September 26, 2012
The story of Earth’s biological and chemical evolution is locked within the geological record of marine sediments, and deciphering these stories requires a means of accessing and calibrating that information. Of the directly targetable marine sedimentary records, the stable isotopes of sulfur are one of the most powerful tools for paleo-environmental reconstructions. This applicability is rooted in the quantitative linkage to surface oxygen budgets (namely atmospheric O2) and inherent biological role with these cycles. In this seminar I outline a suite of projects targeting the multiple sulfur isotope consequences of microbial sulfate reduction (MSR). The sum of these studies allows for the reconstruction of the net rate of sulfate reduction through the Phanerozoic; a prediction that directly relates the quality of organic matter reaching sediments and background changes in O2 over that time. This new calibration and insight is the result of blending physiology and experimental microbiology with a clear Earth history question, all underpinned by the application of a novel isotope system.
October 17, 2012
Biosignature gases are gases emitted by life that can accumulate in an exoplanet atmosphere to remotely detectable levels by future space telescopes. Until now, the dominant focus has been on Earth-like planets, because Earth is the only known planet with life. Yet exoplanets are astonishingly diverse—in terms of their masses, densities, orbits, and host star types—and this diversity motivates a radical extension of what conventionally constitutes a habitable planet. By building a general framework with which to understand a wide range of plausible biosignature gases, we will increase our chances of identifying inhabited worlds.
November 28, 2012
The cell wall is a defining structure of bacterial cells. It provides a protective outer shell and is crucial in pathogenesis as well as the target for important antibiotics. Synthesis of the wall is organised by cytoskeletal proteins homologous to tubulin (FtsZ) and actin (MreB). Because all major branches of the bacterial lineage possess both wall and cytoskeleton, these were probably present in the last common ancestor of the bacteria. L-forms are unusual variants of bacteria that lack the wall and are found in various specialised habitats, possibly responsible for a range of chronic and persistent diseases. We have developed a model system for studying the L-form state in Bacillus subtilis (Leaver et al., 2009, Nature 457, 849-53; Dominguez-Cuevas et al., 2012, Mol Microbiol 83, 52-66; Mercier et al., 2012, Cell Reports 1, 417-23). Unexpectedly, we found that proliferation of L-forms is completely independent of the normally essential FtsZ or MreB cytoskeletal systems and instead occurs by a remarkable membrane blebbing or tubulation process. Results of wide ranging genetic experiments are consistent with the notion that L-form proliferation is based on membrane dynamics, largely driven by biophysical effects. L-forms may provide an interesting model for considering how primitive cells proliferated before the invention of the cell wall. The results also suggest that invention of the cell wall was a pivotal step in the evolution of cellular life.
December 19, 2012
One of the greatest mysteries of life is how a collection of molecules enclosed by a molecular sac self-organizes into a complex system capable of maintaining structural integrity, sensing the environment, propelling itself, self-replicating, and more. In my talk I will illustrate how a combination of cell biology and physics can offer some unique insights into the static organization and dynamic behaviors of cells. Accordingly, the talk will progess from describing the static-mechanical cellular properties to the dynamic processes that underlie cells’ ability to move -- whereas cell micromechanics appears to be conserved across different cell types, the migration strategies differ markedly. The case in point are cancerous cells which, akin to animal predators, can navigate human body in an optimal and thermodynamically most efficient fashion. Such walking strategies are ultimately controlled by the complex signaling networks within the cell -- at the end of my talk I will show how these networks can be "reprogrammed" to exhibit motility patterns ranging from diffusive, through Levy-walking, to ballistic. I will also speculate on the possibility of recreating similar behaviors in artificial chemical systems.
January 23, 2013 - Special Origins Seminar
The lecture will cover recent advances in systems chemistry syntheses of the informational, catalytic and compartment–forming molecules thought necessary for the emergence of life.
February 20, 2013
The talk will provide a general overview of the mission and the first results.
