Continuous production of extracellular ultrafine calcite particles by the marine coccolithophorid alga Pleurochrysis carterae

A new procedure for the production of ultrafine calcite particles by the marine coccolithophorid alga Pleurochrysis carterae is reported. During continuous culture, calcite particles (coccoliths) were detached from the cell surface by optimized air-bubbling, which greatly reduced the damage associated with previous sonication methods. Detached calcite particles could be continuously recovered directly from the culture medium using a nylon mesh membrane filtration module. Cells remained viable and continued to produce coccoliths during culture. The optimum productivity of ultrafine calcite particles was 18 mg/l per day. These results demonstrate the potential for a continuous system for the photosynthetically driven removal of CO₂ and its fixation into ultrafine inorganic calcite particles. Correspondence to: T. Matsunaga     

Incorporation of zinc into the coccoliths of the microalga <Emphasis Type="Italic">Emiliania huxleyi</Emphasis>

The coccolithophore Emiliania huxleyi is covered with elaborated calcite plates, the so-called coccoliths, which are produced inside the cells. We investigated the incorporation of zinc into the coccoliths of E. huxleyi by applying different zinc and calcium amounts via the culture media and subsequently analyzing the zinc content in the cells and the Zn/Ca ratio of the coccoliths. To investigate the Zn/Ca ratio of coccoliths built in the manipulated media, the algae have first to be decalcified, i.e. coccolith free. We used a newly developed decalcification method to obtain ‘naked’ cells for cultivation. E. huxleyi proliferated and produced new coccoliths in all media with manipulated Zn/Ca ratios. The cells and the newly built coccoliths were investigated regarding their zinc content and their Zn/Ca ratio, respectively. High zinc amounts were taken up by the algae. The Zn/Ca ratio of the coccoliths was positively correlated to the Zn/Ca ratio of the applied media. The unique feature of the coccoliths was maintained also at high Zn/Ca ratios. We suggest the following pathway of the zinc ions into the coccoliths: first, the zinc ions are bound to the cell surface, followed by their transportation into the cytoplasm. Obviously, the zinc ions are removed afterwards into the coccolith vesicle, where the zinc is incorporated into the calcite coccoliths which are then extruded. The incorporation of toxic zinc ions into the coccoliths possibly due to a new function of the coccoliths as detoxification sites is discussed.     

Coccolithogenesis In Scyphosphaera apsteinii (Prymnesiophyceae)

Coccolithophores are the most significant producers of marine biogenic calcite, although the intracellular calcification process is poorly understood. In the case of Scyphosphaera apsteinii Lohmann 1902, flat ovoid muroliths and bulky, vase‐shaped lopadoliths with a range of intermediate morphologies may be produced by a single cell. This polymorphic species is within the Zygodiscales, a group that remains understudied with respect to ultrastructure and coccolith ontogeny. We therefore undertook an analysis of cell ultrastructure, morphology, and coccolithogenesis. The cell ultrastructure showed many typical haptophyte features, with calcification following a similar pattern to that described for other heterococcolith bearing species including Emiliania huxleyi. Of particular significance was the reticular body role in governing fine‐scale morphology, specifically the central pore formation of the coccolith. Our observations also highlighted the essential role of the inter‐ and intracrystalline organic matrix in growth and arrangement of the coccolith calcite. S. apsteinii secreted mature coccoliths that attached to the plasma membrane via fibrillar material. Time‐lapse light microscopy demonstrated secretion of lopadoliths occurred base first before being actively repositioned at the cell surface. Significantly, growth irradiance influenced the coccosphere composition with fewer lopadoliths being formed relative to muroliths at higher light intensities. Overall, our observations support dynamic metabolic (i.e., in response to growth irradiance), sensory and cytoskeletal control over the morphology and secretion of polymorphic heterococcoliths. With a basic understanding of calcification established, S. apsteinii could be a valuable model to further study coccolithophore calcification and cell physiological responses to ocean acidification.     

