Aragonite Crystals Research Articles

Thermal-induced crystallographic transformation in shells of Mytilus galloprovincialis Lamarck, 1819

Biominerals in hard parts are widely used as paleoenvironmental archives, employing proxies such as elemental and isotopic composition, microstructure, and crystallography. However, the effective selection and application of these proxies in fossil materials depends on their preservational potential. Here we evaluated the preservational potential of these proxies through controlled experiments on bivalves, one of the commonly used fossils in paleoenvironmental reconstructions. We utilized cultured Mytilus galloprovincialis specimens at five controlled temperatures to investigate the impact of elevated temperatures on various proxies extracted from both aragonitic and calcitic components of the same shell, including isotopic composition, microstructure and crystallography. Elevated temperatures first induced changes in the oxygen isotope composition (in both aragonite and calcite), followed by modification in aragonitic parts, such as changes in the aragonite phase, textures, nacre microstructure, and grain size of aragonite crystals. Despite aragonite's susceptibility to temperature, the microstructure and texture of calcite remained largely unaltered, demonstrating significantly higher resilience. Our findings emphasize the preservational potential of different shell proxies under heat exposure, ranging from oxygen isotopes to microstructure and texture. A ranking of preservation potential provides a practical guide for selecting well-preserved specimens, as textures and microstructures are not always reliable indicators. In distinguishing aragonite from calcite in fossils, our study pioneers a new avenue by proposing the analysis of high-angle boundary (HAB) content, including twin boundaries, as a reference for altered aragonite.

A first-principles study of helium diffusion in aragonite under high pressure up to 40 GPa

Helium diffusion in carbonates under mantle pressure is crucial for understanding thermal and chemical evolution of mantle. Based on the density functional theory (DFT) and the the climbing image nudged elastic band (CI-NEB) method, we performed first-principles calculations of diffusion characteristics of helium in perfect aragonite crystal under high pressure to 40 GPa. Our results show that He diffusion behaviors are controlled by pressure, temperature and crystal size. The activation energy increases, and the diffusion coefficients decrease significantly under high pressure. Ea[100] increases from 176.02 kJ/mol to 278.75 kJ/mol, and Ea[001] increases from 195.89 kJ/mol to 290.43 kJ/mol, with pressure increasing from 20 GPa to 40 GPa. At 700 K, the diffusion coefficients at 40 GPa is 7 orders of magnitude lower than that at 20 GPa; and at 1000 K it decrease 5 orders of magnitude. To ensure that at least 90% helium is not lost, we synthesized the temperature obtained from cooling and heating processes and derive the 'stable temperature range' for helium in aragonite. The obtained results show that the stable temperature range is 22–76 ℃ at 0 GPa and 641–872 °C at 40 GPa, for the crystal of 100–2000 μm size. Besides, the travel time of helium in aragonite under high pressure increases rapidly with pressure increasing. Our calculations indicate that helium can be quantitatively retained in aragonite in the deep mantle as long as the temperature is in the 'stable temperature range'. These results have certain implications for exploring the evolution of mantle and the storage of helium within it.

Analysis of Ca1-xSrxCO3 phases generated by competitive Sr2+ replacement in pre-formed aragonite

The ratio of Sr/Ca ions in marine biogenic minerals is considered advantageous for tracking geochemical and biomineralization processes that occur in the oceans. It is debatable, though, whether the ratio in biominerals such as coral skeleton is simply related to values in the seawater environment or controlled by the organism. Recent data show that coral larvae produce partially disordered immature aragonite in Mg-containing Sr-poor calcifying fluids, which transforms into well-ordered aragonite in Mg-depleted Sr-enriched environments, upon animal metamorphosis into the sessile polyp state.Inspired by the process in young coral, we explored in vitro substitution of Ca by Sr in aragonite by exposing aragonite crystals precipitated a priori to Sr solutions with variable concentrations. The resulting biphasic material, comprised of Sr-doped aragonite and Ca-doped strontianite, was carefully analyzed for foreign cation substitution in each polymorph. This allowed to establish a linear correlation between Sr levels in mineralizing solutions and Sr in aragonite as well as Ca in strontianite. It indicated that ca. 5-fold higher Sr solution concentration is needed for substitution in the crystal to reach the level found in corals. It also provided with Sr levels required for a putative strontianite phase to form.

