Kalisomes in corals: a novel KCl concentrating organelle?

Correlated cryo-SEM and CryoNanoSIMS imaging of biological tissue

BACKGROUND

The development of nanoscale secondary ion mass spectrometry (NanoSIMS) has revolutionized the study of biological tissues by enabling, e.g., the visualization and quantification of metabolic processes at subcellular length scales. However, the associated sample preparation methods all result in some degree of tissue morphology distortion and loss of soluble compounds. To overcome these limitations an entirely cryogenic sample preparation and imaging workflow is required.

RESULTS

Here, we report the development of a CryoNanoSIMS instrument that can perform isotope imaging of both positive and negative secondary ions from flat block-face surfaces of vitrified biological tissues with a mass- and image resolution comparable to that of a conventional NanoSIMS. This capability is illustrated with nitrogen isotope as well as trace element mapping of freshwater hydrozoan Green Hydra tissue following uptake of ¹⁵N-enriched ammonium.

CONCLUSION

With a cryo-workflow that includes vitrification by high pressure freezing, cryo-planing of the sample surface, and cryo-SEM imaging, the CryoNanoSIMS enables correlative ultrastructure and isotopic or elemental imaging of biological tissues in their most pristine post-mortem state. This opens new horizons in the study of fundamental processes at the tissue- and (sub)cellular level.

TEASER

CryoNanoSIMS: subcellular mapping of chemical and isotopic compositions of biological tissues in their most pristine post-mortem state.

Optimization of skeletal protein preparation for LC-MS/MS sequencing yields additional coral skeletal proteins in Stylophora pistillata

Stony corals generate their calcium carbonate exoskeleton in a highly controlled biomineralization process mediated by a variety of macromolecules including proteins. Fully identifying and classifying these proteins is crucial to understanding their role in exoskeleton formation, yet no optimal method to purify and characterize the full suite of extracted coral skeletal proteins has been established and hence their complete composition remains obscure. Here, we tested four skeletal protein purification protocols using acetone precipitation and ultrafiltration dialysis filters to present a comprehensive scleractinian coral skeletal proteome. We identified a total of 60 proteins in the coral skeleton, 44 of which were not present in previously published stony coral skeletal proteomes. Extracted protein purification protocols carried out in this study revealed that no one method captures all proteins and each protocol revealed a unique set of method-exclusive proteins. To better understand the general mechanism of skeletal protein transportation, we further examined the proteins’ gene ontology, transmembrane domains, and signal peptides. We found that transmembrane domain proteins and signal peptide secretion pathways, by themselves, could not explain the transportation of proteins to the skeleton. We therefore propose that some proteins are transported to the skeleton via non-traditional secretion pathways.

Minerals in the pre-settled coral Stylophora pistillata crystallize via protein and ion changes

Aragonite skeletons in corals are key contributors to the storage of atmospheric CO₂ worldwide. Hence, understanding coral biomineralization/calcification processes is crucial for evaluating and predicting the effect of environmental factors on this process. While coral biomineralization studies have focused on adult corals, the exact stage at which corals initiate mineralization remains enigmatic. Here, we show that minerals are first precipitated as amorphous calcium carbonate and small aragonite crystallites, in the pre-settled larva, which then evolve into the more mature aragonitic fibers characteristic of the stony coral skeleton. The process is accompanied by modulation of proteins and ions within these minerals. These findings may indicate an underlying bimodal regulation tactic adopted by the animal, with important ramification to its resilience or vulnerability toward a changing environment.

Coral biomineralization is an important example of natural mineralization and understanding the process will aid biomineralization research. Here, the authors identify the precipitation of amorphous calcium carbonate and small aragonite crystals in pre-settled larva of Stylophora pistillata.

Preparation techniques for transmission electron microscopy of Hydra

Hydra is a classical model organism in developmental and cell biology with a simple body plan reminiscent of a gastrula with one body axis and a limited number of cell types. This rather simple organism exhibits a regeneration capacity that is unique among all eumetazoans and is largely dependent on the stem cell properties of its epithelial stem cell population. Molecular work in the past few years has revealed an unexpected genetic complexity of these simple animals, making them an interesting model for studying the generation of animal form and regeneration. In addition, Hydra has an interstitial stem cell system with a unique population of nematocytes, neuronal cells that are characterized by an explosive exocytotic discharge. Here, we compare classical and modern transmission electron microscopy (TEM) fixation protocols including protocols for TEM immunocytochemistry (post-embedding immunogold labeling). We presume that TEM studies will become an important tool to analyze cell-cell interactions as well as cell matrix interrelationships in Hydra in the future.

Uric acid deposits in symbiotic marine algae

The symbiosis between cnidarians and dinoflagellate algae is not understood at the cell or molecular level, yet this relationship is responsible for the formation of thousands of square kilometres of coral reefs. We have investigated the nature of crystalline material prominent within marine algal symbionts of Aiptasia sp. anemones. This material, which has historically been considered to be calcium oxalate, is shown to be uric acid. We demonstrate that these abundant uric acid stores can be mobilized rapidly, thereby allowing the algal symbionts to flourish in an otherwise N-poor environment. This is the first report of uric acid accumulation by symbiotic marine algae. These data provide new insight and considerations for understanding the physiological basis of algal symbioses, and represent a new and previously unconsidered aspect of N metabolism in cnidarian, and a variety of other, marine symbioses.

Calcium localisation by X-ray microanalysis and fluorescence microscopy in larvae of zooxanthellate and azooxanthellate corals

X-ray microanalysis and fluorescence microscopy (Calcium Orangetrade mark) was used to determine the distribution of intracellular calcium (I(Ca)), in the form of total and ionic calcium respectively, in planulae and settled larvae of a zooxanthellate coral. The distribution of total calcium only was determined in larvae of an azooxanthellate coral. In azooxanthellate planulae and settled larvae, total I(Ca) concentration in the oral ectoderm was high and similar to that in seawater (SW). Calcium concentration did not vary (P > 0.05) between planulae and settled larvae. However, settled larvae accumulated large amounts of calcium in gastrodermal lipid-containing cells. In contrast, zooxanthellate planulae possessed significantly (P < 0.01) lower concentrations of total I(Ca) within ectodermal cells in comparison to settled larvae. In addition, in settled zooxanthellate larvae total calcium concentration in the mesogloea and coelenteron was significantly (P < 0.05) higher than in the oral ectodermal and gastrodermal cells, respectively. Total I(Ca) concentrations in the oral ectoderm of settled larvae were also significantly (P < 0.01) lower than that of the calicoblastic ectoderm. In zooxanthellate settled larvae, ionic I(Ca) levels in the aboral epithelium surrounding rapidly growing septa were high. These levels increased significantly (P < 0.05) within the tissue surrounding growing septa after incubation in high-calcium SW.