Dr. Mary Marsh, Associate Professor

Regulation of CaCO3 Formation in Coccolithophores

Atmospheric pCO2 was 30% lower during glacial periods than during preanthropogenic interglacial times suggesting that pCO2 is a driver or amplifier of glacial cycles. Although the process by which Quaternary pCO2 fluctuations are regulated is still unknown, it is generally accepted that the ocean is the only reservoir capable of absorbing atmospheric pCO2 changes over glacial time scales. CO2 is partitioned between atmosphere and ocean by the biological CO2 pump (eq. 1) which draws CO2 into the ocean, the biological carbonate pump (eq. 2) which liberates CO2 to the atmosphere, and ocean circulation which exchanges carbon between deep and surface waters.

CO2 + H2O CH2O + O2	(1)
Ca2+ + 2HCO3– CaCO3 + CO2 + H2O	(2)

The coccolithophores are marine phytoplankton that produce an outer covering of calcitic (CaCO3) scales known as coccoliths during at least one stage of their life history. They impact the ocean carbon cycle via consumption of CO2 during photosynthesis (eq. 1), but more importantly, the coccolith-bearing stage also increases atmospheric pCO2 during calcification (eq.2). In an effort to understand the role of coccolithophores in Quaternary pCO2 fluctuations, we are studying a) the molecular bases of CaCO3 deposition and b) environmental factors that effect the expression or suppression of genes with a calcifying function and favor the growth of calcifying or noncalcifying morphotypes.

An ocean wide expansion of noncalcifying coccolithophores with a concomitant reduction in the coccolith-bearing population would decrease atmospheric pCO2 by decreasing CaCO3 production, while expansion of the calcifying population at the expense of the noncalcified phase would increase CaCO3 production and atmospheric pCO2. If coccolithophore phase shifting cycles are promoted by environmental consequences of orbital (Milankovitch) cycles, then coccolithophore populations may drive or enhance glacial/interglacial cycles in atmospheric CO2 levels.

Selected References