Live imaging of center of calcification formation during septum development in primary polyps of Acropora digitifera

Recent studies have revealed that stony corals create their extracellular skeletons via biologically controlled calcification, in which amorphous calcium carbonate (ACC), regarded as precursors of aragonite crystals, have been observed at nanoscale using electron microscopy. However, the exact mechanism by which ACC is generated, and how it contributes to skeletal growth in coral calcifying tissue, remains enigmatic. The septal skeleton of an individual polyp is composed of radially aligned plates extending upward from the aboral calcifying tissue. This structure includes microstructure known as the centers of calcification (CoC). However, despite its importance, direct in vivo observation of septal growth has not been reported. Observations under transmitted illumination using polarized light microscopy on calcifying tissue of young Acropora digitifera revealed small crystals, a few micrometers in size, that accompany subtle movements and that emerge exclusively on the inner wall of the pocket in extracellular calcifying fluid (ECF). Crystal growth initiated from small, scattered crystals on a glass plate resembles this phenomenon observed in coral skeletons. Time-lapse photographs of 12 individuals in early primary polyp settlement revealed this process in three individuals, documenting 13 of these crystal events. This phenomenon occurred solely at the bases of subsequently formed septa. These crystals differ notably from fusiform crystals and from dumbbell-like or rod-like crystals growing individually. Upright two-photon microscopy captured movement of sub-micron-sized fluorescent calcein-accumulating particles, emphasizing their presence on the surface of the growing fronts of septa. Methodological advances that facilitate comprehensive in vivo observation of sub-micron-sized structures, calcein-accumulating particles to the skeleton, are needed to develop a more detailed understanding of coral skeletal growth.

Deciphering the Mortar Composition of Ancient Chinese Pagodas: A Comparative Analysis from the Tang to Ming Dynasties in Shaanxi Province

The mortar materials from the internal and external walls of ancient Chinese pagodas in Shaanxi (Luoshan Temple Pagoda (847–858 AD), Daxiang Pagoda (960–1279 AD), and Qianjin Pagoda (1609 AD)) were investigated. Qualitative and semi-quantitative techniques including scanning electronic microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and thermogravimetric analysis with differential scanning calorimetry (TGA-DSC) were employed. The physical and chemical characteristics including microstructural morphology, chemical composition, element distribution, and organic additives of the mortar samples were studied. SEM indicated that the mortar from the external wall exhibits a tightly bonded matrix, with the irregularly shaped particles, and the carbonation of the calcium carbonate in the mortars closely resembles that of aragonite crystals. The results of EDS, XRD, FTIR and TGA-DSC revealed that mortar from external wall of these ancient pagodas were air-hardened lime, and complete carbonization with no traces of calcium hydroxide, while mortars from internal wall primarily comprises local clay and lime. In addition, ATR-FTIR and iodine-starch discoloration tests confirmed that the presence of starch in the external wall of Daxiang Pagoda, whereas no organic additives were detected in the Qianjin and Luoshan Temple Pagoda.

Isotopic and mineralogic bias introduced by pulverization of aragonite

RationaleStable carbon and oxygen isotope data of biogenic and abiogenic aragonite are of fundamental relevance in paleoclimate research. Wet‐chemical analysis of such materials requires well‐homogenized, fine‐grained powder. In the present study, the effect of different grinding/milling methods on sample homogeneity and the potential risk of unintentional calcite formation and isotope shift were evaluated.MethodsShells of Arctica islandica and aragonite sputnik crystals were pulverized using a set of commonly used methods, including a hand‐held drill, a vibromill operated at various settings (with and without liquid nitrogen cooling, changes in ball diameters, frequencies, and processing durations), and an agate mortar and pestle. Stable isotope values were measured using an isotope ratio mass spectrometer operated in continuous flow mode. Identification of mineral phases was obtained by powder X‐ray diffraction (PXRD), Raman spectroscopy, and attenuated total reflectance‐Fourier transform infrared (ATR‐FTIR) spectroscopy. Calcite content was quantified by PXRD Rietveld refinement.ResultsSamples showed substantial homogeneity, in particular after vibromilling (duration 3–10 min). More vigorous grinding resulted in larger fractions of calcite (0.5–4.2 wt%) and a concomitant δ18O and δ13C decrease, specifically in bivalve shells. The only method for producing pure aragonite powder was by pounding the aragonite sputniks manually with an agate mortar and pestle.ConclusionsNone of the studied, commonly used machine‐based pulverization methods produced pure aragonite powder from samples consisting originally of aragonite. These findings have significant implications for light‐stable isotope‐based paleoclimate reconstructions. Except for abiogenic aragonite powder produced by pounding in an agate mortar, paleotemperatures would be overestimated.

