The concept that the formation of CaCO3 coccoliths functions as a photosynthetic adaptation for the use of bicarbonate is evaluated in the coccolithophorids Coccolithus huxleyi and Cricosphaera carterae by two new methods. In the first, carbon fixation is measured at 10‐s intervals in the first 2 min after addition of 14CO2 and H14CO3− to buffered cultures; this method exploits the relatively long half‐time for the hydration or dehydration of dissolved CO2. In the second, shifts in pH and alkalinity resulting from carbon fixation by cells growing in liquid culture are assessed to indicate fluxes of CO2 and HCO3− into cells and these values compared to measurements of 14C incorporation in photosynthesis and carbonate deposition. The data are interpreted in terms of one of several net inorganic reactions of deposition considered. In this reaction, CO2 is the substrate of photosynthesis and HCO3− is the form of carbon supplied to the calcification site. CO2 resulting from carbonate deposition supplements the CO2 from the medium that diffuses into cells as a source of carbon for photosynthesis.

In this paper we report the results of our studies on the stereospecific binding of shorthorn sculpin antifreeze protein (AFP) to (2 -1 0) secondary prism faces of ice. Using ice crystal growth and etching techniques together with molecular modeling, molecular dynamics, and energy minimization, we explain the nature of preferential binding of shorthorn sculpin AFP along the [1 2 2] direction on (2- 1 0) planes. In agreement with ice etching studies, the mechanism of preferential binding suggested by molecular modeling explains why the binding of shorthorn sculpin AFP occurs along [1 2 2] and not along its mirror symmetry-related direction [-1 -2 2] on (2 -1 0). This binding mechanism is based on the protein-crystal surface enantioselective recognition that utilizes both alpha-helical protein backbone matching to the (2 -1 0) surface topography and matching of side chains of polar/charged residues with specific water molecule positions in the ice surface. The mechanisms of winter flounder and shorthorn sculpin antifreeze binding to ice are compared.

Oyster shell protein and polyaspartate bound to calcite have been visualized at the atomic and molecular levels by atomic force microscopy. The identities of potential binding sites have been suggested from atomic force microscopy (AFM) images and have been evaluated by molecular modeling. Energies and conformations of binding to (110) and (110) prism faces, (001) basal calcium planes, and (104) cleavage planes are considered. The interaction with the basal plane is strongest and is essentially irreversible. Binding to (110) prism surfaces is also energetically favored and selective for orientations parallel or perpendicular to the c-axis. Binding to (110) faces is significantly weaker and orientation nonspecific. If carboxyl groups of the protein or peptide replace select carbonate ions of the (110) face, the binding energy increases significantly, favoring binding in the parallel direction. Binding to (104) cleavage surfaces is weak and probably reversible. Specific alignment of oyster shell protein molecules on calcite surfaces is shown by AFM, and the relevance to the binding model is discussed.

No abstract

The inhibition by phosphocitrate of struvite crystal formation and growth has been examined in the present study. Crystal growth in a gel matrix was controlled by phosphocitrate in a dose-dependent manner. The effects of inhibition were followed using scanning electron microscopy, optical microscopy, and single crystal X-ray analysis. The presence of phosphocitrate induced very strong, crystal face specific inhibition of struvite, leading to total cessation of crystal growth when sufficient concentration of the inhibitor was made available. Crystal growth studies and results from molecular modeling indicated strong affinity of phosphocitrate to (101) faces of struvite. This in turn led to an alteration in the expression of these faces and the development of a characteristic arrowhead struvite morphology. Similar changes were not observed in the presence of identical concentrations of citrate, acetohydroxamic acid, and N-sulfo-2 amino tricarballylate (an analog of phosphocitrate), emphasizing the unique interaction of phosphocitrate with the struvite crystal lattice.