Carbon isotope ratios of coccolith-associated polysaccharides of Emiliania huxleyi as a function of growth rate and CO2 concentration

The calcite plates, or coccoliths, of haptophyte algae including Emiliania huxleyi are formed in intracellular vesicles in association with water-soluble acidic polysaccharides. These coccolith-associated polysaccharides (CAPs) are involved in regulating coccolith formation and have been recovered from sediment samples dating back to 180 Ma. Paired measurements of the carbon isotopic compositions of CAPs and coccolith calcite have been proposed as a novel paleo-pCO(2) barometer, but additional proxy validation and development are still required. Here we present culture results quantifying carbon isotopic offsets between CAPs and other cellular components: bulk organic biomass, alkenones, and calcite. E. huxleyi was grown in nitrate-limited chemostat experiments at growth rates (mu) of 0.20-0.62/d and carbon dioxide concentrations of 10.7-17.6 mu mol/kg. We find that CAPs are isotopically enriched by 4.5-10.1%, relative to bulk organic carbon, exhibiting smaller isotopic offsets at faster growth rates and lower CO2 concentrations. This variability suggests that CAPs record a complementary signature of past growth conditions with different sensitivity than alkenones or coccolith calcite. By measuring the isotopic compositions of all three molecules and minerals of self-consistent origin, the ratio mu[CO2(aq)] may be reconstructed with fewer assumptions than current approaches. (C) 2018 Elsevier Ltd. All rights reserved.