NO MECHANISTIC DEPENDENCE OF PHOTOSYNTHESIS ON CALCIFICATION IN THE COCCOLITHOPHORID EMILIANIA HUXLEYI (HAPTOPHYTA)1

There is still considerable uncertainty about the relationship between calcification and photosynthesis. It has been suggested that since calcification in coccolithophorids is an intracellular process that releases CO₂, it enhances photosynthesis in a manner analogous to a carbon‐concentrating mechanism (CCM). The ubiquitous, bloom‐forming, and numerically abundant coccolithophorid Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler was studied in nutrient‐replete, pH and [CO₂] controlled, continuous cultures (turbidostats) under a range of [Ca²⁺] from 0 to 9 mM. We examined the long‐term, fully acclimated photosynthesis‐light responses and analyzed the crystalline structure of the coccoliths using SEM. The E. huxleyi cells completely lost their coccosphere when grown in 0 [Ca²⁺], while thin, undercalcified and brittle coccoliths were evident at 1 mM [Ca²⁺]. Coccoliths showed increasing levels of calcification with increasing [Ca²⁺]. More robust coccoliths were noted, with no discernable differences in coccolith morphology when the cells were grown in either 5 or 9 mM (ambient seawater) [Ca²⁺]. In contrast to calcification, photosynthesis was not affected by the [Ca²⁺] in the media. Cells showed no correlation of their light‐dependent O₂ evolution with [Ca²⁺], and in all [Ca²⁺]‐containing turbidostats, there were no significant differences in growth rate. The results show unequivocally that as a process, photosynthesis in E. huxleyi is mechanistically independent from calcification.     

Polyanion-mediated mineralization — assembly and reorganization of acidic polysaccharides in the Golgi system of a coccolithophorid alga during mineral deposition

Summary Immunolocalization of two highly acidic polysaccharides (PS-1 and PS-2) in a calcifying algaPleurochrysis carterae is described throughout the mineralization process, from before crystal nucleation through the cessation of crystal growth. This unicellular coccolithophorid alga is a useful model for mineralization because it produces calcified scales known as coccoliths in homogeneous cell culture. PS-1 and PS-2 were localized in the crystal coats of mature coccoliths and in electron dense Golgi particles. The polyanions are synthesized in medial Golgi cisternae and co-aggregate with calcium ions into discrete 25 nm particles. Particle-laden vesicles bud from cisternal margins and fuse with a coccolith-forming saccule containing an organic oval-shaped scale which forms the base of the future coccolith. The particles are localized on the base before the onset of mineral deposition and are present in the coccolith saccule throughout the period of crystal (CaCO₃) nucleation and growth. During the final phase of coccolith formation, the particles disappear, and the mature crystals acquire an amorphous coat containing PS-1 and PS-2 polysaccharides which remain with the mineral phase after the coccoliths are extruded from the cell. Postulated mechanisms of polyanion-mediated mineralization are reviewed and their relevance to the calcification of coccoliths is addressed.Abbreviations PS-1 polysaccharide one - PS-2 polysaccharide two - BSA bovine serum albumin - SDS sodium dodecyl sulfate - MES 2-(N-morpholino)-ethanesulfonic acid - EDTA ethylenediaminetetraacetic acid - DHA 3-deoxy-lyxo-2-heptulosaric acid - TCA trichloroacetic acid     