Effect of temperature on binding process of calcium carbonate concrete through aragonite crystals precipitation

This study investigated the impact of temperature on the strength development of calcium carbonate concrete (CCC) comprising calcium carbonate and concrete waste. CCC exhibited its highest compressive strength when manufactured at temperatures between 60 and 70 °C, thereby demonstrating strengths 1.5 and 2.7 times greater than those achieved at 40 and 90 °C, respectively. At this temperature range (60–70 °C), CCC showed the highest amount of precipitated aragonite with large acicular aragonite crystals, which decreased the porosity of CCC. This temperature range governed the homogeneous distribution of calcium carbonate deposition within the CCC specimen. Moreover, the carbonated cement paste particles within the CCC continuously underwent aqueous carbonation, thereby providing an additional Ca source for calcium carbonate precipitation in CCC. At high temperatures, this process promotes the precipitation of Ca ions as needle-like aragonite crystals during reprecipitation with accelerating the transformation of calcium carbonate polymorphs. The CCC strength arose from the deposition of calcium carbonate from input calcium bicarbonate solution and the reprecipitation of calcium carbonate during aqueous carbonation. The calcium carbonate precipitation from aqueous carbonation accounts for 30 % of the total calcium carbonate precipitation of CCC. Needle-like aragonite crystals functioned as interlocking bridges between the particles and frame connections, effectively strengthening the CCC composite.

Ground Improvement with a Single Injection of a High-Performance All-in-one MICP Solution

All-in-one injection strategy has been developed recently as an efficient method in microbially induced carbonate precipitation (MICP). In this method, engineers try to delay the beginning of precipitation and consequently distribute reagents homogeneously on a soil mass. In this article, a series of experiments have been carried out to modify all-in-one solution strategy and promote its uniformity. Primarily, the concentration of all-in-one solutions was optimized and the result showed that all-in-one solution with a 2 M concentration of urea/calcium chloride was completely consumed. The short columns treated by one-time one-pore volume (PV) injection of 2 M all-in-one solution provided approximately 450 kPa in UCS test. In the second step, magnesium substitution for calcium was investigated to improve the strength and homogeneity of biocementation. This substitution raised the UCS by approximately 70%, and 800 kPa was acquired. XRD analysis confirmed that Mg2+ changes the mineralogy of precipitation from vaterite to aragonite. Hence, the higher strength of columns treated with the magnesium-substituted solutions is attributed to the higher hardness of aragonite crystals than that of vaterite. Finally, the bacterial transport of all-in-one solutions showed that magnesium substitution for 20% of calcium cations, enhances its potential to provide a homogeneous biocement on large-scale projects.

Fossilized anuran soft tissues reveal a new taphonomic model for the Eocene Geiseltal Konservat-Lagerstätte, Germany

