Coccolithophores lipid and carbon isotope composition and their variability related to changes in seawater carbonate chemistry

Growth rate, C:N ratio, phospholipid-derived fatty acids (PLFA) distribution and organic carbon isotope composition (∂13C) of both life stages of the Prymnesiophyceae Emiliania huxleyi (Lohmann), Calcidiscus leptoporus (Murray and Blackman) and Syracosphaera pulchra (Lohman) were analyzed. Cultures were grown at two different CO2 partial pressures (pCO2, 400 and 760 μatm) under nutrient-replete conditions. Growth rates (μ) increased at 760 μatm in all species. C:N ratios were unaffected by pCO2 in C. leptoporus and in the diploid stage of E. huxleyi; they significantly decreased in the haploid stage of E. huxleyi and in both life stages of S. pulchra. At 400 μatm, high similarity was found between the lipid composition of C. leptoporus and S. pulchra differing from that of E. huxleyi. The haploid and diploid life stages showed significant differences in mono- and polyunsaturated C18 fatty acids (MUFA and PUFA), which were more abundant in either stage depending on the species. Except for palmitoleic, vaccenic acids and docosahexaeonic acid (DHA), C. leptoporus and S. pulchra fatty acids were lower compared to those of E. huxleyi. Differences in ∂13C between particulate organic carbon (POC) and PLFA (∆∂13Clipid-POC) showed common trends at both pCO2: palmitoleic, oleic acids and DHA were systematically depleted compared to total cell biomass, C18 PUFA were enriched while saturated fatty acids (SAFA) were enriched in C. leptoporus and S. pulchra and depleted in E. huxleyi. Elevated pCO2 influenced PLFA abundance, ∂13C of lipids and POC in a highly species-specific way. Fatty acids were generally more depleted at 760 μatm than at 400 μatm but the effect was variable among PLFA classes. The relationship between coccolithophore isotopic and PLFA composition and CO2 concentration are analyzed and consequent variations in the future web-chain related to changes in carbonate chemistry are discussed.