Chitosan molecular structure as a function of N-acetylation

Molecular dynamics simulations have been carried out to characterize the structure and solubility of chitosan nanoparticle‐like structures as a function of the deacetylation level (0, 40, 60, and 100%) and the spatial distribution of the N‐acetyl groups in the particles. The polysaccharide chains of highly N‐deacetylated particles where the N‐acetyl groups are uniformly distributed present a high flexibility and preference for the relaxed two‐fold helix and five‐fold helix motifs. When these groups are confined to a given region of the particle, the chains adopt preferentially a two‐fold helix with ? and ψ values close to crystalline chitin. Nanoparticles with up to 40% acetylation are moderately soluble, forming stable aggregates when the N‐acetyl groups are unevenly distributed. Systems with 60% or higher N‐acetylation levels are insoluble and present similar degrees of swelling regardless the distribution of their N‐acetyl groups. Overall particle solvation is highly affected by electrostatic forces resulting from the degree of acetylation. The water mobility and orientation around the polysaccharide chains affects the stability of the intramolecular O3‐HO3_(n)···O5_{(n +1)} hydrogen bond, which in turn controls particle aggregation. © 2011 Wiley Periodicals, Inc. Biopolymers 95: 448–460, 2011.     

Formation and mosaicity of coccolith segment calcite of the marine algae Emiliania huxleyi

Coccolithophores belong to the most abundant calcium carbonate mineralizing organisms. Coccolithophore biomineralization is a complex and highly regulated process, resulting in a product that strongly differs in its intricate morphology from the abiogenically produced mineral equivalent. Moreover, unlike extracellularly formed biological carbonate hard tissues, coccolith calcite is neither a hybrid composite, nor is it distinguished by a hierarchical microstructure. This is remarkable as the key to optimizing crystalline biomaterials for mechanical strength and toughness lies in the composite nature of the biological hard tissue and the utilization of specific microstructures. To obtain insight into the pathway of biomineralization of Emiliania huxleyi coccoliths, we examine intracrystalline nanostructural features of the coccolith calcite in combination with cell ultrastructural observations related to the formation of the calcite in the coccolith vesicle within the cell. With TEM diffraction and annular dark‐field imaging, we prove the presence of planar imperfections in the calcite crystals such as planar mosaic block boundaries. As only minor misorientations occur, we attribute them to dislocation networks creating small‐angle boundaries. Intracrystalline occluded biopolymers are not observed. Hence, in E. huxleyi calcite mosaicity is not caused by occluded biopolymers, as it is the case in extracellularly formed hard tissues of marine invertebrates, but by planar defects and dislocations which are typical for crystals formed by classical ion‐by‐ion growth mechanisms. Using cryo‐preparation techniques for SEM and TEM, we found that the membrane of the coccolith vesicle and the outer membrane of the nuclear envelope are in tight proximity, with a well‐controlled constant gap of ~4 nm between them. We describe this conspicuous connection as a not yet described interorganelle junction, the “nuclear envelope junction”. The narrow gap of this junction likely facilitates transport of Ca²⁺ ions from the nuclear envelope to the coccolith vesicle. On the basis of our observations, we propose that formation of the coccolith utilizes the nuclear envelope–endoplasmic reticulum Ca²⁺‐store of the cell for the transport of Ca²⁺ ions from the external medium to the coccolith vesicle and that E. huxleyi calcite forms by ion‐by‐ion growth rather than by a nanoparticle accretion mechanism.     

Effect of Coccolith Polysaccharides Isolated from the Coccolithophorid, <Emphasis Type="Italic">Emiliania huxleyi</Emphasis>, on Calcite Crystal Formation in In Vitro CaCO<Subscript>3</Subscript> Crystallization

Marine coccolithophorids (Haptophyceae) produce calcified scales “coccoliths” which are composed of CaCO₃ and coccolith polysaccharides (CP) in the coccolith vesicles. CP was previously reported to be composed of uronic acids and sulfated residues, etc. attached to the polymannose main chain. Although anionic polymers are generally known to play key roles in biomineralization process, there is no experimental data how CP contributes to calcite crystal formation in the coccolithophorids. CP used was isolated from the most abundant coccolithophorid, Emiliania huxleyi. CaCO₃ crystallization experiment was performed on agar template layered onto a plastic plate that was dipped in the CaCO₃ crystallization solution. The typical rhombohedral calcite crystals were formed in the absence of CP. CaCO₃ crystals formed on the naked plastic plate were obviously changed to stick-like shapes when CP was present in the solution. EBSD analysis proved that the crystal is calcite of which c-axis was elongated. CP in the solution stimulated the formation of tabular crystals with flat edge in the agarose gel. SEM and FIB-TEM observations showed that the calcite crystals were formed in the gel. The formation of crystals without flat edge was stimulated when CP was preliminarily added in the gel. These observations suggest that CP has two functions: namely, one is to elongate the calcite crystal along c-axis and another is to induce tabular calcite crystal formation in the agarose gel. Thus, CP may function for the formation of highly elaborate species-specific structures of coccoliths in coccolithophorids.     

