Preprint volume - SIBM

Pre-print Volume – Introductory lectures
Topic 2: MARINE ORGANISMS AND ECOSYSTEMS AS MODEL SYSTEMS
C. BROWNLEE, G.L. WHEELER, A. CHRACHRI, A.R. TAYLOR,
A. HIGHFIELD, F.J. VERRET, D. SCHROEDER.
Marine Biological Association of the UK, The Laboratory, Citadel Hill, Plymouth PL1 2PB, UK.
cbr@mba.ac.uk
COCCOLITHOPHORE BIOMINERALIZATION:
FROM MOLECULES TO GLOBAL PROCESSES
BIOMINERALIZZAZIONE DEI COCCOLITOFORIDI:
DALLE MOLECOLE AI PROCESSI GLOBALI
Abstract – Coccolithophores are responsible for a major component of biogenic calcite formation in the
oceans. Despite their biogeochemical importance, the molecular and cellular mechanisms of calcification
are poorly understood. However, a deep understanding of the transport and biomineralization processes
underlying coccolithophore biology is essential for understanding their ecological success and for
predicting how they may be affected by or respond or adapt to future ocean acidification scenarios.
Key-words: coccolithophores, calcification, genomics, cell biology.
Introduction - Coccolithophores occur in all of the world’s oceans and are represented
by many different unicellular species. They are characterized by the production of
intricate calcium carbonate (calcite) scales (coccoliths) during at least one phase of
their life cycle. Some species form massive seasonal blooms in temperate oceanic
waters and the reflectance of the calcite in these blooms renders them visible from
space. Estimates suggest that coccolithophores account for approximately half of
global biogenic calcium carbonate production. This group of organisms thus plays an
important role in the biogeochemical cycling of carbon in the oceans. A significant
proportion of the carbon fixed into calcite sinks to the ocean floor where it may form
sediments that give rise to chalk and limestone deposits. This represents a significant
long-term sequestration of inorganic carbon.
Coccolithophores are known to produce two types of calcite scales: Holococcoliths that
have a simple crystal structure, and heterococcliths that are formed by interlocking
calcite crystals made up of calcite crystal elements of complex shape. Significantly,
heterococcolith production has been demonstrated to occur in an intracellular
compartment - the coccolith vesicle (CV). This vesicle is derived from the Golgi body
and encloses the forming coccolith in a membrane-bound isolated compartment,
allowing the chemical composition to be regulated to promote the ordered deposition
of the calcite crystals (Brownlee and Taylor, 2005). A wide rang of experimental
studies have shown that external bicarbonate is the inorganic substrate for calcification.
The intracellular precipitation of carbonate results in the production of protons. It
follows that calcification requires transport of the substrates for calcification
(bicarbonate and calcium) into the cell and removal of the ionic products (protons)
from the cell. Our earlier work has shown that the magnitude of the fluxes involved in
calcification is extremely large, since fixation of inorganic carbon by calcification can
often occur at similar rates to the photosynthetic fixation of carbon.
Some key questions relating to transport processes underlying calcification relate to the
identification of membrane transporters. Do coccolithophore cells have calcificationspecific
transport systems or does calcification recruit the cell’s normal transport
41 st S.I.B.M. CONGRESS Rapallo (GE), 7-11 June 2010
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