PUCCA: Photosynthetic Underpinnings of Coccolithophore CAlcification

Coccolithophores generate over 1 billion tonnes of calcium carbonate every year in the ocean. This calcium carbonate is a foundation of marine ecosystem and the ocean carbon cycle. The generation of calcium carbonate in the surface ocean affects how much CO2 dissolves in those surface waters from the atmosphere. And the added density of calcium carbonate in the plankton and sinking marine particulate matter helps it drop to depth enhancing the transport of carbon dioxide from surface waters (and so the atmosphere) to the deep ocean. Despite the enormous production rate of calcium carbon and its central importance in controlling how much carbon dioxide reaches the deep ocean, there is currently no answer to the question of what limits marine calcification and how this component of the ocean carbon budget will change into the future.

There has been much concern regarding the impact of ocean acidification on the ability of marine calcifiers to produce their shells. All current ocean carbon cycle models infer calcite production rates according to an equation which depends only on the calcite saturation state, which diminishes with acidification, of the form Calcification rate = (1-saturation)^n. But the current ocean is four times supersaturated with respect to calcite so saturation state cannot be the only parameter limiting calcification rates. All marine calcifiers are fueled by sunlight and photosynthesis, with the coccolithophores containing a chloroplast from an ancient secondary endosymbiotic event. So the process of calcification by an organism has two sides to the equation: the energy and carbon that is available to invest in calcification and the energetic and carbon cost of calcification. PUCCA aims to uncover how the environment controls both sides of the equation and their coupling, namely how the energy available from photosynthesis is partitioned between cell growth and calcification. The importance of photosynthesis to these organisms is underlined by the observation that when these cells evolved the ability to calcify in the Mesozoic, they coincidentally lost their ability to feed on any other source of carbon and became fully dependent on photosynthesis, or autotrophic.

The ability to succeed in an environment is determined by an organism's growth rate under conditions of resource limitation. PUCCA aims to document and generate a mechanistic understanding at the molecular, cell and community level, of how environmental change selects for different coccolithophore strategies of cell resource allocation between photosynthesis, calcification and growth. PUCCA will use the geological record and modern ocean as a natural laboratory to show how evolution has selected for these traits over the 60 million year history of climate and across the latitudes of the modern ocean. For the first time there are sufficient strains of a single species, Emiliania huxleyi, isolated from across the world's oceans and with diverse calcification phenotypes, to obtain candidates for key calcification and photosynthetic genes and to identify the metabolic pathways linking photosynthetic products with calcification and their efficiency. This unique library of near genetically identical isolates also allows a detailed investigation of the biochemical basis for differing cellular resource allocation between photosynthesis and calcification which underpins the diverse phenotypes. PUCCA will test how these biochemical strategies function over time and leave an imprint in the geological record by developing cutting edge techniques for the extraction and characterisation of metabolites from the fossil record. Our results will transform predictions of the coccolithophore calcification response to global change in the future and will generate a novel, mechanistically grounded, parameterisation of marine calcite production to implement into models of the past and future carbon cycles.