NEW EVIDENCE FOR MORPHOLOGICAL AND GENETIC VARIATION IN THE COSMOPOLITAN COCCOLITHOPHORE EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) FROM THE COX1b‐ATP4 GENES1

Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler is a cosmopolitan coccolithophore occurring from tropical to subpolar waters and exhibiting variations in morphology of coccoliths possibly related to environmental conditions. We examined morphological characters of coccoliths and partial mitochondrial sequences of the cytochrome oxidase 1b (cox1b) through adenosine triphosphate synthase 4 (atp4) genes of 39 clonal E. huxleyi strains from the Atlantic and Pacific Oceans, Mediterranean Sea, and their adjacent seas. Based on the morphological study of culture strains by SEM, Type O, a new morphotype characterized by coccoliths with an open central area, was separated from existing morphotypes A, B, B/C, C, R, and var. corona, characterized by coccoliths with central area elements. Molecular phylogenetic studies revealed that E. huxleyi consists of at least two mitochondrial sequence groups with different temperature preferences/tolerances: a cool‐water group occurring in subarctic North Atlantic and Pacific and a warm‐water group occurring in the subtropical Atlantic and Pacific and in the Mediterranean Sea.     

The Role of Autotrophic Picocyanobacteria in Calcite Precipitation in an Oligotrophic Lake

A 1-year field study monitoring depth profiles of picoplankton and physicochemical data in the oligotrophic Lake Lucerne (Switzerland) showed that picocyanobacteria play an important role in the CaCO₃ precipitation process. Laboratory experiments with Mychonastes and Chlorella, isolated from Lake Lucerne and Synechococcus using ion selective electrodes, scanning electron microscopy and X-ray powder diffraction clearly demonstrated the potential of picoplankton for fast and effective CaCO₃ precipitation. The combination of a field study with laboratory experiments confirmed the previous reports of picocyanobacteria triggering the CaCO₃ precipitation in hardwater oligotrophic lakes. Electron micrographs of particles from the water column often reveal the size and shape of picoplankton cells covered by calcite. In addition the results from the laboratory approach indicated that algae and bacteria induced different precipitation mechanisms. Experiments with Mychonastes and Chlorella produced crystalline calcite completely covering the cells. Experiments with the cyanobacteria Synechococcus, however, yielded amorphous, micritic CaCO₃, indicating a different precipitation process.     

Adaptive evolution in the coccolithophore Gephyrocapsa oceanica following 1,000 generations of selection under elevated CO2

Coccolithophores are important oceanic primary producers not only in terms of photosynthesis but also because they produce calcite plates called coccoliths. Ongoing ocean acidification associated with changing seawater carbonate chemistry may impair calcification and other metabolic functions in coccolithophores. While short‐term ocean acidification effects on calcification and other properties have been examined in a variety of coccolithophore species, long‐term adaptive responses have scarcely been documented, other than for the single species Emiliania huxleyi. Here, we investigated the effects of ocean acidification on another ecologically important coccolithophore species, Gephyrocapsa oceanica, following 1,000 generations of growth under elevated CO₂ conditions (1,000 μatm). High CO₂‐selected populations exhibited reduced growth rates and enhanced particulate organic carbon (POC) and nitrogen (PON) production, relative to populations selected under ambient CO₂ (400 μatm). Particulate inorganic carbon (PIC) and PIC/POC ratios decreased progressively throughout the selection period in high CO₂‐selected cell lines. All of these trait changes persisted when high CO₂‐grown populations were moved back to ambient CO₂ conditions for about 10 generations. The results suggest that the calcification of some coccolithophores may be more heavily impaired by ocean acidification than previously predicted based on short‐term studies, with potentially large implications for the ocean's carbon cycle under accelerating anthropogenic influences.     

