Biological control on calcite crystallization: AFM investigation of coccolith polysaccharide function

Abstract

Calcite crystals grown by organisms can be elaborate and enigmatic. One of the best examples is the tiny calcite shields known as coccoliths that are produced by unicellular algae. Coccoliths consist of interlocking single crystals of highly modiÞ ed morphology, and complex organic molecules called CAPs (coccolith associated polysaccharides) are known to be intimately associated with their formation. Here, we show how a CAP can regulate crystal morphology to enhance precipitation of speciÞ c faces, a crucial aspect of the biomineralization process.Using atomic force microscopy (AFM), we investigated the interaction of CAP from the species Emiliania huxleyi with the calcite surface during dissolution, precipitation, and dynamic equilibrium. We were able to see the polysaccharide adsorbed to the surface and observe its impact on mineral behavior. These experiments demonstrate that CAP preferentially interacts with surface sites deÞ ned by acute, rather than obtuse, angles and blocks acute sites during dissolution and growth. Therefore, CAP makes the calcite face that is most stable in the pure system, {101-4}, extend preferentially on the obtuse edges, promoting development of faces with lower angles to c-axis. AFM images of E. huxleyi at micrometer and atomic scales established that this is precisely the type of faces that deÞ ne the morphology of the coccolith crystals. Therefore, we propose that crystal shape regulation by CAP is a fundamental aspect of coccolith biomineralization, and that preferential growth inhibition by sitespeciÞ c functional groups is the mechanism causing CAP functionality.

Insight into <i>Emiliania huxleyi</i> coccospheres by focused ion beam sectioning

“…Moreover it is reasonable to assume that the non-calcite space in the coccosphere (i.e. the extracellular matrix) has the density of seawater, because it actually is seawater in a polysaccharide matrix, and even if the polysaccharides present (Henriksen et al, 2004) should lower the density, this would only affect the absolute value of overall cell density and not the relationship of overall cell density and the PIC / POC ratio (Fig. 7a).…”

Influence of changing carbonate chemistry on morphology and weight of coccoliths formed by <i>Emiliania huxleyi</i>

“…The inner side of the CV membrane always remains in close contact with the coccolith and is actively expanded from the outside by the cytoskeleton located within the cytosol so that the growing calcite crystals fill the space defined by the expanding vesicle (Westbroek et al, 1984(Westbroek et al, , 1989Didymus et al, 1994;Marsh et al, 1994;Young et al, 2009). Inside the CV, coccolithassociated polysaccharides (CAPs) bound to the inner side of the membrane, have a crucial role in controlling CaCO 3 precipitation due to their potential to bind Ca 2+ (De Jong et al, 1976) and inhibit precipitation at places where they cover the calcite (Borman et al, 1982;Henriksen et al, 2004). Considering the pathway described above, the cytoskeleton and CAPs seem to be two major cellular components controlling the correct growth of calcite crystals within E. huxleyi (Young et al, 1999;Langer et al, 2006).…”

Coccolith strontium to calcium ratios in Emiliania huxleyi: The dependence on seawater strontium and calcium concentrations

“…In the case of no polysaccharides at the cell surface, ␥ ϭ 1. It should be noted that a very small amount of polysaccharides, the coccolith-associated polysaccharides (CAP; Henriksen et al 2004), is always present at the cell surface of E. huxleyi. The amount of CAP, however, is negligible compared with the amount of polysaccharides produced and extruded additionally during nitrogen limitation (Engel et al 2004 (Stoll et al 2002b), whereas the second shows the temperature dependence of in inorganic calcite precipitation K D Sr experiments (Malone and Baker 1999 (Stoll et al 2002b).…”

“…Because the bulk of the coccolith is thought to be precipitated by an inorganic crystal growth mechanism (Young et al 1999), the application of results from inorganic precipitation experiments to coccolithogenesis may provide a useful framework. However, such inferences are doubtful, because precipitation of coccolith calcite takes place within a specialized Golgi-derived vesicle-the so-called coccolithvesicle-and is organically modulated (Young et al 1999;Henriksen et al 2004). Determination of exchange coefficients for coccoliths is problematic, because the Sr/Ca of seawater is used for calculation, although the chemistry of seawater differs from that of the solution within the coccolith vesicle.…”