The Eocene Geiseltal Konservat-Lagerstätte (Germany) is famous for reports of three dimensionally preserved soft tissues with sub-cellular detail. The proposed mode of preservation, direct replication in silica, is not known in other fossils and has not been verified using modern approaches. Here, we investigated the taphonomy of the Geiseltal anurans using diverse microbeam imaging and chemical analytical techniques. Our analyses confirm the preservation of soft tissues in all body regions but fail to yield evidence for silicified soft tissues. Instead, the anuran soft tissues are preserved as two layers that differ in microstructure and composition. Layer 1 comprises sulfur-rich carbonaceous microbodies interpreted as melanosomes. Layer 2 comprises the mid-dermal Eberth–Katschenko layer, preserved in calcium phosphate. In addition, patches of original aragonite crystals define the former position of the endolymphatic sac. The primary modes of soft tissue preservation are therefore sulfurization of melanosomes and phosphatization of more labile soft tissues, i.e., skin. This is consistent with the taphonomy of vertebrates in many other Konservat-Lagerstätten. These findings emphasize an emerging model for pervasive preservation of vertebrate soft tissues via melanosome films, particularly in stagnation-type deposits, with phosphatization of more labile tissues where tissue biochemistry is favorable.

Structural and Functional Analysis of the Amorphous Calcium Carbonate-Binding Protein Paramyosin in the Shell of the Pearl Oyster, Pinctada fucata.

Amorphous calcium carbonate (ACC) is an important precursor phase for the formation of aragonite crystals in the shells of Pinctada fucata. To identify the ACC-binding protein in the inner aragonite layer of the shell, extracts from the shell were used in the ACC-binding experiments. Semiquantitative analyses using liquid chromatography-mass spectrometry revealed that paramyosin was strongly associated with ACC in the shell. We discovered that paramyosin, a major component of the adductor muscle, was included in the myostracum, which is the microstructure of the shell attached to the adductor muscle. Purified paramyosin accumulates calcium carbonate and induces the prism structure of aragonite crystals, which is related to the morphology of prism aragonite crystals in the myostracum. Nuclear magnetic resonance measurements revealed that the Glu-rich region was bound to ACC. Activity of the Glu-rich region was stronger than that of the Asp-rich region. These results suggest that paramyosin in the adductor muscle is involved in the formation of aragonite prisms in the myostracum.

Microzone analysis of stalagmite chemical components in caves based on confocal controlled Raman and laser induced breakdown spectroscopy

Cave stalagmites are recognized as excellent archives for studying contemporary and historical climate variations. They present a valuable opportunity for accurately identifying past climate signals through a combination of O and C isotopes(δ18O, δ13C), as well as trace elements. This study introduces confocal controlled Raman coupled to laser induced breakdown spectroscopy (LIBS) technology for analyzing of climate change using stalagmites for the first time. With a spatial resolution of 20 μm, this technology enables the detection of various elements and mineral. Correlation and principal component analysis of the ratio between elements, Raman, gray, fluorescence, δ13C, and δ18O revealed that the data are correlated and show significant grouping. These factors are related to the micro-environmental changes caused by the aragonite crystals in stalagmites. The increase/decrease in the Sr-Mg/Ca ratio and the decrease/increase in the (Ba-K-Fe)/Ca ratio were closely related to the water-rock interaction and prior calcite precipitation, the decrease/increase in soil biological activity, and the influx of detrital material caused by precipitation. This also corresponded to the decrease/increase in temperature. These findings indicate that the confocal controlled Raman-LIBS technique for detecting microscale chemical compositions in stalagmites holds significant promise and potential for studying past climate change.

Aragonite crystallization in a sulfate-rich hypersaline wetland under dry Mediterranean climate (Laguna Honda, eastern Guadalquivir basin, S Spain)

This research investigates the influence of water composition, the presence of seasonal algal mats, detrital inputs and the activity of microorganisms on the crystallization of aragonite in the sediments deposited in the hypersaline Laguna Honda wetland (S of Spain). The high alkaline and hypersaline waters (pH > 9.2 and C.E. > 70 mS/cm) of the wetland lake are rich in SO42− (>24,000 mg/l), Cl− (>21,000 mg/l), Na+ (>11,000 mg/l) Mg2+ (>8400 mg/l) and Ca2+ (>1000 mg/l), and are supersaturated for dolomite, calcite and aragonite. Sediments have lower pH values than column waters, oscillating from 8.54 in the low Eh (up to −80.9 mV) central deep sediments and 6.33 in the shallower higher Eh (around −13.6 mV) shore sediments. Erosion of the surrounding olive groves soils produced detrital silicates rich sediments with concretions of carbonate or sulfate. Aragonite (up to 19 %) and pyrite (up to 13 %) are mainly concentrated in the organic matter rich samples from the upper part of the sediment cores, whereas gypsum is preferably concentrated in low organic matter content samples. Mineral crusts containing a MgAl silicate phase, epsomite, halite and gypsum are precipitated on the floating algal mats covering the wetland waters. Floating algal mats deposit increased the organic matter content of the upper sediments which promoted the presence of fermentative microorganisms, sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB) communities and variations of Eh that influence the authigenesis of carbonate and S-bearing minerals. Replacement of poorly crystalline MgSi phases infilling algal cells by aragonite was favored in the organic matter rich sediments with low Eh values and important SRB communities that promoted sulfate bioreduction processes to form pyrite. Aragonite precipitation was favored by the increase of carbonate and bicarbonate concentration produced by the SRB oxidation of organic matter, the CO2 degassing by high summer temperatures and the CO2 uptake by photosynthesis of the algal mats.

