Role of macromolecules in coccolithophore biomineralization
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Date
09/07/2018Item status
Restricted AccessEmbargo end date
31/12/2100Author
Walker, Jessica Mary
Metadata
Abstract
Biomineralization refers to the production of mineralized tissues by organisms. The
fine control which organisms can exert over this process produces crystals with morphologies
and properties contrasting to that of non-biogenic crystals and specifically altered to suit the
required functional need. A key model system of biomineralization are a unicellular marine
algae, coccolithophores, which produce calcium carbonate scales known as coccoliths. These
coccoliths are comprised of arrangements of single crystals of calcite interlocked to form a
plate-shaped structure. Coccoliths are developed intracellularly in a specialised compartment
called the coccolith vesicle, before being extruded to the cell surface.
In this work, two vital components of the coccolith biomineralization process are
investigated – a soluble polysaccharide thought to act as a habit modifier and an insoluble
organic scaffold known as a baseplate that provides the surface for nucleation and growth of
the crystals. Whilst both these elements are thought to play a key part in the biomineralization
process, the role of each is not fully understood.
To investigate the effect of coccolith-associated polysaccharides (CAPs) on
nucleation and polymorph selection, two systems that promote different polymorphs of
calcium carbonate were utilised. In both systems, the intracrystalline polysaccharide fraction
extracted from one species, Gephyrocapsa oceanica, was able to promote calcite nucleation
in vitro, even under conditions favouring the kinetically-privileged polymorphs of calcium
carbonate: vaterite and aragonite. As this property is not observed with CAPs extracted from
its ‘sister species’, Emiliania huxleyi, the in vivo function of CAPs may differ between the two
species. Both cryo-transmission electron microscopy (cryoTEM) and scanning electron
microscopy (SEM) were used to determine the mechanism of calcite growth in the presence
of G. oceanica CAPs, showing its impact on the forming amorphous calcium carbonate
(ACC), decreasing the size of the particles and producing irregular, angular particles.
Using cryo-electron tomography (cryoET), it was possible to create a 3D
representation of the structure of the baseplate from the coccolithophore Pleurochrysis
carterae, revealing its two-sided organisation. Examination of several stages of the coccolith
growth process demonstrated the interlocking nature of the calcite crystals that make up the
coccolith and the progression of the crystal morphologies over time, and the interaction of
these crystals with the baseplate rim. Additionally, the effect of inhibiting carbonic anhydrase (CA), an enzyme involved
in the regulation of carbonate species, revealed that inhibition of CA can affect
coccolithogenesis as well as cell proliferation.