B43G-01
BORON ISOTOPIC COMPOSITION CORRELATES WITH ULTRA-STRUCTURE IN A DEEP- SEA CORAL LOPHELIA PERTUSA: IMPLICATIONS FOR BIOMINERALIZATION AND PALEO- PH
The geochemistry (stable isotopes and trace elements) of biogenic carbonates has been widely used for more than fifty years to reconstruct past climatic variability. During this time, the studies were mainly based on bulk sampling limiting sometimes the interpretations of the geochemical data as paleoclimatic proxies. Recently, high spatial resolution sampling techniques, such as micro-mill and SIMS, have been employed in the study of C, O and B isotopic compositions and trace elements (Mg, Sr) in the skeletons of a variety of (deep-sea) coral species. These studies have documented dramatic ‘vital effects' and uncovered a systematic relationship between skeletal ultra-structure and stable isotopic composition. The formation of skeleton corals follows a universal two-step growth process. At the tips of the skeletal structures, the mineralizing cell layer produces centers of calcification (COC) or, equivalently, Early Mineralization Zone (EMZ). These EMZ are subsequently overgrown by fibrous aragonite(FA) consisting of cyclically added layers. The EMZ are characterized by systematically lighter C and O isotopic compositions compared with the adjacent FA. A number of geochemical models have been proposed, in which this systematic stable isotopic difference between EMZ and FA is ascribed to a biologically induced variation in the pH of a proposed Extra-cytoplasmic Calcifying Fluid (ECF) reservoir. In these models, relatively high pH conditions during the formation of EMZ result in relatively light C and O isotopic compositions compared with FA, which form under generally lower pH conditions. A direct test of such models would be possible if the Boron isotopic composition, which is pH sensitive, of EMZ and FA could be measured. We performed ion microprobe d11B measurements for EMZ and FA in Lophelia pertusa, a deep-sea coral common in the North-East Atlantic Ocean. We observe a systematic difference in B isotopic composition between the EMZ and FA skeleton. In EMZ, the measured δ11B values are consistently low. Fibrous aragonite is characterized by systematically higher d11B values, but also display B isotopic heterogeneity associated with specific growth bands in the calyx wall. The magnitude of the observed B isotopic variations cannot be explained by changes in environmental conditions and are likely caused by biological processes involved in the biomineralization of new skeleton; i.e. ‘vital' effects. The observed B isotopic variations are opposite to the predictions of geochemical models for vital effects. Our data indicate that pH variations are not responsible for the observed stable isotopic fractionations. Geochemical models therefore do not provide an adequate framework within which to understand coral skeletal formation. Without a better understanding of these processes, which require experiments, the use of B isotopic composition to reconstruct paleo-pH variations in the oceans must be considered problematic - at least as far as Lophelia pertusa is concerned.
B43G-02
New insights into the molecular-level control of silica mineralization by diatoms
Marine diatoms are arguably the most important silica-mineralizing organisms in modern seas. They incorporate gigatons of Si into their siliceous cell walls annually, and control the concentration and bioavailability of silicon in marine environments. Although the roles that diatoms assume in regulating the global carbon cycle often go unrecognized, their ecological success places them alongside marine calcifiers as major players in the sequestration of organic carbon in the surface ocean. Consequently, most investigations have focused upon understanding how calcifying organisms mineralize their skeletons, and the silica biomineralization literature remains minimal. However, understanding how silicifying organisms control the deposition of silica is becoming increasingly important for a variety of forefront issues in science and technology. The increasing utilization of δO18 values obtained from biologically formed silica in paleoclimate research has recently raised questions about how the structural relationship between diatom silica and the macromolecular organic components of the cell wall might influence the quality of isotopic measurements. Biochemical investigations have begun to yield information about structural and chemical properties of organic macromolecules involved in biosilicification processes. Molecules that have been identified as part of the silicification mechanism either possess regions of locally concentrated positive and negative charge (silaffins), or require the presence of specific counter ions such as phosphate to control the assembly of the polycationic constituents (polyamines) of the organic matrix; however, the mechanisms by which these molecules control the spatial and temporal onset of biosilica formation remain unclear. As a first step towards quantifying the kinetic and thermodynamic drivers behind heterogeneous nucleation in biological systems, we have developed a new and novel approach that marries tapping mode atomic force microscopy with elements of modern materials chemistry, to directly measure the rate of amorphous silica nucleation on COOH, NH3+, and COOH / NH3+-terminated surfaces under controlled solution conditions. Our results provide new insights into the molecular-level control of silica mineralization in diatoms. We show that differences between substrate-specific nucleation rates are controlled largely by kinetic factors rather than thermodynamic drivers, and that amine-terminated surfaces are not capable of triggering the onset of silica deposition without the synergistic activity of neighboring negatively charged species on the surface or in solution (e.g. carboxyl or phosphoryl groups). In light of this result we conclude that sites on the organic matrix that have phosphate and amine moieties in close proximity serve not only as contact points between the constituent macromolecules in the matrix, but also as initial sites of silica deposition.