March 13, 2013
Minerals, because of their reactivity and ubiquity, likely contributed to the transformation from prebiotic geochemistry to biochemistry. We propose two central hypothesis: that mineral surfaces acted as (1) an “evolutionary selection stress” in selecting lipid or amphiphile membrane compositions that were stable (survival of the fittest protocells) and (2) as “prebiotic enzymes” for photocatalyzed trans-membrane energy transduction in the emergence of metabolism. We use phospholipid and single chain amphiphile vesicles and planar bilayers as model protocells, and metal oxides and sulfides as model minerals to examine our hypotheses. Results of this work have implications for prebiotic chemistry as well as for other fields, such as biomedical sciences and energy-related technologies.
The plasma membranes of most extant organisms are composed predominantly of zwitterioinc lipids with embedded negatively-charged proteins and other biomolecules, which evolved to facilitate metabolic and genetic functions. We examine the effects of mineral and lipid head-group charge and solution conditions on vesicle stability and integrity. The combined results of quantitative adsorption isotherms, Atomic Force Microscopy, Neutron Reflectivity and Fluorescence methods show that bilayers of the zwitterionic lipid, dipalmitoyl phosphocholine (DPPC) have a greater affinity for and membrane integrity at the surface of the positively-charged mineral, corundum (a-Al2O3) than at the quartz (a-SiO2) surface with a negative-charge. Bilayers of charged lipids were unstable at both types of mineral surfaces. Thus, charged mineral surfaces could provide an environmental stress survival and selection of zwitterionic lipids over negatively-charged lipids in protocell membranes. The preferred stability of neutral vesicles with with or without negatively charged head-groups is also confirmed by bulk selection experiments using single-chain amphiphiles under variable environmental solution conditions, including pH, ionic strength, and multivalent cations.
Metal sulfide clusters at the active core of many metabolic enzymes have been proposed as “molecular fossils” reflecting a role for photocatalytic sulfide minerals in early trans-membrane energy transduction. Preliminary results show that CdS trapped in a DPPC membrane can promote a trans-membrane redox reaction involving methylviologen as oxidant and methanol as reductant.
April 17, 2013
The property of chirality has fascinated scientists and laymen alike since Pasteur’s first painstaking separation of the enantiomorphic crystals of a tartrate salt over 150 years ago. Chiral molecules – nonsuperimposable forms that are mirror images of one another, as are left and right hands – in living organisms in Nature exist almost exclusively as single enantiomers, as exemplified by D-sugars and L-amino acids. Single chirality is critical for molecular recognition and replication processes and would thus seem to be a a signature of life. Yet left and right-handed molecules of a compound will form in equal amounts (a racemic mixture) when we synthesize them in the laboratory in the absence of some type of directing template. Our work has led to the development of several plausible mechanisms for how one enantiomer might have come to dominate over the other in the prebiotic world, highlighting mechanisms for enantioenrichment by either chemical or physical processes.
May 15, 2013
NASA’s Kepler Space Telescope has been in operation for more than 4 years, and its dataset on transiting exoplanets is becoming more and more complete. It is now becoming feasible to make estimates for Eta_Earth—the fraction of Sun-like stars that have at least one rocky planet within their habitable zone. The habitable zone, as conventionally defined, is the region around a star where liquid water can exist on a planet’s surface. Published estimates for Eta_Earth for FGK stars, based on much earlier versions of the Kepler dataset, range from 0.03 to 0.3. More recent published estimates of Eta_Earth for M stars range from 0.15 to 0.6. Why do these estimates differ by so much, and are any of them believable? One reason for disagreement has to do with how the habitable zone boundaries are defined. Theoretical limits derived from 1-D climate modeling tend to be pessimistic, whereas empirical limits based on Venus and Mars are more optimistic. I will talk about what our group has been doing on this problem recently and what we hope to accomplish in the near future.