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-pCO2 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 (µ) of 0.20-0.62/d and carbon dioxide concentrations of 10.7-17.6 µmol/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 µ/[CO2(aq)] may be reconstructed with fewer assumptions than current approaches.