In This Issue
APPLIED PHYSICAL SCIENCES, EVOLUTION
Flagella help algae eat
As unicellular organisms evolved into multicellular ones, they required increasing amounts of nutrition. For the volvocine green algae, which consist of spherical colonies containing up to 50,000 individual cells, diffusion of nutrient molecules alone cannot keep up with metabolic demands. Martin Short et al. report that flagella, typically used for motility, play a crucial role in metabolism by concentrating nutrients and enhancing nutrient uptake. The authors developed a mathematical model encoding flagella length, beating frequency, and waveform parameters into the shear stress that a colony’s surface exerts on the surrounding fluid. The fluid velocity around a colony was determined from this equation, and theoretical calculations agreed with the experimental measurements pooled from 10 colonies. The modeling identified a thin nutrient concentration boundary layer, which formed around the leading edge of the colony as it swam. A plume of nutrient depletion or waste product trailed behind. Surprisingly, this nutrient current was proportional to the surface area of the colony. The authors suggest that this proportionality allows larger colonies to acquire nutrients at a faster rate than smaller ones, facilitating green algae’s evolution from a unicellular to multicellular organism. — F.A.
Green algae colony and superimposed fluid streamlines.
“Flows driven by flagella of multicellular organisms enhance long-range molecular transport” by Martin B. Short, Cristian A. Solari, Sujoy Ganguly, Thomas R. Powers, John O. Kessler, and Raymond E. Goldstein (see pages 8315–8319)
CHEMISTRY
Identifying golden buckyballs
Since the discovery of the C60 buckyball and other carbon fullerenes, research has sought to identify and synthesize other molecular hollow-cage structures. Bare elemental metal cages have been particularly elusive, principally because such clusters tend to compact due to metallic properties, but now Satya Bulusu et al. report theoretical and experimental evidence of hollow golden cages. Recent work has shown that gold anion clusters of 13 atoms or fewer possess planar structures, whereas a cluster of 20 (Au20) possesses a pyramidal structure. Bulusu et al. used photoelectron spectroscopy and theoretical approaches to examine the intermediate structures. The authors found that for Au16, Au17, and Au18, all but one of the candidate lowest energy isomers exist as hollow-cage structures. These cages typically have an empty interior diameter of >5.5 Å, which can easily contain a foreign atom. At Au19, the structures shift to pyramids as the hollow cages no longer become energetically competitive. The authors note that gold has some unique properties such as strong relativistic effects and aurophilic attraction, which likely play a key role in allowing the formation of these unusual golden cages. — N.Z.
“Evidence of hollow golden cages” by Satya Bulusu, Xi Li, Lai-Sheng Wang, and Xiao Cheng Zeng (see pages 8326–8330)
GEOLOGY
Mechanism behind bifurcation of tidal basins
Coastal tidal basins, such as Italy’s Venice Lagoon, are invariably dominated by two distinct habitats: tidal flats, which typically lie 0.5–1.0 m below mean sea level, and salt marshes, which lie at elevations of 0.2 m above sea level or higher. Intermediate areas are rare, and Sergio Fagherazzi et al. show that this bifurcation is due to the inherent instability of this elevation range (−0.5 to 0.2 m). Using a conceptual model that takes into account both wave-induced sediment deposition and erosion, Fagherazzi et al. studied bottom shear stress as a function of elevation. The authors found that the elevations around sea level encompass an unstable equilibrium point, where even a slight perturbation will shift the environment toward a higher or lower elevation because either deposition or erosion becomes dominant. Thus, these regions rapidly transition to one of the two stable outcomes. The authors applied this model to several flats and marshes within the Venice Lagoon and found that it accurately predicted the evolution of these locales over the last century. — N.Z.
Venice Lagoon tidal flats.
“Critical bifurcation of shallow microtidal landforms in tidal flats and salt marshes” by Sergio Fagherazzi, Luca Carniello, Luigi D’Alpaos, and Andrea Defina (see pages 8337–8341)
BIOPHYSICS
Structural disorder creates functional diversity
Alternative splicing of pre-mRNA allows a small number of genes to generate large numbers of proteins. Because the mechanism is prevalent only in multicellular eukaryotes, researchers believe that alternative splicing was seminal to the development of multicellular life. Despite its evolutionary importance, few studies have examined in detail the relationship between alternative splicing sites of pre-mRNA and protein structure. Pedro Romero et al. provide evidence that regions of pre-mRNA removed by alternative splicing often code for regions of intrinsic disorder in the corresponding protein products. This association leads to functional and regulatory diversity in proteins and, by removing regions of disorder, avoids potential structural pitfalls. The authors analyzed 46 differentially spliced genes of known function that contained both structured and disordered amino acid segments. Over 80% of the alternatively spliced fragments in these proteins associated with disordered protein regions. In silico prediction data on proteins with known isoforms produced by alternatively spliced fragments lent further support to the authors’ hypothesis. Romero et al. suggest that the protein variations brought about by alternative splicing in structurally disordered regions have facilitated the diversification of multicellular organisms over short periods of geologic time. — F.A.
Alternative splicing + protein disorder = signaling diversity.
“Alternative splicing in concert with protein intrinsic disorder enables increased functional diversity in multicellular organisms” by Pedro R. Romero, Saima Zaidi, Ya Yin Fang, Vladimir N. Uversky, Predrag Radivojac, Christopher J. Oldfield, Marc S. Cortese, Megan Sickmeier, Tanguy LeGall, Zoran Obradovic, and A. Keith Dunker (see pages 8390–8395)
MICROBIOLOGY
Role of adaptive bacterial mutations in cystic fibrosis
The bacterium Pseudomonas aeruginosa colonizes the lungs of most cystic fibrosis (CF) patients by their early teens, causing chronic and often fatal respiratory infections. In furthering the understanding of how P. aeruginosa infects the lungs, Eric Smith et al. identified genetic changes in the bacterium over the course of chronic colonization, which may offer avenues for treating CF. In bacterial isolates taken from a single CF patient over an 8-year period, the authors found numerous genetic adaptations favoring the bacterium’s survival and continued expansion. Bacteria isolated late in infection showed genetic changes that appeared to decrease the bacterium’s virulence, as well as several mutations in genes involved in antibiotic resistance. A subset of genes mutated in this initial patient was also commonly mutated during P. aeruginosa infections in other CF patients. These commonly mutated genes may offer therapeutic targets for treating the chronic phase of this disease, the authors suggest. — M.M.
Mutation tree of patient with chronic airways infection.
“Genetic adaptation by Pseudomonas aeruginosa to the airways of cystic fibrosis patients” by Eric E. Smith, Danielle G. Buckley, Zaining Wu, Channakhone Saenphimmachak, Lucas R. Hoffman, David A. D’Argenio, Samuel I. Miller, Bonnie W. Ramsey, David P. Speert, Samuel M. Moskowitz, Jane L. Burns, Rajinder Kaul, and Maynard V. Olson (see pages 8487–8492)
