Coalescence and directed anisotropic growth of starch granule initials in subdomains of Arabidopsis thaliana chloroplasts.

Living cells orchestrate enzyme activities to produce myriads of biopolymers but cell-biological understanding of such processes is scarce. Starch, a plant biopolymer forming discrete, semi-crystalline granules within plastids, plays a central role in glucose storage, which is fundamental to life. Combining complementary imaging techniques and Arabidopsis genetics we reveal that, in chloroplasts, multiple starch granules initiate in stromal pockets between thylakoid membranes. These initials coalesce, then grow anisotropically to form lenticular granules. The major starch polymer, amylopectin, is synthesized at the granule surface, while the minor amylose component is deposited internally. The non-enzymatic domain of STARCH SYNTHASE 4, which controls the protein's localization, is required for anisotropic growth. These results present us with a conceptual framework for understanding the biosynthesis of this key nutrient.

Integrated waveguide PIN photodiodes exploiting lateral Si/Ge/Si heterojunction.

Germanium photodetectors are considered to be mature components in the silicon photonics device library. They are critical for applications in sensing, communications, or optical interconnects. In this work, we report on design, fabrication, and experimental demonstration of an integrated waveguide PIN photodiode architecture that calls upon lateral double Silicon/Germanium/Silicon (Si/Ge/Si) heterojunctions. This photodiode configuration takes advantage of the compatibility with contact process steps of silicon modulators, yielding reduced fabrication complexity for transmitters and offering high-performance optical characteristics, viable for high-speed and efficient operation near 1.55 µm wavelengths. More specifically, we experimentally obtained at a reverse voltage of 1V a dark current lower than 10 nA, a responsivity higher than 1.1 A/W, and a 3 dB opto-electrical cut-off frequency over 50 GHz. The combined benefits of decreased process complexity and high-performance device operation pave the way towards attractive integration strategies to deploy cost-effective photonic transceivers on silicon-on-insulator substrates.

Nutritional input from dinoflagellate symbionts in reef-building corals is minimal during planula larval life stage.

Dispersion of larval offspring is of fundamental ecological importance to sessile marine organisms. Photosymbiotic planulae emitted by many reef-forming corals may travel over large distances before settling to form a new colony. It is not clear whether the metabolic requirements of these planula larvae are met exclusively with lipid and protein reservoirs inherited from the mother colony or when metabolic inputs from their endosymbiotic dinoflagellates become important. Pulse-chase experiments using [(13)C]bicarbonate and [(15)N]nitrate, combined with subcellular structural and isotopic imaging of freshly emitted symbiotic larvae from the coral Pocillopora damicornis, show that metabolic input from the dinoflagellates is minimal in the planulae compared with adult colonies. The larvae are essentially lecithotrophic upon emission, indicating that a marked shift in metabolic interaction between the symbiotic partners takes place later during ontogeny. Understanding the cellular processes that trigger and control this metabolic shift, and how climate change might influence it, is a key challenge in coral biology.

Subcellular investigation of photosynthesis-driven carbon assimilation in the symbiotic reef coral Pocillopora damicornis.

UNLABELLED: Reef-building corals form essential, mutualistic endosymbiotic associations with photosynthetic Symbiodinium dinoflagellates, providing their animal host partner with photosynthetically derived nutrients that allow the coral to thrive in oligotrophic waters. However, little is known about the dynamics of these nutritional interactions at the (sub)cellular level. Here, we visualize with submicrometer spatial resolution the carbon and nitrogen fluxes in the intact coral-dinoflagellate association from the reef coral Pocillopora damicornis by combining nanoscale secondary ion mass spectrometry (NanoSIMS) and transmission electron microscopy with pulse-chase isotopic labeling using [(13)C]bicarbonate and [(15)N]nitrate. This allows us to observe that (i) through light-driven photosynthesis, dinoflagellates rapidly assimilate inorganic bicarbonate and nitrate, temporarily storing carbon within lipid droplets and starch granules for remobilization in nighttime, along with carbon and nitrogen incorporation into other subcellular compartments for dinoflagellate growth and maintenance, (ii) carbon-containing photosynthates are translocated to all four coral tissue layers, where they accumulate after only 15 min in coral lipid droplets from the oral gastroderm and within 6 h in glycogen granules from the oral epiderm, and (iii) the translocation of nitrogen-containing photosynthates is delayed by 3 h. IMPORTANCE: Our results provide detailed in situ subcellular visualization of the fate of photosynthesis-derived carbon and nitrogen in the coral-dinoflagellate endosymbiosis. We directly demonstrate that lipid droplets and glycogen granules in the coral tissue are sinks for translocated carbon photosynthates by dinoflagellates and confirm their key role in the trophic interactions within the coral-dinoflagellate association.

Chip-to-chip optical interconnections between stacked self-aligned SOI photonic chips.

Photonic silicon devices are key enabling technologies for next generation High Performance Computers. In this paper, we report the possibility to stack and optically interconnect SOI based photonic chips for future System-In-Package photonic architecture. Combining vertical grating couplers and state-of-the-art flip-chip technology, we demonstrated low loss penalties and wide spectral range optical interconnections between stacked photonic chips.