B43G-03
Coccolith Chemistry: A Flow-Through Analysis
Coccoliths not only hold much promise as a source of new and exciting geochemical data, but also provide an elegant test case for models of biomineral trace-metal incorporation due to their simple construction. Coccoliths are formed from a small number of single crystals growing from a ring of alternating vertical and radially oriented calcite nucleation sites. Microprobe and electron backscatter analysis hint at crystallographically discrete trace metal distribution coefficients. Here we investigate inhomogeneities within the calcite of coccolithophore monocultures using continuous-dissolution trace-metal analysis, combined with repeated AFM imaging of individual dissolving coccoliths. We present coccolith trace-metal distribution coefficients, explore the potential of coccolith calcite as a recorder of palaeoenvironmental data, and consider crystallographic controls over trace- metal chemistry.
B43G-04
The Role of Ca2+/CO32- Ratio in Calcite Growth: An Underestimated Factor in Calcium Carbonate Biomineralization
Laboratory investigations of calcium carbonate growth often employ stoichiometric solution compositions, with equivalent activities of calcium and carbonate species, or simply consider precipitation kinetics in the context of saturation state alone. However, nonstoichiometric solutions, with nonequivalent Ca2+ and CO32- activities, are the rule in natural biomineralizing systems where metabolic reactions produce microenvironments exhibiting disparate Ca2+/CO32- ratios proximal to biological membranes. Additionally, recent evidence suggests that the calcium/carbonate ratio of the oceans has varied over geologic time, making the examination of this solution parameter important for understanding paleoenvironmental signatures in biominerals, as well as for predicting the response of biomineralizing systems to anthropogenic forcing of seawater chemistry under elevated atmospheric carbon dioxide levels. Here we investigate the role of Ca2+/CO32- ratio, at constant saturation state, in determining calcite growth using atomic force microscopy (AFM) and vertical scanning interferometry (VSI). These coupled techniques provide kinetic measurements across multiple length-scales. Our results indicate that changes in Ca2+/CO32- ratio significantly affect the overall growth rate as well as the anisotropy of growth features on the crystal surface. We further demonstrate that carbonate biomineralization cannot be understood in terms of bulk solution chemistry alone, but requires specific knowledge of both the structure of the biomineral surface and the interaction of solution species with elementary steps on the surface. These findings suggest that differences in Ca2+/CO32- ratio may account for some of the complexity associated with carbonate biomineral proxies and may be an underestimated aspect of biological control over CaCO3 mineralization.
