Understanding the cellular mechanisms and constraints of coccolithophore calcification in relation to ocean pH.

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Ocean acidification represents one of the most significant global changes occurring in response to increased atmospheric carbon dioxide from the burning of fossil fuels. Such rapid environmental change is likely to have far-reaching impacts on the ecology and chemistry of the oceans. Certain processes, such as calcification are particularly sensitive to the levels of carbonate, bicarbonate and dissolved carbon dioxide and pH in the surface ocean, which is changing as a consequence of rapid increase in atmospheric carbon dioxide. Organisms which produce external calcium carbonate skeletons, such as foraminifera and corals are affected directly by ocean acidification because it lowers the critical concentration of carbonate on which these organisms depend for calcification. Coccolithophores, photosynthetic unicellular microalgae that account for up to 50% of global calcification, are unique in that they precipitate calcium carbonate (calcite) internally in a specialised organelle that is isolated from the external seawater. Moreover, the use of bicarbonate rather than carbonate as the substrate for calcification by these organisms potentially leads to the production of protons and intracellular acidification. Regulated intracellular pH is critical for correct cellular function and this unique biology raises some fundamental questions: -Does intracellular proton production significantly influence pH homeostasis and photosynthetic metabolism? Are there any unique pH regulatory mechanisms associated with this physiology? How does calcification in an isolated intracellular compartment respond to changes in external pH and inorganic carbon chemistry? We will apply a range of powerful cellular approaches to characterize the mechanisms of pH regulation in calcifying coccolithophores and determine how these may be affected by, and adapt to, increased ocean acidity in the short and longer term. While there is significant evidence that decreased ocean acidity may lead to reduced coccolithophore calcification, the picture is by no means clear with evidence for and against significant effects and considerable variability between different coccolithophore species. In particular we will use cellular biophysical approaches, intracellular and extracellular pH imaging and direct monitoring of cell surface proton fluxes to characterize the role in intracellular pH regulation of a novel proton efflux mechanism at the cell membrane that we have recently discovered and which we propose is pivotal in linking changes in extracellular pH and inorganic carbon chemistry with the intracellular calcification mechanism. We will also monitor calcification/photosynthesis ratios under fluctuating light conditions designed to induce repetitive short-term uncoupling of calcification and photosynthesis. Since photosynthesis is a net consumer of protons in the cell, we will test whether such uncoupling leads to fluctuating intracellular acidic loads that may impact on calcification, particularly under conditions that do not favour H+ removal from the cell surface. These combined studies will establish a framework to model the major dissolved inorganic carbon (DIC), and H+ fluxes underlying coccolithophore calcification. Longer-term continuous culture experiments will be established to assess the adaptive changes in homoestatic mechanisms that may offset the impact of decreased ocean pH on the calcification process. Overall, these studies will facilitate the interpretation of coccolithophore distribution patterns in relation to ocean inorganic carbon chemistry and will allow us to construct better models to predict more accurately how intracellular calcification may be affected by and adapt to increased ocean acidity on a global scale.