Farfan
Gabriela A.
Farfan
Gabriela A.
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ArticleMineralogy of deep-sea coral aragonites as a function of aragonite saturation state(Frontiers Media, 2018-12-10) Farfan, Gabriela A. ; Cordes, Erik E. ; Waller, Rhian G. ; DeCarlo, Thomas M. ; Hansel, Colleen M.In an ocean with rapidly changing chemistry, studies have assessed coral skeletal health under projected ocean acidification (OA) scenarios by characterizing morphological distortions in skeletal architecture and measuring bulk properties, such as net calcification and dissolution. Few studies offer more detailed information on skeletal mineralogy. Since aragonite crystallography will at least partially govern the material properties of coral skeletons, such as solubility and strength, it is important to understand how it is influenced by environmental stressors. Here, we take a mineralogical approach using micro X-ray diffraction (XRD) and whole pattern Rietveld refinement analysis to track crystallographic shifts in deep-sea coral Lophelia pertusa samples collected along a natural seawater aragonite saturation state gradient (Ωsw = 1.15–1.44) in the Gulf of Mexico. Our results reveal statistically significant linear relationships between rising Ωsw and increasing unit cell volume driven by an anisotropic lengthening along the b-axis. These structural changes are similarly observed in synthetic aragonites precipitated under various saturation states, indicating that these changes are inherent to the crystallography of aragonite. Increased crystallographic disorder via widening of the full width at half maximum of the main (111) XRD peaks trend with increased Ba substitutions for Ca, however, trace substitutions by Ba, Sr, and Mg do not trend with crystal lattice parameters in our samples. Instead, we observe a significant trend of increasing calcite content as a function of both decreasing unit cell parameters as well as decreasing Ωsw. This may make calcite incorporation an important factor to consider in coral crystallography, especially under varying aragonite saturation states (ΩAr). Finally, by defining crystallography-based linear relationships between ΩAr of synthetic aragonite analogs and lattice parameters, we predict internal calcifying fluid saturation state (Ωcf = 11.1–17.3 calculated from b-axis lengths; 15.2–25.2 calculated from unit cell volumes) for L. pertusa, which may allow this species to calcify despite the local seawater conditions. This study will ideally pave the way for future studies to utilize quantitative XRD in exploring the impact of physical and chemical stressors on biominerals.
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ArticleThe origin and role of organic matrix in coral calcification: insights from comparing coral skeleton and abiogenic aragonite(Frontiers Media, 2018-05-15) DeCarlo, Thomas M. ; Ren, Haojia ; Farfan, Gabriela A.Understanding the mechanisms of coral calcification is critical for accurately projecting coral reef futures under ocean acidification and warming. Recent suggestions that calcification is primarily controlled by organic molecules and the biological activity of the coral polyp imply that ocean acidification may not affect skeletal accretion. The basis for these suggestions relies heavily on correlating the presence of organic matter with the orientation and disorder of aragonite crystals in the skeleton, carrying the assumption that organic matter observed in the skeleton was produced by the polyp to control calcification. Here we use Raman spectroscopy to test whether there are differences in organic matter content between coral skeleton and abiogenic aragonites precipitated from seawater, both before and after thermal annealing (heating). We measured the background fluxorescence and intensity of C-H bonding signals in the Raman spectra, which are commonly attributed to coral polyp-derived skeletal organic matrix (SOM) and have been used to map its distribution. Surprisingly, we found no differences in either fluorescence or C-H bonding between abiogenic aragonite and coral skeleton. Annealing reduced the molecular disorder in coral skeleton, potentially due to removal of organic matter, but the same effect was also observed in the abiogenic aragonites. The presence of organic molecules in the abiogenic aragonites is further supported by measurements of N content and δ15N. Together, our data suggest that some of what has been interpreted in previous studies as polyp-derived SOM may actually be seawater-sourced organic matter or some other signal not unique to biogenic aragonite. Finally, we create a high-resolution Raman map of a Pocillopora skeleton to demonstrate how patterns of fluorescence and elevated calcifying fluid aragonite saturation state (ΩAr) along centers of calcification are consistent with both biological and physico-chemical controls. Our aim is to advance discussion on biological mediation of calcification and the implications for coral resilience in a high-CO2 world.
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PreprintCoupled x-ray fluorescence and x-ray absorption spectroscopy for microscale imaging and identification of sulfur species within tissues and skeletons of scleractinian corals( 2018-10) Farfan, Gabriela A. ; Apprill, Amy ; Webb, Samuel M. ; Hansel, Colleen M.Identifying and mapping the wide range of sulfur species within complex matrices presents a challenge for under-standing the distribution of these important biomolecules within environmental and biological systems. Here, we present a coupled micro X-ray fluorescence (μXRF) and X-ray absorption near edge structure (XANES) spectroscopy method for determining the presence of specific sulfur species in coral tissues and skeletons at high spatial resolution. By using multiple energy stacks and principal component analysis of a large spectral database, we were able to more accurately identify sulfur species components and distinguish different species and distributions of sulfur formerly unresolved by previous studies. Specifically, coral tissues were domi-nated by more reduced sulfur species, such as glutathione disulfide, cysteine and sulfoxide, as well as organic sulfate as represented by chondroitin sulfate. Sulfoxide distributions were visually correlated with the presence of zooxanthellae endosymbionts. Coral skeletons were composed primarily of carbonate-associated sulfate (CAS), along with minor contributions from organic sulfate and a separate inorganic sulfate likely in the form of adsorbed sulfate. This coupled XRF-XANES approach allows for a more accurate and informative view of sulfur within biological systems in situ, and holds great promise for pairing with other techniques to allow for a more encompassing understanding of elemental distributions within the environment.