B43G-05
The Role of Microbes in the Precipitation of Microbialites in Cuatro Cienegas, Mexico: A Genomic and Stable Isotopic Perspective
Ancient biologically-mediated sedimentary carbonate deposits, including stromatolites and other microbialites, provide insight into conditions on early Earth. However, interpretation of the environmental and evolutionary significance of microbialites throughout the geological record is dependent upon an understanding of the complex linkages between biological and chemical processes, and isotopic properties associated with the formation of modern microbialites. Here we present the results of metagenomic and isotopic analyses two types of actively accreting freshwater microbialites in Cuatro Cienegas: oncolites and thrombolites. To determine the genetic capabilities of the microbial communities and to identify the dominant metabolic pathways present in the samples, total DNA was purified from microbialite surface samples and pyrosequenced. Genes identified in the metagenome included both autotrophic and heterotrophic processes. In addition, 16S rDNA sequences recovered from the metagenome included both cyanobacteria and heterotrophic bacteria. d13C of intra-crystalline organic matter (IC- OM) in the microbialites is -26‰, consistent with the enzymatic fractionation associated with oxygenic photosynthesis. The microscopy-based occurrence of cyanobacteria on the microbialites, the visible presence of bubbling O2, and the metagenomic confirmation of genes attributed to photosynthesis confirm the photoautotrophic origin of IC-OM. Carbonate d13C values of the oncolite and thrombolite are depleted relative to equilibrium considerations indicating that a 13C-depeleted source of carbon is strongly influencing the DIC at the site of carbonate precipitation. Respiration of photoautotrophic biomass by heterotrophic organisms would release 13C-depleted CO2 to the DIC reservoir resulting in a localized negative shift in the 13C-DIC. Our results indicate that the microbialites are precipitating at sites in direct association with heterotrophic respiration where the remineralization of photoautotrophic carbon is important. Because aerobic respiration leads to carbonate dissolution, our results further suggest that anaerobic heterotrophic respiration processes may be critical for microbialite formation in Cuatro Cienegas. Metagenomic results confirm the presence of genes capable of assimilatory nitrate reduction, denitrification, and sulfate reduction in the microbialites. d15N values of IC-OM from the microbialites are 3-5‰ depleted relative to 15??DIN (+13 to +16‰), consistent with enzymatic fractionation associated with nitrate assimilation by aquatic photoautotrophs where [NO3=] is not limited. Reduced organic sulfur from the thrombolite has a d34S value of - 25‰, a value that can only be associated with dissimilatory sulfate reduction- an anaerobic, heterotrophic process. Together, the metagenomic and isotopic data suggest that the coupling of aerobic and anaerobic autotrophic and heterotrophic processes is a critical component to the formation of microbialites in the CCB.
B43G-06
Investigating the Physical Basis of Amorphous Precursor Transformation to Calcite Using Patterned Alkanethiol Surfaces
Increasing evidence from X-ray Absorption Spectroscopy and Environmental Scanning Electron Microscopy (ESEM) studies of biominerals extracted from calcifying organisms show that amorphous calcium carbonate (ACC) plays a key role in the initial formation of carbonate minerals and in shaping them into complex morphologies. Echinoderms and possibly a wide variety of other organisms, use ACC as a precursor phase. The ACC is first formed within spatial and temporally controlled environments such as vesicles, followed by a subsequent onset of mineralization that transforms the precursor into a fully crystalline material. Recent studies on sea urchin embryos have shown that during this transformation, ACC develops short-range order that resembles calcite before fully crystallizing. While this "non-traditional" process is recognized, the mechanisms and factors that govern this transformation remain poorly understood. Of particular interest are the roles of water, and the functional group chemistry of surfaces and macromolecules within mineralization environments. To investigate these questions, we have developed an experimental approach using ESEM that allows us to control impurity concentration, surface functionality and water content through the degree of water condensation. Patterned self-assembled monolayers (SAM) of hydrophilic moieties with domains of approximately 25 microns in diameter are used to form an array of micro-reactors. ACC particles with known composition are then deposited on the patterns. Condensing water in the ESEM initializes the transformation of ACC to calcite. Our results show that in saturated water vapor, ACC swells, but no obvious faceting of the material occurs. It is only in bulk water, via dissolution/crystallization, where the calcite grown on carboxyl-terminated surfaces is found with the often-observed \{013\} nucleation face. We use this insight to understand the role of the different chemical moieties on ACC to calcite transformation by measuring nucleation rates with time resolved experiments in the ESEM using bulk water. Preliminary evidence shows a markedly lower nucleation rate on hydroxyl versus carboxyl surfaces suggesting that hydroxyl functionalized surfaces do not promote the nucleation of calcite thus stabilizing the amorphous phase.
B43G-07
Bridging the Reef gaps: first evidence for corals surviving under low pH conditions