The Chloroplast of Chlamydomonas reinhardtii as a Testbed for Engineering Nitrogen Fixation into Plants

Eukaryotic organisms such as plants are unable to utilise nitrogen gas (N₂) directly as a source of this essential element and are dependent either on its biological conversion to ammonium by diazotrophic prokaryotes, or its supply as chemically synthesised nitrate fertiliser. The idea of genetically engineering crops with the capacity to fix N₂ by introduction of the bacterial nitrogenase enzyme has long been discussed. However, the expression of an active nitrogenase must overcome several major challenges: the coordinated expression of multiple genes to assemble an enzyme complex containing several different metal cluster co-factors; the supply of sufficient ATP and reductant to the enzyme; the enzyme’s sensitivity to oxygen; and the intracellular accumulation of ammonium. The chloroplast of plant cells represents an attractive location for nitrogenase expression, but engineering the organelle’s genome is not yet feasible in most crop species. However, the unicellular green alga Chlamydomonas reinhardtii represents a simple model for photosynthetic eukaryotes with a genetically tractable chloroplast. In this review, we discuss the main advantages, and limitations, of this microalga as a testbed for producing such a complex multi-subunit enzyme. Furthermore, we suggest that a minimal set of six transgenes are necessary for chloroplast-localised synthesis of an ‘Fe-only’ nitrogenase, and from this set we demonstrate the stable expression and accumulation of the homocitrate synthase, NifV, under aerobic conditions. Arguably, further studies in C. reinhardtii aimed at testing expression and function of the full gene set would provide the groundwork for a concerted future effort to create nitrogen-fixing crops.     

A novel polymer infiltrated ceramic for dental simulation

Simulation of tooth preparation using rotary cutting instruments is viewed as beneficial and essential in dental training. Various types of materials have been used for simulation systems in dental preclinical training. However, the phantom tooth materials used for simulation have not changed significantly for decades and they are acknowledged to be different from natural teeth. This study investigated the mechanical properties and microstructure of a widely used phantom tooth material and compared them with a novel, polymer infiltrated, ceramic. It was concluded that the polymer infiltrated ceramic has mechanical properties more similar to natural teeth than current phantom tooth materials, suggesting that it might be a good candidate material for phantom teeth for trainees to acquire initial tactile sense for tooth preparation.     

The effect of sand grading on the calcium silicate brick reaction

Calcium silicate brick specimens with different sand gradings were autoclaved at 1.20 MN/m² gauge steam pressure for periods of 2, 4, 6, 8, 12 and 16 hours and tested for compressive strength and drying shrinkage. Their composition has been investigated by X-ray diffraction and differential thermal analysis.Results were in agreement with existing knowledge that the reaction was faster, the higher the content of fine sand in the specimen. The development of tobermorite as indicated by X-ray diffraction was not related to any other properties of the specimens examined. Drying shrinkage was correlated with the amount of hydrosilicate formed but the strength (whose maximum was achieved with a gap grading) was more probably related to the degree to which the cementing material filled the voids.     

Application of thermal analysis in investigations on calcium silicate bricks

The principles of differential thermal analysis and thermogravimetric analysis are outlined and two examples of their application to autoclaved calcium silicate products are given. The proportions of calcium hydroxide and calcium carbonate in samples can be estimated. Useful information on calcium hydrosilicate binding materials can be obtained in experiments where the number of variables is strictly controlled.     

The composition of the binding material in calcium silicate bricks

Results from the chemical analyses of commercial calcium silicate brick samples are presented. The composition of the binding material in the bricks is consistent with the formation of C? S? H(I) . The strength of the bricks appears to be related to the proportion of voids filled with cementing material.     

Comparison of the binding properties of hydrated lime and cement in brick manufacture

The effect of replacing cement by hydrated lime in autoclaved brick specimens with binder contents of 7.5–25% has been studied on a laboratory scale and the results have been compared with non-autoclaved cement-bound specimens. It has been shown that the strength of cement-bound specimens autoclaved for 8 h was similar to that of specimens air-cured for 28 days. Lime was shown to be superior to cement in autoclaved specimens for strengths up to the maximum of 23 MN/m² obtainable with the lime binder. For a given compressive strength up to this value, half as much binder was required in the mix and, in addition, the green strength was greater and the drying shrinkage was lower when lime was used instead of cement. Maximum strengths of up to 42 MN/m² were achieved at high binder contents with blends of lime and cement.     

A note on volume changes in the lime-silica reaction

Volume changes occuring during the formation of calcium hydrosilicate phases in autoclaved quartzlime products have been calculated. An increase in the volume of the solid phase (and a corresponding decrease in porosity) occurs with the formation of C-S-H(II) and C-S-H(I) and is followed by a subsequent decrease in the volume of the solid phase with the formation of tobermorite, gyrolite or xonotlite. These changes correspond to strength changes in the system.     

Use of massively parallel molecular dynamics simulations for radiation damage in pyrochlores

DL_POLY_3 is a general purpose molecular dynamics (MD) simulation package designed to simulate systems of the order of tens of millions of particles and beyond by efficiently harnessing the power of modern computer clusters. Here we discuss the package design, functionality and report on performance and capability limits. We then report the application of DL_POLY_3 to study radiation cascades in Gd₂Ti₂O₇ and Gd₂Zr₂O₇, potential materials for high-level radioactive waste storage and discuss problems associated with the analysis of the cascades. We see little direct amorphisation but rather the start of a transition to the fluorite structure which is more pronounced for the Zr than the Ti compound.

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The effect of serrations on carbon fibre posts-retention within the root canal, core retention, and post rigidity

The retention in root canals of serrated carbon fibre Composiposts and stainless steel Paraposts was tested under tensile load. Twenty unrestored human roots were endodontically prepared and root filled. Two groups of 10 roots received 1.4-mm Composiposts or 1.25-mm Paraposts luted with a resin cement. The specimens were then embedded in acrylic resin and mounted in an Instron machine. The tensile force (kg) required to dislodge the posts was recorded and analysed with Student's test. The results revealed that there was no significant difference in the retention of either post (P > .05). The rigidty of 10 1.4-mm serrated Composiposts was tested in a three-point bend test in an Instron machine, and the retention of composite cores to 10 of these posts under tensile force was also tested. The results from these tests were compared to previous data from the authors' laboratory and revealed that the serrations significantly reduced the rigidity of the post (P < .001) and increased the retention of a core material (P < .001).