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ArticleCrystallographic and chemical signatures in coral skeletal aragonite(Springer, 2021-11-29) Farfan, Gabriela A. ; Apprill, Amy ; Cohen, Anne L. ; DeCarlo, Thomas M. ; Post, Jeffrey E. ; Waller, Rhian G. ; Hansel, Colleen M.Corals nucleate and grow aragonite crystals, organizing them into intricate skeletal structures that ultimately build the world’s coral reefs. Crystallography and chemistry have profound influence on the material properties of these skeletal building blocks, yet gaps remain in our knowledge about coral aragonite on the atomic scale. Across a broad diversity of shallow-water and deep-sea scleractinian corals from vastly different environments, coral aragonites are remarkably similar to one another, confirming that corals exert control on the carbonate chemistry of the calcifying space relative to the surrounding seawater. Nuances in coral aragonite structures relate most closely to trace element chemistry and aragonite saturation state, suggesting the primary controls on aragonite structure are ionic strength and trace element chemistry, with growth rate playing a secondary role. We also show how coral aragonites are crystallographically indistinguishable from synthetic abiogenic aragonite analogs precipitated from seawater under conditions mimicking coral calcifying fluid. In contrast, coral aragonites are distinct from geologically formed aragonites, a synthetic aragonite precipitated from a freshwater solution, and mollusk aragonites. Crystallographic signatures have future applications in understanding the material properties of coral aragonite and predicting the persistence of coral reefs in a rapidly changing ocean.
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ThesisThe mineralogy and chemistry of modern shallow-water and deep-sea corals(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2019-02) Farfan, Gabriela A.The architecture of coral reef ecosystems is composed of coral skeletons built from the mineral aragonite (CaCO3). Coral reefs are currently being threatened by ocean acidification (OA), which may lower calcification rates, reduce skeletal density, and increase aragonite dissolution. Crystallography and chemistry are what govern the materials properties of minerals, such solubility and strength. Thus, understanding the mineralogical nature of coral aragonite and how it forms are important for predicting bulk skeletal responses under climate change. Different models based on geochemical versus biological controls over coral skeleton biomineralization propose conflicting predictions about the fate of corals under OA. Rather than investigating the mechanism directly, I use a mineralogical approach to study the aragonite end-products of coral biomineralization. I hypothesize that coral mineralogy and crystallography will lend insights into how coral aragonite crystals form and how sensitive coral aragonite material properties may be to OA. Here I compare the crystallography, bonding environments, and compositions of coral aragonite with aragonite produced by other organisms (mollusk), synthetically (abiogenic precipitation in aragonite-supersaturated seawater and freshwater), and in natural geological settings (abiogenic). Coral aragonite crystallography does not resemble mollusk aragonite (aragonite formed with a strong biological influence), but rather is identical to abiogenic synthetic aragonite precipitated from seawater. I predict that the material properties of coral aragonite are similar to that of abiogenic synthetic seawater aragonites and that coral aragonite formation is sensitive to surrounding seawater chemistry. To test the effect OA on coral aragonites, I studied deep-sea corals from a natural Ωsw gradient (1.15–1.44) in the Gulf of Mexico and shallow-water corals across a natural Ωsw (2.3–3.7) and pH (7.84–8.05) gradient in Palau. Minor shifts in crystallography are expressed by coral aragonite in these natural systems, likely governed by skeletal calcite contents, density, and Ω of the coral calcifying fluid. My results are most consistent with a geochemical model for biomineralization, which implies that coral calcification may be sensitive to OA. However, further work is required to determine whether the modest crystallographic shifts I observe are representative on a global scale and whether they could influence bulk skeletal material properties.
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ArticleSpatial heterogeneity in particle-associated, light-independent superoxide production within productive coastal waters(American Geophysical Union, 2020-10-06) Sutherland, Kevin M. ; Grabb, Kalina C. ; Karolewski, Jennifer S. ; Plummer, Sydney ; Farfan, Gabriela A. ; Wankel, Scott D. ; Diaz, Julia M. ; Lamborg, Carl H. ; Hansel, Colleen M.In the marine environment, the reactive oxygen species (ROS) superoxide is produced through a diverse array of light‐dependent and light‐independent reactions, the latter of which is thought to be primarily controlled by microorganisms. Marine superoxide production influences organic matter remineralization, metal redox cycling, and dissolved oxygen concentrations, yet the relative contributions of different sources to total superoxide production remain poorly constrained. Here we investigate the production, steady‐state concentration, and particle‐associated nature of light‐independent superoxide in productive waters off the northeast coast of North America. We find exceptionally high levels of light‐independent superoxide in the marine water column, with concentrations ranging from 10 pM to in excess of 2,000 pM. The highest superoxide concentrations were particle associated in surface seawater and in aphotic seawater collected meters off the seafloor. Filtration of seawater overlying the continental shelf lowered the light‐independent, steady‐state superoxide concentration by an average of 84%. We identify eukaryotic phytoplankton as the dominant particle‐associated source of superoxide to these coastal waters. We contrast these measurements with those collected at an off‐shelf station, where superoxide concentrations did not exceed 100 pM, and particles account for an average of 40% of the steady‐state superoxide concentration. This study demonstrates the primary role of particles in the production of superoxide in seawater overlying the continental shelf and highlights the importance of light‐independent, dissolved‐phase reactions in marine ROS production.