In This Issue
APPLIED PHYSICAL SCIENCES
Explaining water's anomalies
Water still has many secrets to divulge before its density changes with temperature can be fully explained. Francesco Mallamace et al. report the density of liquid water at temperatures from its boiling point to 30 K, using optical scattering data, Raman and Fourier transform infrared. By analyzing OH-stretching spectra, the authors could measure the relative contributions of the low- and high-density phases in supercooled water and calculate the density of H2O at any temperature. The authors found that the density matched theoretical predictions and molecular dynamic (MD) simulations, indicating that, at supercooled temperatures, water is a mix of low- and high-density phases. Water has a well known maximum density at 277 K, just above its freezing point. The authors found a minimum density near 203 K, which had been predicted by MD simulations. From the analysis of these data, they show that water's temperature derivative is characterized by a maximum at ≈240 K. The occurrence of this maximum explains some of water's intriguing thermodynamical anomalies and supports the hypothesis of a liquid–liquid phase transition. — P.D.
Low- and high-density regions of seawater.
“The anomalous behavior of the density of water in the range 30 K < T < 373 K” by Francesco Mallamace, Caterina Branca, Matteo Broccio, Carmelo Corsaro, Chung-Yuan Mou, and Sow-Hsin Chen (see pages 18387–18391)
GEOLOGY, EVOLUTION
Rudderless evolution
Paleontologists have historically looked for evidence of gradual evolutionary trends in fossil species. The theory of punctuated equilibrium contradicts this idea, instead claiming that evolution within species consists of long periods of stasis interrupted by brief bursts of change associated with speciation. Gene Hunt presents a statistical study that supports punctuated equilibrium. Hunt analyzed published data on 251 traits in 53 fossil lineages, including the variation over geological time of microfossils, mollusks, and mammals. Hunt compared trait evolution to three possible modes: a random walk, directional change, and stasis, in which phenotypes fluctuate about a constant mean. Traits were assigned weights corresponding to how well they fit each of the three modes. Hunt found that directional change was a good fit for only 5% of the traits. The other 95% were nearly equally split between random walk and stasis. However, traits related to body shape, rather than size, were much more likely to remain in stasis. Hunt suggests that most intervals of directional evolutionary change occur too quickly to be caught in the fossil record and that his results corroborate a key claim of punctuated equilibrium. — K.M.
“The relative importance of directional change, random walks, and stasis in the evolution of fossil lineages” by Gene Hunt (see pages 18404–18408)
MEDICAL SCIENCES
Zinc transporter key in pancreatic cancer
Several studies have linked breast cancer to abnormalities in cellular zinc transport, which is controlled by the ZnT and ZIP protein families. Zinc is required by many enzymes, including those critical to aspects of tumor growth. Min Li et al. report that overexpression of the zinc importer ZIP4 also is connected tohuman pancreatic cancer. The authors had previously identified high levels of ZIP4 mRNA in human pancreatic cancer tissues. Here, they assayed tissue samples from cancer patients in vitro as well as pancreatic cancer cell lines in vivo for ZIP4. In 16 of 17 tissue samples and 7 of 8 cell lines, the ZIP4 mRNA was significantly overexpressed. Antibody labeling further identified high levels of ZIP4 protein. The authors used a retroviral vector to insert a strongly promoted ZIP4 gene into the one pancreatic cancer cell line that did not already overexpress ZIP4. Compared to the vector control line, the new cells accumulated 73% more zinc and proliferated twice as quickly. Injected into immunodeficient nude mice, the cancer cells overexpressing ZIP4 grew into larger and heavier tumors than those from the vector control cell line. ZIP4 may prove to be a useful marker for pancreatic cancer, the authors say, or an important drug target. — K.M.
ZIP4 expression in normal (Left) and cancerous (Right) tissue.
“Aberrant expression of zinc transporter ZIP4 (SLC39A4) significantly contributes to human pancreatic cancer pathogenesis and progression” by Min Li, Yuqing Zhang, Zijuan Liu, Uddalak Bharadwaj, Hao Wang, Xinwen Wang, Sheng Zhang, Juan P. Liuzzi, Shou-Mei Chang, Robert J. Cousins, William E. Fisher, F. Charles Brunicardi, Craig D. Logsdon, Changyi Chen, and Qizhi Yao (see pages 18636–18641)
SUSTAINABILITY SCIENCE
Rise in atmospheric carbon accelerating
Three major factors contribute to the growing rate of increase in atmospheric CO2: increasing global economic output, rising carbon intensity, and a growing fraction of emissions that accumulate in the atmosphere. Carbon intensity, the quantity of CO2 released per unit economic output, depends largely on fossil fuel consumption. Josep Canadell et al. analyzed data from the UnitedNations and the U.S. National Oceanic and Atmospheric Administration to provide a breakdown of global CO2 dynamics. The authors show that, although the carbon intensity of the global economy was decreasing before 2000, it has since turned upward. They report that the efficiency of CO2 sinks on land and in the ocean to absorb emissions is declining because of human-dependent changes in Southern Ocean wind circulation as well as a series of droughts in 2002–2005. The authors suggest that, even though the rapid economic growth of China and India has contributed to the recent acceleration of CO2 emissions, it is only one of several factors that are boosting accumulation of CO2 faster than expected in the atmosphere. — K.M.
Smokestack emissions are only one factor in the acceleration of atmospheric CO2.
“Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks” by Josep G. Canadell, Corinne Le Quéré, Michael R. Raupach, Christopher B. Field, Erik T. Buitehuis, Philippe Ciais, Thomas J. Conway, Nathan P. Gillett, R. A. Houghton, and Gregg Marland (see pages 18866–18870)
SUSTAINABILITY SCIENCE
Bacterial reactor generates hydrogen fuel
Around the world, public transportation systems are switching to hydrogen-fueled engines as an emissions-free alternative to gasoline. However,most hydrogen available today is inefficiently generated from nonrenewable fossil fuels such as natural gas. Shaoan Cheng and Bruce Logan report a method of hydrogen production that obviates the need for such fossil fuels, relying instead on electron-generating bacteria and biomass. The authors grew soil- or wastewater-derived bacteria in compact fuel cells modified with ammonia-treated graphite granule anodes and an anion exchange membrane (AEM). Ammonia treatment of the anodes increased bacterial adhesion, while the AEM enhanced proton conduction, which achieved high current densities. Using acetic acid, a common dead-end product of glucose fermentation, along with a small applied voltage of 0.2–0.8 V, the reactor generated hydrogen gas to nearly 99% of the theoretical maximum yield. An additional advantage of their microbial electrolysis cell is that sustainable hydrogen can be produced at energy yields of up to 82% from any type of biodegradable organic matter, including acetic acid, cellulose, and glucose. — F.A.
Microbial electrolysis cell and power supply.
“Sustainable and efficient biohydrogen production via electrohydrogenesis” by Shaoan Cheng and Bruce E. Logan (see pages 18871–18873)