The intrinsic twinning and enigmatic twisting of aragonite crystals

It is generally accepted that aragonite crystals of biogenic origin are characterized by significantly higher twin densities compared to samples formed during geological processes. Based on our single crystal X-ray diffraction (SCXRD) and transmission electron microscopy (TEM) study of aragonite crystals from various localities, we show that in geological aragonites, the twin densities are comparable to those of the samples from crossed lamellar zones of molluscs shells. The high twin density is consistent with performed calculations, according to which the Gibbs free energy of twin-free aragonite is close to that of periodically twinned aragonite structure. In some cases, high twin densities result in the appearance of diffuse scattering in SCXRD patterns. The obtained TEM and optical micrographs show that besides the twin boundaries (TBs) of growth origin, there are also TBs and especially stacking faults that were likely formed as the result of local strain compensation. SCXRD patterns of the samples from Tazouta, in addition to diffuse scattering lines, show Debye arcs in the [Formula: see text] plane. These Debye arcs are present only on one side of the Bragg reflections and have an azimuthal extent of nearly 30°, making the whole symmetry of the diffraction pattern distinctly chiral, which has not yet been reported for aragonite. By analogy with biogenic calcite crystals, we associate these arcs with the presence of misoriented subgrains formed as a result of crystal twisting during growth.

Pearl Farming Micro-Nanoplastics Affect Oyster Physiology and Pearl Quality.

Pearl farming is crucial for the economy of French Polynesia. However, rearing structures contribute significantly to plastic waste, and the widespread contamination of pearl farming lagoons by microplastics has raised concerns about risks to the pearl industry. This study aimed to evaluate the effects of micro-nanoplastics (MNPs, 0.4-200 μm) on the pearl oyster (Pinctada margaritifera) over a 5-month pearl production cycle by closely mimicking ecological scenarios. MNPs were produced from weathered plastic pearl farming gear and tested at environmentally relevant concentrations (0.025 and 1 μg L-1) to decipher biological and functional responses through integrative approaches. The significant findings highlighted the impacts of MNPs on oyster physiology and pearl quality, even at remarkably low concentrations. Exposure to MNPs induced changes in energy metabolism, predominantly driven by reduced assimilation efficiency of microalgae, leading to an alteration in gene expression patterns. A distinct gene expression module exhibited a strong correlation with physiological parameters affected by MNP conditions, identifying key genes as potential environmental indicators of nutritional-MNP stress in cultured oysters. The alteration in pearl biomineralization, evidenced by thinner aragonite crystals and the presence of abnormal biomineral concretions, known as keshi pearls, raises concerns about the potential long-term impact on the Polynesian pearl industry.

