sung-Kwan Mo, a physicist and staff scientist at Berkeley Lab’s Advanced Light Source (ALS) who co-led the study, published in Nature Physics.

“The flow of electrons is completely linked with the direction of their spins, and is limited only to the edges of the material,” Mo said. “The electrons will travel in one direction, and with one type of spin, which is a useful quality for spintronics devices.” Such devices could conceivably carry data more fluidly, with lesser power demands and heat buildup than is typical for present-day electronic devices.

“We’re excited about the fact that we have found another family of materials where we can both explore the physics of 2-D topological insulators and do experiments that may lead to future applications,” said Zhi-Xun Shen, a professor in Physical Sciences at Stanford University and the Advisor for Science and Technology at SLAC National Accelerator Laboratory who also co-led the research effort. “This general class of materials is known to be robust and to hold up well under various experimental conditions, and these qualities should allow the field to develop faster,” he added.

The material was fabricated and studied at the ALS, an X-ray research facility known as a synchrotron. Shujie Tang, a visiting postdoctoral researcher at Berkeley Lab and Stanford University, and a co-lead author in the study, was instrumental in growing 3-atom-thick crystalline samples of the material in a highly purified, vacuum-sealed compartment at the ALS, using a process known as molecular beam epitaxy.

The high-purity samples were then studied at the ALS using a technique known as ARPES (or angle-resolved photoemission spectroscopy), which provides a powerful probe of materials’ electron properties.

“After we refined the growth recipe, we measured it with ARPES. We immediately recognized the characteristic electronic structure of a 2-D topological insulator,” Tang said, based on theory and predictions. “We were the first ones to perform this type of measurement on this material.”

But because the conducting part of this material, at its outermost edge, measured only a few nanometers thin — thousands of times thinner than the X-ray beam’s focus — it was difficult to positively identify all of the material’s electronic properties.

So collaborators at UC Berkeley performed additional measurements at the atomic scale using a technique known as STM, or scanning tunneling microscopy. “STM measured its edge state directly, so that was a really key contribution,” Tang said.

The research effort, which began in 2015, involved more than two dozen researchers in a variety of disciplines. The research team also benefited from computational work at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC).

Two-dimensional materials have unique electronic properties that are considered key to adapting them for spintronics applications, and there is a very active worldwide R&D effort focused on tailoring these materials for specific uses by selectively stacking different types.

“Researchers are trying to sandwich them on top of each other to tweak the material as they wish — like Lego blocks,” Mo said. “Now that we have experimental proof of this material’s properties, we want to stack it up with other materials to see how these properties change.”

A typical problem in creating such designer materials from atomically thin layers is that materials typically have nanoscale defects that can be difficult to eliminate and that can affect their performance. But because 1T’-WTe2 is a topological insulator, its electronic properties are by nature resilient.

“At the nanoscale it may not be a perfect crystal,” Mo said, “but the beauty of topological materials is that even when you have less than perfect crystals, the edge states survive. The imperfections don’t break the key properties.”

Going forward, researchers aim to develop larger samples of the material and to discover how to selectively tune and accentuate specific properties. Besides its topological properties, its “sister materials,” which have similar properties and were also studied by the research team, are known to be light-sensitive and have useful properties for solar cells and for optoelectronics, which control light for use in electronic devices.