Prospecting macroalgae (seaweeds) as feedstocks for bioconversion into biofuels and commodity chemical compounds is limited primarily by the availability of tractable microorganisms that can metabolize alginate polysaccharides. Here, we present the discovery of a 36-kilo-base pair DNA fragment from Vibrio splendidus encoding enzymes for alginate transport and metabolism. The genomic integration of this ensemble, together with an engineered system for extracellular alginate depolymerization, generated a microbial platform that can simultaneously degrade, uptake, and metabolize alginate. When further engineered for ethanol synthesis, this platform enables bioethanol production directly from macroalgae via a consolidated process, achieving a titer of 4.7% volume/volume and a yield of 0.281 weight ethanol/weight dry macroalgae (equivalent to ~80% of the maximum theoretical yield from the sugar composition in macroalgae).
The increasing demands placed on natural resources for fuel and food production require that we explore the use of efficient, sustainable feedstocks such as brown macroalgae. The full potential of brown macroalgae as feedstocks for commercial-scale fuel ethanol production, however, requires extensive re-engineering of the alginate and mannitol catabolic pathways in the standard industrial microbe Saccharomyces cerevisiae. Here we present the discovery of an alginate monomer (4-deoxy-L-erythro-5-hexoseulose uronate, or DEHU) transporter from the alginolytic eukaryote Asteromyces cruciatus. The genomic integration and overexpression of the gene encoding this transporter, together with the necessary bacterial alginate and deregulated native mannitol catabolism genes, conferred the ability of an S. cerevisiae strain to efficiently metabolize DEHU and mannitol. When this platform was further adapted to grow on mannitol and DEHU under anaerobic conditions, it was capable of ethanol fermentation from mannitol and DEHU, achieving titres of 4.6% (v/v) (36.2 g l(-1)) and yields up to 83% of the maximum theoretical yield from consumed sugars. These results show that all major sugars in brown macroalgae can be used as feedstocks for biofuels and value-added renewable chemicals in a manner that is comparable to traditional arable-land-based feedstocks.
We review the use of pair distribution function analysis for characterization of atomic structure in nanomaterials.
Most studies on iron oxide nanocrystals (NCs) suggest that the magnetic properties depend strongly on size for diameters below 10 nm, but there is less agreement about how the structure of the NC surface influences magnetic properties. Because the magnetic properties of iron oxide NCs hold promise for applications from cancer detection and therapeutics to environmental remediation, it is imperative to understand how size influences those properties. In most cases, the effective magnetic size is significantly lower than the measured physical size, a finding attributed to spin canting or disorder at the NC surface. A complicating factor is that the reaction conditions used to produce samples influence their magnetic properties. Thus, we employed a continuous growth method involving layer-by-layer addition of precursor to produce single-crystalline, spherical cores with subnanometer precision over a range of sizes under the same reaction conditions. Analysis of the NCs by small-angle X-ray scattering, transmission electron microscopy, and powder X-ray diffraction showed that the NCs possess the spinel structure (primarily maghemite) and are crystalline, defect-free, and uniform in size. The saturation magnetization values for a series of eight distinct diameters between 4 and 10 nm increase smoothly with increasing size, from 55 to 78 Am 2 /kg. Magnetic sizes of the NCs determined by fitting magnetization curves to the Langevin function are nearly identical to the physical sizes, suggesting low levels of strain-producing defects and a very thin nonmagnetic surface layer on the NCs. The results suggest that syntheses that permit slower growth at reduced temperatures through a single reaction mechanism can enhance, and offer fine control over, magnetic properties.
Spinel iron oxide nanocrystals (NCs) have been reported to have atomic-level core and surface structural features that differ from those of the bulk material. Recent advances in a continuous growth synthesis of metal oxide NCs make it possible to prepare a series of NCs with subnanometer control of size with diameters below 10 nm that are well-suited for investigating size-dependent structure and reactivity. Here, we study the evolution of size-dependent structure in spinel iron oxide and determine how nanoscale structure influences the growth of NCs. We synthesized spinel iron oxide NCs via a continuous growth method that permits layer-by-layer control of size in order to monitor nanoscale structure over 16 core sizes between 3 and 10 nm. X-ray total scattering data were collected and analyzed with pair distribution function (PDF) analysis in order to refine quantitative structural features including cation occupancies that could be used to detect changes both in the oxidation state and the presence of tetrahedrally coordinated cation vacancies in the NCs. We find that the average iron oxidation state increases as core diameters decrease from 8 down to 3 nm. The trend in iron oxidation state can be explained by the oxidation of surface layers in the NCs. For samples exposed to air for several weeks, oxidation appears to cease when a volume equivalent to that of an ∼1.3 nm shell is converted to the more oxidized maghemite. The number of tetrahedrally coordinated cation vacancies also increases as the NC core size decreases. The correlation between the number of these vacancies and the faster growth for smaller NCs suggests that these reactive vacancies may be responsible for the rapid growth observed for nanocrystals with diameters smaller than 8 nm.
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