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The magnetic properties of these crystals
have been studied for their potential ap-
plications—for instance, enabling preci-
sion drug delivery in cancer treatments ( 7 ).
Another distinctive example is the guanine
nanocrystals found in the skin of chame-
leons, which play a role in the color change
ability of chameleons ( 8 ). In addition, the
deeper skin cells contain slightly larger gua-
nine crystals that can reflect sunlight in the
infrared range, which means that guanine
crystals also provide thermal protection for
the chameleons. This, like the other bio-
genic crystals mentioned, has also provoked
a search for possible applications, such as
a chameleon-inspired electronic skin with
touch-controlled color change ( 9 ). Biogenic
crystals can also be found in some patho-
logical processes related to human health.
An example is the struvite crystal, which is

formed in the urinary tract when infected

by urease-positive bacteria. Bacteria actively

participate in the process of growing these

crystals, such as by affecting their porosity

and the characteristic surface structure ( 10 ,

11 ). Through these crystals, bacteria can en-

hance their adhesion to the epithelium of

urinary tract, making them more difficult

to be dislodged by the urine stream ( 12 ).

Therefore, the study of biogenic crystals is

also of interest to the medical community.

Adding to the diverse roster of biogenic

crystals, Avrahami et al. dive into micro-

scopic detail of coccoliths grown by the ma-

rine algae Calcidiscus leptoporus. Coccoliths

are micrometer-sized plates of calcite crys-

tals grown around an organic substrate

known as the base plate. Researchers have

created models to explain how organisms

crystallize such complex structures. One

such popular model is the V/R model, which

prescribes the coccoliths to have two differ-

ent crystal units: a radial (R) unit with the

crystallographic c axis oriented parallel to

the coccolith plane, and a vertical (V) unit

with the c axis perpendicular to the cocco-

lith plane ( 13 ).

After examining the stages of coccolith

formation using various three-dimensional

imaging techniques, Avrahami et al. pro-

pose that the orientations of the R and V

units are determined by having their dif-

ferent edges attached to the base plate. In

this model, R and V units are one and the

same rhombohedron-shape calcite crystal.

Rhombohedra placed on their acute edges

are R units, and those on obtuse edges are

V units. Through careful characterization of

the morphology and orientation of calcite

crystals at various stages of growth, the au-

thors conclude that these rhombohedron-

shape calcite crystals are built of only one

facet set—the {104}, which contains six

symmetry-related facets.

Avrahami et al. show that the facets of the

{104} calcite rhombohedron found in cocco-

liths can have different growth rates, break-

ing the symmetry of the rhombohedron. The

structure of the coccolith can be explained

by the anisotropic growth rates of the {104}

rhombohedron facets, which depend on

their orientation in relation to the concen-

tration gradient of calcium and carbonate

ions. A similar symmetry-breaking phe-

nomenon has also been observed, for ex-

ample, in the case of the magnetite crystals

from the aforementioned magnetotactic

bacteria ( 14 ). Such anisotropy of the growth

rates may occur due to an anisotropy in the

environment or of the growth sites, which

may be the result from an uneven flux of

ions passing through the intracellular mem-

brane surrounding the crystal.

If the idea of symmetry breaking and

anisotropy of the growth rates of symmetri-

cally equivalent facets is correct, then other

interesting phenomena can be expected—

for example, an increase in the size of fast-

growing facets. Growing crystals adopt a

morphology such that the bounding facets

of the crystal have a low surface energy,

which corresponds to the slow-growing fac-

ets, whereas the fast-growing facets tend to

disappear because of a higher surface energy.

By looking at the {104} calcite rhombohe-

dron as seen in coccoliths, one can see how

the growth anisotropy can alter the overall

appearance of a crystal ( 1 ). Although it is lit-

tle wonder that the slow-growing facets can

grow larger, the geometry of the {104} rhom-

bohedron is such that the fast-growing facets

can grow larger as well. The phenomenon of

increasing the size of fast-growing facets,

rather than their disappearance, is of great

interest but not very often observed and is

related to the geometry of the crystal ( 15 ).

In the process of biogenic precipitation of

calcite in the form of coccoliths, coccolitho-

phores release carbon dioxide, and simul-

taneously, in the process of photosynthesis,

they capture carbon dioxide. This process

takes place on an enormous scale in the

oceans and is one of the basic factors regulat-

ing the carbon dioxide (CO 2 ) and carbon cy-

cle in nature. The difference between capture

and excretion of CO 2 by coccolithophores is

an important consideration for modeling

carbon cycling in the oceans and therefore

has relevance for climate change discussions.

The research presented by Avrahami et al. is

thus in line with the current research trends

and can help to assess the role of coccolitho-

phore species C. leptoporus with respect to

the global carbon cycle. j

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10.1126/science.abo2781

15 APRIL 2022 • VOL 376 ISSUE 6590 241

Biogenic crystals, such as coccoliths (pictured here)

formed by single-celled algae, play an essential role in

many biological processes.

Institute of Physics, Lodz University of
Technology, ul. Wólczańska 217/221, 93-005
Łódź, Poland Email: [email protected]