Sunlight, Clouds, and Dimethylsulfide
Dimethylsulfide is produced in abundance by marine organisms and is a precursor for a major fraction of cloud-forming aerosols. Thus, it has been suggested that oceanic primary productivity could be part of an important feedback loop involving clouds. Vallina and Simó (p. 506) report that dimethylsulfide concentrations over the remote ocean are strongly correlated with the amount of solar radiation received by the upper mixed layer of the ocean. This response could produce a negative feedback in which dimethylsulfide emissions are inhibited as the increased cloudiness they cause reduces the amount of light received by the pelagic ecosystem.
Making a Stand
Many responsive materials, such as hydrogels, are made from polymers that can swell or shrink with changing exposure to a solvent. Although these materials can respond rapidly, there is a limit to the complexity in their shape change, as well as to the amount of stress that can be exerted on such soft materials. Sidorenko et al. (p. 487) constructed two architectures in which silicon nanocolumns are either freely embedded in a polymer hydrogel or attached to the underlying substrate. When the hydrogel changes dimension upon exposure to water vapor, the nanocolumns are lifted up and stand up off the surface. This actuation is fast (on the order of seconds) and reversible. The hydrogel can also be templated so that the motion of the silicon nanocolumns occurs in a specific pattern.
Japan installed seismic networks on the Nankai subduction zone after the Kobe earthquake, and the resulting measurements have revealed a wealth of new phenomena, including nonvolcanic tremor, long-period volcanic events, and slow earthquakes. Ito et al. (p. 503, published online 30 November; see the Perspective by Dragert) identify yet another type of seismic reverberation, that of very-low-frequency events with equivalent magnitudes of 3 to 3.5 and long periods of tens of seconds. The very-low-frequency earthquakes accompany and migrate with nonvolcanic tremor and slow slip events. The coincidence of these three phenomena improves the detection and characterization of slow earthquakes, which are thought to increase the stress on the updip megathrust earthquake rupture zone.
The role of water in many atmospheric reactions remains poorly understood at the molecular level because it is difficult to distinguish nonspecific effects, such as collisional activation, from direct participation of water as a catalyst. Vöhringer-Martinez et al. (p. 497; see the Perspective by Smith) combined precise kinetic measurements with quantum-chemical calculations to implicate a catalytic role for individual water molecules in the gas-phase reaction of OH radicals with acetaldehyde. Their results suggest that water complexation to acetaldehyde lowers the subsequent barrier to attack by OH. The increasing instability of such complexes with rising temperature accounts for an unusual negative dependence on temperature of the magnitude of the catalytic effect.
Beating on a Graphene Drumskin
A resonator vibrates in response to an applied external force, and making a resonator thinner and less massive will generally cause it to operate at higher frequencies and have better sensitivity. Bunch et al. (p. 490) exfoliated ultrathin sheets of graphene from graphite and suspended the two-dimensional layers across a trench. They then actuated the thin sheets optically and electrically, thereby realizing the ultimate limit of resonators only one atom in thickness.
Fewer Escapist Tendencies
Early in its history, Mars was once wet and humid and had a denser atmosphere than it has today. This atmosphere was supposedly battered by the solar wind and lost into space. Barabash et al. (p. 501) find that the escape rate today for gases in the martian atmosphere is very low, based on measurements from the orbiting Mars Express spacecraft. Propagating these rates backward over a period of 3.5 Gy would result in the removal of 0.2 to 4 mbar of CO2 and a few centimeters of water. Rather than having left the planet, CO2 and water could instead be locked away beneath its surface.
A Species by Another Name?
There is considerable uncertainty about what constitutes a species for bacteria because of the apparently rampant horizontal gene exchange that occurs between microorganisms, which seems to result in microbial populations being blended. Fraser et al. (p. 476) focus on the distribution and effect of mutation and recombination frequencies and, by combining a modeling approach with a review of the existing data, reveal the conditions under which bacterial speciation may occur.
Like Mother, Like Daughter
Tissues derived from embryonic stem cells may prove therapeutically useful, but a likely problem will be the rejection of stem cells that carry surface antigens dissimilar from those of the recipient. In order to create embryonic stem cell lines that carry matched surface antigens, Kim et al. (p. 482, published online 14 December; see the cover) have analyzed parthenogenetic derivation of embryonic stem cells in the mouse. Using oocytes from two separate phases of mitosis, they induced parthenogenetic development, in which the cells carry two copies of the maternal genome. Stem cells derived from these embryos would be a specific match for the donor of the oocyte. Such parthenogenetically derived stem cells could produce many, but not all, tissues, possibly because of the absence of the paternally imprinted genome.
