Coccolithophore particulate inorganic and organic carbon ratios: An evaluation of acid decalcification methods

Abstract Accurate direct measurements of coccolithophore particulate inorganic and organic carbon ratios (PIC/POC) are crucial for understanding the physiology and role these microbes play in global ocean carbon flux. Direct measurements of POC are commonly obtained by acid fuming or applying acid directly to filters to dissolve PIC (CaCO 3 ) prior to CN analysis. Review of the literature revealed considerable variation in methods for this decalcification step demonstrating a clear need for a validated standard operating procedure. Importantly, visual verification of complete decalcification after acid treatment was absent. We therefore systematically tested the efficacy of two common acid decalcification methods for several species of coccolithophores. We examined acid fuming wet or dry filters, time spent fuming, and whether there are significant differences in PIC/POC determined using decalcification by acid fuming or direct addition. Scanning electron microscopy was used to visually verify acid fuming completely decalcified filters and there was no statistically significant difference in PIC/POC obtained by fuming for 5 min to 24 h. Direct addition of 0.5 M HCl to wet filters also yielded complete decalcification across all species and cell densities tested with less variation in subsequent CN‐analysis. There were no statistically significant differences in PIC/POC obtained from samples treated either by direct acid addition, 30 min acid fuming, or 24 h acid fuming. Moreover, by utilizing the direct acid addition method we were able to reliably resolve PIC/POC for haploid coccolithophores. This study provides important recommendations for obtaining accurate and precise PIC/POC utilizing either acid fuming or direct addition protocols.