Biomineralization Within Vesicles: The Calcite of Coccoliths

200 Young & HenriksenThe Coccolithus structure is similar, but more complex—the V-unit again forms thedistal shield but also extends onto the proximal surface as a lower tube element. The R-unit forms the proximal shield, but extends on the distal surface as an upper tube element(Fig. 3A-B). As a result, the two crystal unit types are intergrown and the proto-coccolithring is embedded within the coccolith structure and only visible on broken specimens(Fig. 4D-E). Additional complexity is seen on the proximal shield with development ofseparate upper and lower elements growing at different angles. The upper unit isapproximately radially directed (and elongate approximately parallel to the c-axis), whilethe lower element is oblique to this (Fig. 4F). Finally on complete coccoliths, the centralarea elements on both the proximal and distal surfaces appear to be composed of a massof minute elements, but interconnections between them are often visible, and atintermediate growth phases the morphology is simpler; hence these are evidently aproduct of less regular growth at a late phase.The Emiliania structure (Westbroek et al. 1989; Young 1989; Didymus et al. 1994) isunusual in being composed essentially of R-units, with the V-units confined to the protococcolithring (Fig. 6A). As a result, the individual R-units have particularly complex form,being constructed of proximal shield, distal shield, inner tube and central area elements,each of which represents a separate growth direction from the nucleation site.Ophiaster has a simple rim of alternating V- and R-units but the central area isfloored by separate elements. These elements interdigitate with the rim units, but arecrystallographically separate and have sub-tangential c-axis orientations. This structurewith three separate crystal unit types has been described in detail for Syracosphaerapulchra (Young et al. in press).NucleationThe most remarkable common feature of the heterococcoliths is the development ofthe rim from a proto-coccolith ring of simple calcite crystals with alternating sub-verticaland sub-radial c-axis orientations—the V/R structure of Young et al. (1992). This isshown by each of the illustrated examples and has been demonstrated in numerous others(e.g., Young and Bown 1997; Marsh 1999; Young et al. 1999). In some cases, it has onlybeen possible to demonstrate that the rim consists of two alternating crystal unit types,not to prove that these have different c-axis orientations. For instance, in thePapposphaeraceae the rim is clearly formed from two alternating crystal unit types (e.g.,Manton and Oates 1975), but because the entire coccoliths are usually only 1-2 micronslong and specimens are rare, it has not been possible for crystallographic orientations tobe determined. However, it seems reasonable to predict that these fit the basic V/Rmodel. This means that the first stage of biomineralization is nucleation of a ring ofcalcite nuclei with alternating orientations. This is a distinctive feature ofheterococcoliths with no obvious parallels in any other biomineralization systems and hasdemonstrably remained constant through more than 200 million years of coccolithevolution (Young et al. 1992, 1999). Moreover, nucleation controls total calciteorientation, i.e., not simply c-axis orientation, but also a-axis orientation. This isevidenced by the fact that crystal faces on adjacent elements are sub-parallel (e.g., Fig. 1,3C−G, 5A). It has also been demonstrated using SAED study of Emiliania huxleyicoccoliths (Davis et al. 1995) and AFM study of Coccolithus pelagicus and Oolithotusfragilis (Henriksen et al. in press a).A final distinctive, nucleation-related feature is that all heterococcoliths showchirality. This is seen for instance in element orientations and cross-polarized lightextinction crosses. The lower proximal shield elements of Coccolithus (Figs. 3A, 4F) area good example. These grow at an oblique angle and always show clockwise offset from