Plaguing the Lungs
Plague is not just transmitted by fleas; hypervirulent pneumonic plague can be transmitted directly among people by coughing. Lathem et al. (p.509) show that a specific virulence factor from Yersinia pestis, plasminogen activator, is injected into host cells and promotes proliferation of the bacteria and massive lung inflammation. It is possible that this bacterial protease converts host plasminogen into plasmin, thereby releasing trapped bacteria from fibrin clots. Thus, inhibiting plasminogen activator during therapy might slow disease progression and allow antibiotics time to take effect.
The region of the brain called the insula has received relatively little attention in drug addiction literature. Naqvi et al. (p. 531) now report the results of a retrospective anatomical analysis of a large cohort of brain-damaged patients. The analysis was prompted by their observation of a single patient with insula damage who quit a severe nicotine addiction immediately upon recovery from his acute neurological damage without any apparent difficulty or relapse. Damage to the insula appears to reduce the urge to smoke rather than, for example, reducing the reward or reinforcement associated with smoking.
A three-way endosymbiosis involving a grass, a fungus, and a virus has been discovered in the hot spots of Yellowstone National Park by Márquez et al. (p. 513). A virally infected fungus infects the host grass's roots and confers heat tolerance to both grass and fungus. Furthermore, it is the virus in the fungus that allows it to endow this tolerance. Fungus lacking the virus did not provide heat resistance to its host grass until it was reinfected.
Heads, I Win
One of the enduring observations in support of human irrationality is that we are disproportionately sensitive to losses relative to gains. When offered a 50-50 gamble, the potential gain needs to be twice as large as the potential loss in order for subjects to exhibit an equal willingness to accept or to reject the gamble. Tom et al. (p. 515) have mapped a set of brain regions that respond parametrically to the sizes of potential gains and losses, and show that these regions are more sensitive to losses. Furthermore, the between-subject differences in neural responses reflect the between-subject differences in their behavioral aversion to losses.
Inside B Cell Central
B cells become effective factories for antibody production only after they have gone through a series of maturation steps that select clones of B cells carrying somatic mutations for high-affinity antibodies. This process takes place in the germinal center, where B cells are also thought to compete vigorously for available antigen. Using intravital microscopy, Allen et al. (p. 528; published online 21 December) observe that the behavior of germinal center B cells is more consistent with a competition for the attention of helper T cells than for scarce antigen. This finding could prove useful in considering how best to stimulate robust immune responses with vaccines.
Reduction Runs Skin Deep
One route for improving the activity of oxygen reduction reaction at the platinum electrode of polymer electrolyte membrane fuel cells is to alloy Pt with other metals, such as ruthenium. In an effort to understand how alloying can improve activity, Stamenkovic et al. (p. 493, published online 11 January with the news story by Service) have prepared single-crystal surfaces of a nickel alloy, Pt3Ni, in ultrahigh conditions and then examined them further in solution under reaction conditions. They find that relative to Pt, the (111) close-packed surface of this alloy is 10 times more active for oxygen reduction. This higher activity appears to originate in changes in electronic structure (negative shift of the d-band center) and the formation of Pt-rich outer layer and third layer that sandwich a Ni-rich subsurface atomic layer. The formation of hydrogenated species such as OH is inhibited on this surface and allows for higher surface coverages of O2.
Exploiting Your Niche
In Drosophila, male germline stem cells orient the mitotic spindle to generate a daughter stem cell and a cell destined for germ cell differentiation after asymmetric cell division. Using gene mutation and ultra-structural studies, Yamashita et al. (p. 518; see the Perspective by Spradling and Zheng) now identify the mechanism by which the oriented spindle is established. The mother and daughter centrosomes are differentially oriented, with the mother centrosome anchored close to the niche-stem cell junction by microtubules, whereas the daughter centrosome migrates to the other side of the cell. This differential centrosome identity and migration may provide a mechanism for asymmetric cell division in the generation of daughter cells with different developmental fates.
Making the Gradient
The role of gradients of morphogens in developmental biology is well established, but the mechanisms involved in the establishment and maintenance of these gradients are controversial. Kicheva et al. (p. 521) present a quantitative study of the kinetics of morphogen transport and the formation of morphogen gradients in the wing disc of Drosophila. The steady-state gradients of two morphogens can be described by a simple exponential decay defined by four key kinetic parameters.
Not the Same Difference
The 20 common amino acids in proteins are represented in the genome by a triplet code of four DNA bases giving a total of 64 (43) different codons, with the majority of amino acids being encoded by more than one codon. Different synonymous codons affect RNA structure and stability and also affect protein translation rates. In bacteria, they are also known to affect protein folding. Can they have a similar effect in eukaryotes? Kimchi-Sarfaty et al. (p. 525, published online 21 December with the Perspective by Komar) look at the human multidrug resistance 1 (MDR1) gene and show that a relatively common single nucleotide polymorphism—which changes ATC for ATT (both isoleucine), in combination with two other polymorphisms—causes a change in the conformation of the protein and also underlies its altered drug and inhibitor interactions.