Unravelling the microbial role in ooid formation – results of an in situ experiment in modern freshwater Lake Geneva in Switzerland

The microbial role in the formation of the cortex of low‐Mg calcite freshwater ooids in western part of Lake Geneva in Switzerland has been suggested previously, but not demonstrated conclusively. Early work mostly concentrated in hypersaline milieus, and hence little is known about their genesis in freshwater environments. We designed an in situ experiment to mimic the natural process of low‐Mg calcite precipitation. A special device was placed in the ooid‐rich bank of the lake. It contained frosted glass (SiO₂) slides, while quartz (SiO₂) is the most abundant mineral composition of ooid nuclei that acted as artificial substrates to favour microbial colonization. Microscopic inspection of the slides revealed a clear seasonal pattern of carbonate precipitates, which were always closely associated with biofilms that developed on the surface of the frosted slides containing extracellular polymeric substance, coccoid and filamentous cyanobacteria, diatoms and heterotrophic bacteria. Carbonate precipitation peaks during early spring and late summer, and low‐Mg calcite crystals mostly occur in close association with filamentous and coccoid cyanobacteria (e.g. Tolypothrix, Oscillatoria and Synechococcus, Anacystis, respectively). Further scanning electron microscope inspection of the samples revealed low‐Mg calcite with crystal forms varying from anhedral to euhedral rhombohedra, depending on the seasons. Liquid cultures corroborate the in situ observations and demonstrate that under the same physicochemical conditions the absence of biofilms prevents the precipitation of low‐Mg calcite crystals. These results illustrate that biofilms play a substantial role in low‐Mg calcite ooid cortex formation. It further demonstrates the involvement of microbes in the early stages of ooid development. Combined with ongoing microbial cultures under laboratory‐controlled conditions, the outcome of our investigation favoured the hypothesis of external microbial precipitation of low‐Mg calcite as the main mechanism involved in the early stage of ooid formation in freshwater Lake Geneva.     

Hepatitis E virus capsid protein assembles in 4M urea in the presence of salts

The hepatitis E virus (HEV) capsid protein has been demonstrated to be able to assemble into particles in vitro. However, this process and the mechanism of protein–protein interactions during particle assembly remain unclear. In this study, we investigated the assembly mechanism of HEV structural protein subunits, the capsid protein p239 (aa368–606), using analytical ultracentrifugation. It was the first to observe that the p239 can form particles in 4M urea as a result of supplementation with salt, including ammonium sulfate [(NH₄)₂SO₄], sodium sulfate (Na₂SO₄), sodium chloride (NaCl), and ammonium chloride (NH₄Cl). Interestingly, it is the ionic strength that determines the efficiency of promoting particle assembly. The assembly rate was affected by temperature and salt concentration. When (NH₄)₂SO₄ was used, assembling intermediates of p239 with sedimentation coefficient values of approximately 5 S, which were mostly dodecamers, were identified for the first time. A highly conserved 28‐aa region (aa368–395) of p239 was found to be critical for particle assembly, and the hydrophobic residues Leu³⁷², Leu³⁷⁵, and Leu³⁹⁵of p239 was found to be critical for particle assembly, which was revealed by site‐directed mutagenesis. This study provides new insights into the assembly mechanism of native HEV, and contributes a valuable basis for further investigations of protein assembly by hydrophobic interactions under denaturing conditions.     

