In This Issue
CHEMISTRY
Source of galaxy's red glow
Candidate molecules have been proposed as being responsible for the distinctive red glow observed in many parts of the galaxy, but none match all of the available observational constraints. Young Min Rhee et al. provide an explanation for the mystery of the origin of the red glow, known as the galactic extended red emission, suggesting that charged polycyclic aromatic hydrocarbon (PAH) clusters are the source. The authors calculated the structures and binding energies of PAHs and simulated the spectra of charged PAH clusters. Closed-shell cationic PAH dimers were found to be consistent with all observational constraints for the galactic extended red emission. These unusual dimers were held together by an interaction several times stronger than typical van der Waals forces but weaker than normal chemical bonds. As such, these dimers can form and reform in PAH-rich regions of space and could contribute to the nucleation of carbonaceous clusters. Rhee et al. suggest that laboratory studies of these dimers could confirm their hypothesis regarding the source of the galactic extended red emission. — P.D.
Understanding the galactic extended red emission.
“Charged polycyclic aromatic hydrocarbon clusters and the galactic extended red emission” by Young Min Rhee, Timothy J. Lee, Murthy S. Gudipati, Louis J. Allamandola, and Martin Head-Gordon (see pages 5274–5278)
ECOLOGY
Volatile communication in plants
Upon attack by herbivores, plants release a complex bouquet of volatile organic compounds (VOCs). Experiments under controlled laboratory conditions have shown that VOCs may function to repel herbivores or to attract predators that defend the plant. The signals may also be used by other, “eavesdropping” plants as a warning of impending attack. Martin Heil and Juan Carlos Silva Bueno show that VOCs released by lima bean plants serve as signals to the attacked plant itself, and to its neighbors, under natural conditions. The authors grew lima bean plants in the field and damaged the plants' leaves either mechanically or by using beetles. Exposure to damaged leaves increased the production of extrafloral nectar in the intact leaves of the damaged plant and in intact neighboring plants as well. The sweet extrafloral nectar attracted ants that protected the plants from herbivores. When the damaged leaves were covered with plastic bags to trap the VOCs, neighboring leaves and plants did not change their nectar production. The authors also observed that lima bean plants exposed to beetle-damaged leaves in the field sustained less damage from herbivores and grew faster than plants exposed to intact leaves. — F.A.H.
Plants exposed to herbivore-induced emitter tendrils (VOCs) and to undamaged emitters (C).
“Within-plant signaling by volatiles leads to induction and priming of an indirect plant defense in nature” by Martin Heil and Juan Carlos Silva Bueno (see pages 5467–5472)
ENVIRONMENTAL SCIENCES, GEOPHYSICS
Predicting coral bleaching events
Warm ocean temperatures can cause coral bleaching, in which reef-building corals lose color due to a breakdown in symbiosis with the dinoflagellate Symbiodinium. In a study of past and future coral bleaching events in the Caribbean, Simon Donner et al. report that bleaching may occur more often than currently observed, unless the corals can adapt to warmer ocean temperatures. The authors used global climate models, with and without human impact, to examine the probability of coral bleaching events, such as the widespread bleaching that occurred in the Eastern Caribbean in 2005. Human-induced climate change was found to have increased the probability of coral bleaching in the Caribbean by an order of magnitude. Coral bleaching events like that which occurred in 2005 were predicted to become relatively frequent in 20–30 years, possibly occurring every other year, unless corals can adapt to rising ocean temperatures. Donner et al. say that if corals can adapt to a temperature increase of 1–1.5°C, future bleaching events could be postponed until the second half of this century, permitting time to alter the course of greenhouse gas emissions. If corals are unable to adapt, however, Caribbean coral reef ecosystems may be severely stressed as a result of predicted warming, even if carbon dioxide levels are stabilized in this century. — P.D.
Satellite-observed thermal stress on Eastern Caribbean coral reefs, 2005.
“Model-based assessment of the role of human-induced climate change in the 2005 Caribbean coral bleaching event” by Simon D. Donner, Thomas R. Knutson, and Michael Oppenheimer (see pages 5483–5488)
EVOLUTION
Early Cretaceous lizard glided with its ribs
In most gliding animals, the membrane that keeps the animal airborne, called the patagium, is stretched between the toes or between the legs and the body. Among lizards, an alternative patagium evolved: a membrane spread alongside the body by elongated ribs. Currently only the dragon lizards of Southeast Asia and a fossil specimen from the Late Triassic era, distantly related to modern lizards, are known to possess such a patagium. Pi-Peng Li et al. report finding a fossil lizard from the Early Cretaceous period with a rib-supported patagium. The specimen, named Xianglong zhaoi, was found at the Zhuanchengzi locality in Liaoning Province, northeastern China. The fossil is 15.5 cm long and shows the whole skeleton, as well as imprints of the animal's skin. The most striking features are eight elongated ribs that helped to spread the wing-like patagium to the left and right of the lizard's body. The feet and claws of the specimen show adaptations for climbing, indicating that the lizard lived in trees. Based on the shape and size of the patagium, the animal appears to have possessed great maneuverability. Li et al. suggest that the rib-supported patagia of different lizard species arose by convergent evolution. — F.A.H.
Early Cretaceous gliding lizard fossil from China.
“A gliding lizard from the Early Cretaceous of China” by Pi-Peng Li, Ke-Qin Gao, Lian-Hai Hou, and Xing Xu (see pages 5507–5509)