Magnetization coupled reverse osmosis (RO): Enhanced inhibition scaling mechanisms and operation optimization

In this study, magnetization pretreatment was applied to the reverse osmosis process. The scale inhibition effect and the mechanism of magnetization treatment were analyzed. The results show that after magnetization, the decay of water flux tended to slow down during RO and the flux recovery increased after cleaning. The Ryznar stability index of the solutions was calculated, which showed that magnetization could increase the solubility of carbonate solution and reduce the trend of carbonate scaling to a certain extent. The infrared spectra of the solutions showed that the hydrogen bonds of water molecules in the solution decreased after magnetization, indicating that the magnetization made the hydrogen bonds of water molecules break to form smaller clusters of water molecules, which might make the water molecules pass through the membrane more easily. After magnetization, the crystal size of the carbonate precipitated decreased and the sedimentation rate slowed down. Different flow rates and magnetization times were selected during the magnetization process. The results showed that when the flow velocity was greater than 0.24 m/s, the RSI, scale size and precipitation rate of the magnetized solution changed significantly compared with that of the unmagnetized solution. At the flow velocity of 0.48 m/s, the optimal magnetization time is 5 min. Moreover, the SEM characterization of the fouled membrane after filtration showed that after magnetization, the calcium carbonate deposited on the membrane surface appeared aragonite crystal, and the fouling was more loose, so it was easier to wash away during the membrane cleaning process.

Light-driven nucleation, growth, and patterning of biorelevant crystals using resonant near-infrared laser heating

Spatiotemporal control over crystal nucleation and growth is of fundamental interest for understanding how organisms assemble high-performance biominerals, and holds relevance for manufacturing of functional materials. Many methods have been developed towards static or global control, however gaining simultaneously dynamic and local control over crystallization remains challenging. Here, we show spatiotemporal control over crystallization of retrograde (inverse) soluble compounds induced by locally heating water using near-infrared (NIR) laser light. We modulate the NIR light intensity to start, steer, and stop crystallization of calcium carbonate and laser-write with micrometer precision. Tailoring the crystallization conditions overcomes the inherently stochastic crystallization behavior and enables positioning single crystals of vaterite, calcite, and aragonite. We demonstrate straightforward extension of these principles toward other biorelevant compounds by patterning barium-, strontium-, and calcium carbonate, as well as strontium sulfate and calcium phosphate. Since many important compounds exhibit retrograde solubility behavior, NIR-induced heating may enable light-controlled crystallization with precise spatiotemporal control.

Unravelling calcite-to-aragonite evolution from a subsurface fluid - Formation pathway, interfacial reactions and nucleation effects

Calcite-aragonite alternations are documented in sedimentary deposits worldwide, but their formation is still poorly understood and individual CaCO3 precipitation pathways are rarely confirmed experimentally. Therefore, (sub)recent CaCO3 sinter formation in a historic subsurface adit at Erzberg (Austria) was used as a natural laboratory to monitor and assess the calcite-to-aragonite evolution pathway and intergrowth mechanism in terms of solid-liquid-atmosphere dynamics and their relevance for solid-liquid interface reactions and nucleation effects. Our results indicate an initial homogeneous nucleation of low-Mg calcite (LMC: ∼3 ± 1 mol% MgCO3), induced by CO2 degassing from the percolating geogenic fluid, which is originated from seepage of local meteoric water and incongruent dissolution of Mg-Ca-Fe-bearing minerals from the host rock. Progressive LMC growth leads to an increase in the aqueous molar ratio of Mg/Ca, causing a Mg/Ca zonation pattern with transitions to high-Mg calcite (HMC: up to 7 mol% MgCO3). At a critical Mg concentration, the available Mg calcite crystal surfaces are acting as a nucleation site for heterogenous aragonite formation. In this way, fast growing acicular aragonite crystals are initiated, which impede further calcite growth. The calcite-to-aragonite transition is thus controlled by the reaction kinetics and mechanisms of Mg-calcite formation and the chemical evolution of the precipitating solution at the nano- to micro-spatial scale, creating Mg-enriched HMC surface sites for aragonite to be nucleated and preferentially grown. In the present case, the dynamics of the formation of calcite-aragonite sequences are triggered by distinct local environmental changes, in particular seasonal variations in seepage fluid flow behavior and progress in CO2 degassing. These considerations are relevant for a better understanding of proxy signal development and preservation in calcareous sedimentary sequences forming under highly dynamic environmental conditions.