The Effect of cytoskeleton inhibitors on coccolith morphology in Coccolithus braarudii and Scyphosphaera apsteinii

The calcite platelets of coccolithophores (Haptophyta), the coccoliths, are among the most elaborate biomineral structures. How these unicellular algae accomplish the complex morphogenesis of coccoliths is still largely unknown. It has long been proposed that the cytoskeleton plays a central role in shaping the growing coccoliths. Previous studies have indicated that disruption of the microtubule network led to defects in coccolith morphogenesis in Emiliania huxleyi and Coccolithus braarudii. Disruption of the actin network also led to defects in coccolith morphology in E. huxleyi, but its impact on coccolith morphology in C. braarudii was unclear, as coccolith secretion was largely inhibited under the conditions used. A more detailed examination of the role of actin and microtubule networks is therefore required to address the wider role of the cytoskeleton in coccolith morphogenesis. In this study, we have examined coccolith morphology in C. braarudii and Scyphosphaera apsteinii following treatment with the microtubule inhibitors vinblastine and colchicine (S. apsteinii only) and the actin inhibitor cytochalasin B. We found that all cytoskeleton inhibitors induced coccolith malformations, strongly suggesting that both microtubules and actin filaments are instrumental in morphogenesis. By demonstrating the requirement for the microtubule and actin networks in coccolith morphogenesis in diverse species, our results suggest that both of these cytoskeletal elements are likely to play conserved roles in defining coccolith morphology.     

Involvement of Acidic Polysaccharide Ph-PS-2 and Protein in Initiation of Coccolith Mineralization,as Demonstrated by In Vitro Calcification on the Base Plate

Coccolithophorids, unicellular marine microalgae, have calcified scales with elaborate structures, called coccoliths, on the cell surface. Coccoliths generally comprise a base plate, CaCO₃, and a crystal coat consisting of acidic polysaccharides. In this study, the in vitro calcification conditions on the base plate of Pleurochrysis haptonemofera were examined to determine the functions of the base plate and acidic polysaccharides (Ph-PS-1, -2, and -3). When EDTA-treated coccoliths (acidic polysaccharide-free base plates) or low pH-treated coccoliths (whole acidic polysaccharide-containing base plates) were used, mineralization was not detected on the base plate. In contrast, in the case of coccoliths which were decalcified by lowering of the pH and then treated with urea (Ph-PS-2-containing base plates), distinct aggregates, probably containing CaCO₃, were observed only on the rim of the base plates. Energy dispersive X-ray spectroscopy (EDS) confirmed that the aggregates contained Ca and O, although X-ray diffraction analysis did not reveal any evidence of crystalline materials. Also, in vitro mineralization experiments performed on EDTA-treated coccoliths using isolated acidic polysaccharides demonstrated that the Ca-containing aggregates were markedly formed only in the presence of Ph-PS-2. Furthermore, in vitro mineralization experiments conducted on protein-extracted base plates suggested that the coccolith-associated protein(s) are involved in the Ca deposition. These findings suggest that Ph-PS-2 associated with the protein(s) on the base plate rim initiates Ca²⁺ binding at the beginning of coccolith formation, and some other factors are required for subsequent calcite formation.     

COCCOLITH FUNCTION AND MORPHOGENESIS: INSIGHTS FROM APPENDAGE‐BEARING COCCOLITHOPHORES OF THE FAMILY SYRACOSPHAERACEAE (HAPTOPHYTA)1

The morphology of three remarkable genera of coccolithophores, Ophiaster, Michaelsarsia, and Calciopappus, is reviewed based on new images using field emission scanning electron microscopy. Each of these genera characteristically forms coccospheres with long appendages formed of highly modified coccoliths, which radiate from either the circum‐flagellar pole of the coccosphere (Calciopappus and Michaelsarsia) or the antapical pole (Ophiaster). For each genus, it is shown that the appendage coccoliths can also occur in an alternative orientation appressed to the main coccosphere. It is hypothesized that the appendage coccoliths are initially deployed in the appressed orientation and that extension of the appendages is a dynamic response to environmental stress. The observations suggest that coccoliths are more sophisticatedly adapted to specific functions than has been assumed and that the cytoskeleton plays more active roles in coccolith morphogenesis and deployment than has previously been inferred.     