Zero-bias 40Gbit/s germanium waveguide photodetector on silicon.

We report on lateral pin germanium photodetectors selectively grown at the end of silicon waveguides. A very high optical bandwidth, estimated up to 120GHz, was evidenced in 10 µm long Ge photodetectors using three kinds of experimental set-ups. In addition, a responsivity of 0.8 A/W at 1550 nm was measured. An open eye diagrams at 40Gb/s were demonstrated under zero-bias at a wavelength of 1.55 µm.

A single-cell view of ammonium assimilation in coral-dinoflagellate symbiosis.

Assimilation of inorganic nitrogen from nutrient-poor tropical seas is an essential challenge for the endosymbiosis between reef-building corals and dinoflagellates. Despite the clear evidence that reef-building corals can use ammonium as inorganic nitrogen source, the dynamics and precise roles of host and symbionts in this fundamental process remain unclear. Here, we combine high spatial resolution ion microprobe imaging (NanoSIMS) and pulse-chase isotopic labeling in order to track the dynamics of ammonium incorporation within the intact symbiosis between the reef-building coral Acropora aspera and its dinoflagellate symbionts. We demonstrate that both dinoflagellate and animal cells have the capacity to rapidly fix nitrogen from seawater enriched in ammonium (in less than one hour). Further, by establishing the relative strengths of the capability to assimilate nitrogen for each cell compartment, we infer that dinoflagellate symbionts can fix 14 to 23 times more nitrogen than their coral host cells in response to a sudden pulse of ammonium-enriched seawater. Given the importance of nitrogen in cell maintenance, growth and functioning, the capability to fix ammonium from seawater into the symbiotic system may be a key component of coral nutrition. Interestingly, this metabolic response appears to be triggered rapidly by episodic nitrogen availability. The methods and results presented in this study open up for the exploration of dynamics and spatial patterns associated with metabolic activities and nutritional interactions in a multitude of organisms that live in symbiotic relationships.

²6Mg labeling of the sea urchin regenerating spine: Insights into echinoderm biomineralization process.

This paper reports the results of the first dynamic labeling experiment with regenerating spines of sea urchins Paracentrotus lividus using the stable isotope ²6Mg and NanoSIMS high-resolution isotopic imaging, which provide a direct information about the growth process. Growing spines were labeled twice (for 72 and 24 h, respectively) by increasing the abundance of ²6Mg in seawater. The incorporation of ²6Mg into the growing spines was subsequently imaged with the NanoSIMS ion microprobe. Stereom trabeculae initially grow as conical micro-spines, which form within less than 1 day. These micro-spines fuse together by lateral outgrowths and form a thin, open meshwork (inner stereom), which is subsequently reinforced by addition of layered thickening deposits (outer stereom). The (longitudinal) growth rate of the inner stereom is ca. 125 µm/day. A single (ca. 1 µm) thickening layer in the stereom trabeculae is deposited during 24h. The thickening process is contemporaneous with the formation micro-spines and involves both longitudinal trabeculae and transverse bridges to a similar degree. Furthermore, the skeleton-forming cells remain active in the previously formed open stereom for at least 10 days, and do not migrate upwards until the end of the thickening process. The experimental capability presented here provides a new way to obtain detailed information about the skeleton formation of a multitude of marine, calcite producing organisms.

Calcareous sponge biomineralization: ultrastructural and compositional heterogeneity of spicules in Leuconia johnstoni.

In contrast to siliceous sponge spicules, the biomineralization in calcareous sponges is poorly understood. In particular, the existence of a differentiated central core in calcareous spicules is still controversial. Here we combine high-spatial resolution analyses, including NanoSIMS, Raman, SXM, AFM, SEM and TEM to investigate the composition, mineralogy and ultrastructure of the giant tetractines of Leuconia johnstoniCarter, 1871 (Baeriidae, Calcaronea) and the organization of surrounding cells. A compositionally distinct core is present in these spicule types. The core measures 3.5-10 µm in diameter and is significantly depleted in Mg and lightly enriched in S compared with the adjacent outer layer in the spicule. Measured Mg/Ca ratios in the core range from 70 to 90 mmol/mol compared to 125-130 mmol/mol in the adjacent calcite envelope. However, this heterogeneous distribution of Mg and S is not reflected in the mineralogy and the microstructure. Raman spectroscopy demonstrates a purely calcitic mineralogy. SEM examination of slightly etched spicules indicates an ultrastructure organized hierarchically in a concentric pattern, with layers less than 250 nm in width inside layers averaging 535 ± 260 nm. No change in structural pattern corresponds to the Mg/Ca variation observed. AFM and TEM observations show a nanogranular organization of the spicules with a network of intraspicular organic material intercalated between nanograins 60-130 nm in diameter. Observations of sclerocyte cells in the process of spiculogenesis suggest that the compositionally distinct core is produced by a sub-apical sclerocyte "founder cell" that controls axial growth, while the envelope is secreted by lateral sclerocytes "thickener cells", which control radial growth.