Editors' Choice

Science  18 Feb 2005:
Vol. 307, Issue 5712, pp. 1015

    Rete Mirabile

    1. H. Jesse Smith

    The atmospheric concentration of CO2 and the climate are connected by an intricate web of positive and negative feedbacks. The CO2 content of the atmosphere is increased by volcanic and metamorphic degassing and decreased by the chemical weathering of silicate rocks; yet another important influence is the vascular land plants. A fundamental difficulty in understanding the role of plants, however, is that long-term changes in CO2 and climate affect terrestrial plant development and evolution, which in turn has consequences for the burial of organic matter in sediments and chemical weathering.

    Beerling and Berner present a systems analysis of the physiological and geochemical processes linking plants and CO2 on geological time scales and pay special attention to how this wondrous network prevents runaway changes in CO2 and catastrophic planetary warming. By incorporating processes that affect CO2 on million-year time scales, such as evolution and weathering, and ones occurring on much shorter time scales, such as how terrestrial ecosystems regulate the land/ atmosphere exchange of water vapor and recycling of precipitation, they uncover important feedback loops not previously identified. They also find that the biota exerted a destabilizing influence on climate regulation in the Paleozoic, and this quickened the rates of terrestrial plant and animal evolution, which accelerated the diversification of terrestrial tetrapods and insects, and caused a large rise in the concentration of atmospheric oxygen. — HJS

    Proc. Natl. Acad. Sci. U.S.A. 102, 1302 (2005).


    How Wet Does It Get?

    1. Marc S. Lavine

    The ability to manipulate small volumes of liquids has opened up the possibility of designing a lab on a chip with micrometer-scale channels. The architectures for these microfluidic chambers can either be closed—with the typical channels, pumps, valves, and reservoirs— or open, where the flow is controlled by local changes in either the wettability of the substrate or its topography.

    Seemann et al. patterned a set of rectangular grooves into chemically modified silicon to give a system that could be described by two parameters: the aspect ratio of the grooves, defined as the depth/width, and the contact angle formed between the fluid and the substrate. At contact angles greater than 45°, the fluid formed droplets that would spill over the walls of shallow channels and that transformed into filaments in deeper trenches. At lower contact angles, the wetting was complicated by pinned wedges that formed along the corners of the channels, and the filaments could take on either a positive or negative Laplace pressure. This suggests that dynamic changes in the properties of the substrate can be used to drive a fluid through a chip, keeping in mind that the chemistry performed on a chip will affect the wettability and hence the dynamics and shape of the moving fluid. — MSL

    Proc. Natl. Acad. Sci. U.S.A. 102, 1848 (2005).


    Hiding with Ease

    1. Caroline Ash

    Catching an intractable disease while in the hospital is a worrying prospect and has become of greater concern mostly owing to persistent Staphylococcus epidermidis attaching to indwelling devices such as prosthetic heart valves. Its more aggressive relative S. aureus sports an arsenal of virulence factors, but how a ubiquitous skin commensal causes pathology is less clear. One useful defensive component appears to be poly-γ-DL-glutamic acid (PGA), which Kocianova et al.found is synthesized by all 74 strains of S. epidermidis they tested. In support of its commensal lifestyle, S. epidermidis relies on PGA to resist the wild swings in salt concentration that occur on human skin. PGA is known to protect other Gram-positive bacteria (such as Bacillus anthracis, which takes shelter in a capsule of PGA), from phagocytosis by host cells. PGA-nonproducing mutants of S. epidermidis, in which the cap gene locus was replaced, were wiped out by antibacterial peptides known as defensins and by neutrophil attack, whereas cap-intact bacteria survived. — CA

    J. Clin. Invest. 10.1172/JCI200523523 (2005).


    Unequal Fission

    1. Lisa D. Chong

    Despite the apparent symmetry of cell division, the rod-shaped bacterium Escherichia coli does not produce progeny that are identical. Upon division, each daughter cell acquires a pre-existing end (old pole) from its ancestor as well as a newly created end (new pole) where the septum forms. In a present-day reenactment of the heroic lineage mapping of the nematode, Stewart et al.followed individual bacteria for nine generations of growth and reproduction; computerized analysis of about 35,000 cells revealed that the cell that inherited the parent's older pole grew more slowly than the cell bequeathed the younger pole. Cells with older poles produced less biomass (summed across their offspring) and had an increased probability of death. Because these asymmetric characteristics are hallmarks of cellular aging in multicellular organisms and in yeast, the study suggests that asymmetric cell division and fundamental mechanisms of aging may be evolutionarily conserved in bacteria. — LDC

    PLoS Biol. 3, e45 (2005).


    Gaining Quadruple Points

    1. Brooks Hanson

    The most important binary mixture on Earth, as well as a significant one in industrial applications, is that of H2O and CO2. Their binary chemistry affects the atmosphere and ocean; determines volatiles in Earth's mantle and crust, including volcanoes; and plays a role in numerous industrial reactions. Although there have been many studies of the relation between CO2 and H2O at temperatures above the freezing point of water—involving gases, liquids, or supercritical fluids—the phase relations at lower temperatures, those at or below the freezing point of water, are less well documented. At low temperatures, the mixture is important in the upper atmosphere; in ice cores (where CO2 is trapped as a gas); in clathrates in the deep ocean; and on other planets, notably Mars with its polar caps of water and dry ice.

    Longhi explores this parameter space using thermodynamic data on the various pure phases and the H2O + CO2 clathrate and shows that the phase diagram is richer than previously thought. The analysis suggests that CO2 clathrate will be stable in only some regions of the deep ocean, a critical issue with respect to carbon sequestration, and that conditions may be appropriate in some polar ice sheets for the accumulation of liquid CO2. — BH

    Geochim. Cosmochim. Acta 69, 529 (2005).


    Death by Any Other Name

    1. Stella M. Hurtley

    An enormous amount of molecular detail about the mechanisms of programmed cell death (apoptosis) has been amassed, and it is now recognized as an integral cellular pathway involved in development and in disease. During apoptosis, an orchestrated series of events leads to the inhibition of protein synthesis. Another cell death pathway, much less well understood, is known as necrosis. It occurs both when cells are subjected to physical damage and during certain pathologies, including cardiac ischemia and stroke. Saelens et al. show that in necrosis, cellular protein synthesis continues unabated right up until the point at which the cell membrane ruptures. This means that a necrotic cell remains an attractive abode for incoming viruses, which will be able to exploit the cellular protein synthesis machinery to generate progeny. It also means that after necrotic cell death, many more intact cellular proteins are released locally and may thereby trigger an inflammatory response. — SMH

    J. Cell Biol. 10.1083/jcb.200407162 (2005).


    A Folding Ruler

    1. Gilbert J. Chin

    The development of a suite of fluorescent probes that can be introduced into cells has made it feasible to estimate intracellular distances (static and dynamic) via the technique of Förster resonance energy transfer (FRET). To a first approximation, the efficiency of energy transfer from a donor to an acceptor fluorophore depends on the inverse sixth power of the distance separating them. Converting efficiency into units of distance requires a calibrating ruler with a donor fixed at one end and an acceptor at the other. Schuler et al. use contemporary single-molecule technology to reexamine the classical ruler, a rigid rod of 12 proline residues, introduced originally by Stryer and Haugland almost four decades ago. A number of factors conspire to make the observed efficiencies deviate from the values predicted by Förster theory, especially for much longer rulers that, surprisingly, turn out to be much less rigid than the canonical polyproline helix. — GJC

    Proc. Natl. Acad. Sci. U.S.A. 102, 2754 (2005).