COVER

Ultrastructural analysis reveals how the calcifying haptophyte Scyphosphaera apsteinii engineers and secretes its polymorphic calcite coccoliths to construct the external coccosphere. [Vol. 48, No. 6, pp. 1343–1361]     

Ti‐Gradient Doping to Stabilize Layered Surface Structure for High Performance High‐Ni Oxide Cathode of Li‐Ion Battery

High‐Ni layered oxide cathodes are considered to be one of the most promising cathodes for high‐energy‐density lithium‐ion batteries due to their high capacity and low cost. However, surfice residues, such as NiO‐type rock‐salt phase and Li₂CO₃, are often formed at the particle surface due to the high reactivity of Ni³⁺, and inevitably result in an inferior electrochemical performance, hindering the practical application. Herein, unprecedentedly clean surfaces without any surfice residues are obtained in a representative LiNi_0.8Co_0.2O₂ cathode by Ti‐gradient doping. High‐resolution transmission electron microscopy (TEM) reveals that the particle surface is composed of a disordered layered phase (≈6 nm in thickness) with the same rhombohedra structure as its interior. The formation of this disordered layered phase at the particle surface is electrochemically favored. It leads to the highest rate capacity ever reported and a superior cycling stability. First‐principles calculations further confirm that the excellent electrochemical performance has roots in the excellent chemical/structural stability of such a disordered layered structure, mainly arising from the improved robustness of the oxygen framework by Ti doping. This strategy of constructing the disordered layered phase at the particle surface could be extended to other high‐Ni layered transition metal oxides, which will contribute to the enhancement of their electrochemical performance.     

Strain improvement of <Emphasis Type="Italic">Sporosarcina pasteurii</Emphasis> for enhanced urease and calcite production

Phenotypic mutants of Sporosarcina pasteurii (previously known as Bacillus pasteurii) (MTCC 1761) were developed by UV irradiation to test their ability to enhance urease activity and calcite production. Among the mutants, Bp M-3 was found to be more efficient compared to other mutants and wild-type strain. It produced the highest urease activity and calcite production compared to other isolates. The production of extracellular polymeric substances and biofilm was also higher in this mutant than other isolates. Microbial sand plugging results showed the highest calcite precipitation by Bp M-3 mutant. Scanning electron micrography, energy-dispersive X-ray and X-ray diffraction analyses evidenced the direct involvement of bacteria in CaCO₃ precipitation. This study suggests that calcite production by the mutant through biomineralization processes is highly effective and may provide a useful strategy as a sealing agent for filling the gaps or cracks and fissures in any construction structures.     

Surface‐Plasmon Assisted Energy Conversion in Dye‐Sensitized Solar Cells

In this study, the effect of plasmonic core‐shell structures, consisting of dielectric cores and metallic nanoshells, on energy conversion in dye‐sensitized solar cells (DSSCs) is investigated. The structure of the core‐shell particles is controlled to couple with visible light so that the visible component of the solar spectrum is amplified near the core‐shell particles. In core‐shell particle – TiO₂ nanoparticle films, the local field intensity and light pathways are increased due to the surface plasmons and light scattering. This, in turn, enlarges the optical cross‐section of dye sensitizers coated onto the mixed films. When 22 vol% of core‐shell particles are added to a 5 μm thick TiO₂ film, the energy conversion efficiency of DSSCs increases from 2.7% to 4.0%, in spite of a more than 20% decrease in the amount of dyes adsorbed on the composite films. The correlation between core‐shell particle content and energy conversion efficiency in DSSCs is explained by the balance among near‐field effects, light scattering efficiency, and surface area in the composite films.