and the vesicle membrane ( 8 , 27 , 32 ). Such con-
finement manifests directly in crystal morphol-
ogy as noncrystallographic surfaces that result
from a physical block of growth by the delimit-
ing membrane of the coccolith vesicle ( 27 ). We
propose that confinement also influences crys-
tal growth indirectly by creating a graded nano-
environment within the coccolith vesicle. For
example, a concentration gradient may arise
from localized ion fluxes generated by ion trans-
porters on the vesicle membrane ( 33 ). It re-
mains imperative to characterize, chemically
and structurally, the cellular environment and
its interactions with the growing crystals.
Figure 4, C to E, illustrates how such a con-
centration gradient differentially affects growth
steps at the atomic scale, leading to different
growth kinetics of equivalent facets that result
in an anisotropic growth. For example, when
one facet of a crystal experiences a higher ion
concentration than another, it will grow faster
toward the ion source (Fig. 4C). Even more in-
teresting is when two neighboring facets of dif-
ferent crystals present different geometries of
their atomic steps toward the ion gradient (Fig.
4D), resulting in faster growth of one of the
crystals. The differences between step orienta-
tions in the presence of a nanoscale gradient
(Fig. 4E) break the symmetry between the ad-
jacent crystals and can explain their aniso-
tropic growth.
Coccolith crystal growth is not a process
that stems from multiple manipulations of
crystallographic growth; rather, it hinges on the
various consequences that emerge from the
stable habit of calcite and its rhombohedral
geometry. Such a growth regime can be con-
trolled by the rate and location of ion transport,
rather than by“tailored”modifications for
specific crystal facets. One can envision how
alterations in the initial conditions of coccolith
assembly (e.g., unit orientation, unit spacing,
ion flux direction, or membrane position during
growth) can markedly affect the final coccolith
morphology.
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ACKNOWLEDGMENTS
We thank E. Shimoni for dedicated assistance with HAADF-STEM
acquisitions, L. Addadi for helpful discussions, and E. Komendacka
for designing the schematic 3D model of a coccolith ring.Funding:
Supported by Israel Science Foundation grant 697/19.Author
contributions:E.M.A. performed coccolith extractions, HAADF-STEM
data collection, computational reconstructions, segmentations
and 3D visualizations; L.H. performed the NBED measurements;
L.A. performed HAADF-STEM imaging at cryogenic conditions; E.M.A.
analyzed the results with the guidance of A.G; A.G. supervised the
research; A.G. and E.M.A. wrote the paper with the assistance
of L.H.Competing interests:The authors declare no competing
interests.Data and materials availability:Electron microscopy raw
data, as well as volume renderings and segmentation files, are
available through the Dryad digital repository ( 34 ).SUPPLEMENTARY MATERIALS
science.org/doi/10.1126/science.abm1748
Materials and Methods
Supplementary Text
Figs. S1 to S11
Movies S1 to S5
31 August 2021; accepted 15 February 2022
10.1126/science.abm1748316 15 APRIL 2022•VOL 376 ISSUE 6590 science.orgSCIENCE
Fig. 4. Development of crystal anisotropy due to differential growth rates
of equivalent facets.(AandB) Two sets of crystals at different growth stages
(same as in Fig. 3). In (A), the former is schematically superimposed on the
latter. In addition, a schematic {104} rhombohedron (in white) that is
superimposed over a silhouette of the early unit (asterisks) shows the
differentially growing facets. Continuous arrows indicate fast-growing facets;
dashed arrows designate slow-growing facets. (CandD) Illustrations of the
possible conditions inside the cell leading to the observed growth regimes in (A)
and (B). A putative gradient in ion concentration is shown in purple and with
black arrows. In scenario (C) the directional flux leads to accelerated growth of
one facet toward it, unlike a different facet on the same crystal that is farther
from the ion source. In scenario (D), two different units have different step
orientations facing a similar gradient. In this case, their differential growth rate
will determine which facet grows faster. a, acute step; o, obtuse step. (E)Amodel
showing local ion concentration gradients (purple) in the confined environment
of the coccolith vesicle. This anisotropic environment enables different atomic steps
of adjacent crystals to experience different solution conditions. In all panels, red
arrows and lines at crystal apexes indicatec-axis directions.RESEARCH | REPORTS