Ni-rich mineral nepouite explains the exceptional green color of speleothems

Speleothems are secondary mineral structures typically found in karstic caves and usually composed of calcite or aragonite. Despite being naturally white, some might exhibit unusual colors, such as blue, black, red, yellow or green. The causes of these exceptional colorations are poorly understood, especially for green speleothems, which are barely reported. Here we describe the occurrence of the green Ni-bearing serpentine nepouite in green aragonite and calcite speleothems, in the Aven du Marcou (Hérault, France). Nepouite is mainly found as flat lamellar crystals in the outer rim of green speleothems and crystallized alongside radially grown aragonite crystals. This supports nepouite beginning to crystallize recently, due to a change in the chemical composition of the water. Nepouite also exhibits extensive substitution between Ni, Mg and Zn. The various elements responsible for nepouite precipitation are thought to come from the weathering of pyrite crystals in the overlying rocks, which is consistent with the pH conditions of the cave and the Al-free composition of nepouite. This study explains the crystallization mechanisms and stability conditions of silicate minerals in colored caves.

A Study on Bio-Stabilisation of Sub-Standard Soil by Indigenous Soil Urease-Producing Bacteria

Sub-standard soils are of great concern worldwide due to diverse economic losses and the possibility of severe environmental hazards ranging from catastrophic landslides, building collapse, and erosion to loss of lives and properties. This study explored the potential of urease-producing bacteria, <i>Bacillus cereus</i> and <i>Bacillus paramycoides</i>, to stabilise sub-standard soil bio-stabilisation. The maximum urease activity measured by <i>B. cereus</i> and <i>B. paramycoides</i> was 665 U/mL and 620 U/mL, respectively. <i>B. cereus</i> and <i>B. paramycoides</i> precipitated 943 ± 57 mg/L and 793 ± 51 mg/L of CaCO<sub>3</sub> at an optical density (425 nm) of 1.01 and 1.09 and pH 8.83 and 8.59, respectively, after 96 hours of incubation. SEM microstructural analysis of the precipitated CaCO<sub>3</sub> revealed crystals of various sizes (2.0–23.0 µm) with different morphologies. XRD analysis confirmed that the precipitated CaCO<sub>3</sub> comprised calcite and aragonite crystals. SEM analysis of the microstructure of organic and sandy clay soils treated with <i>B. cereus</i> and <i>B. paramycoides</i> showed the formation of bio-precipitated calcium carbonate deposits on the soil particles (biocementing soil grains), with <i>B. cereus</i> precipitating more CaCO<sub>3</sub> crystals with a better biocementing effect compared to <i>B. paramycoides</i>. Overall, the experimental results attributed CaCO<sub>3</sub> formation to bacterial-associated processes, suggesting that soil ureolytic bacteria are potentially useful to stabilise sub-standard soil.

A comparison of SNARF-1 and skeletal δ11B estimates of calcification media pH in tropical coral

Coral skeletal boron geochemistry offers opportunities to probe the pH of the calcification media (pHCM) of modern and fossil specimens, to estimate past changes in seawater pH and to explore the biomineralisation response to future ocean acidification. In this research we grew 2 Stylophora pistillata coral microcolonies over glass coverslips to allow analysis of the pH sensitive dye SNARF-1, in the extracellular calcification medium at the growing edge of colonies where the first aragonite crystals are formed, under both light and dark conditions. We use secondary ion mass spectrometry (SIMS) to measure the boron isotopic composition (δ11B) of the skeleton close to the growth edge after 2 to 3 days of additional calcification had enlarged the crystals until they joined, generating a continuous sheet of aragonite. Mean skeletal δ11B-pHCM estimates are higher than those by SNARF-1 by 0.35–0.44 pH units. These differences either reflect real variations in the pH of the calcification media associated with each measurement technique or indicate other changes in the biomineralisation process which influence skeletal δ11B. SNARF-1 measures directly the pH of the extracellular calcification medium while skeletal δ11B analyses aragonite potentially formed via both extracellular and intracellular biomineralisation pathways. Analysis of a third coral specimen, also growing on a glass slide but with a 5 cm long branch, indicated good agreement between the δ11B value of the apex of the branch and the skeletal growth edge. The tissues overlying both these regions were transparent indicating they had low symbiont densities. This suggests that the biomineralisation process is broadly comparable between these sites and that studies growing corals over glass slides/coverslips provide representative data for the colony apex.