Abstract
Authentic biomarker standards were obtained from algal cultures in an attempt to accurately determine blank C added during sample processing for compound-specific radiocarbon analysis. Emiliania huxleyi and Thalassiosira pseudonana were grown under manipulated Δ14C dissolved inorganic carbon (DIC) levels and chlorophyll a and either alkenones (E. huxleyi) or low molecular weight (LMW) alkanoic acids (T. pseudonana) were isolated from the respective biomass using preparative liquid chromatography (LC), wet chemical techniques or preparative gas chromatography, respectively. DI14C in the seawater medium was determined pre- and post-growth. Biomarker Δ14C values mostly agree within 1-2 analytical uncertainties. In those cases where biomarker Δ14C values differ significantly, chlorophyll a is up to 104‰ more 14C-depleted than alkenones or LMW alkanoic acids, consistent with a larger LC blank compared to the other purification methods. However, in the majority of experimental setups pre- and post-growth DIC Δ14C values seem to be compromised by an unknown and variable blank C contribution. DIC Δ14C values deviate strongly from the anticipated Δ14C values (by up to ca. 560‰), pre- and post-growth Δ14C values differ significantly (by up to ca. 460‰), and changes are not unidirectional. Accordingly, since the substrate Δ14C value cannot unequivocally be constrained, blank C contributions for the different biomarker purification methods cannot be accurately calculated. This study illustrates the challenges and problems of producing authentic standards that are not readily commercially available and exemplifies how a laborious and time-consuming culturing approach may enter a vicious cycle of blank C contamination hampering accurate blank C determination.
Highlights
Compound-specific radiocarbon analysis (CSRA) has revolutionized our understanding of carbon cycling in the ocean including both sedimentary and metabolic processes in the water column and sediments (e.g., Pearson et al, 2005; Ingalls et al, 2006; Mollenhauer and Eglinton, 2007; Mollenhauer et al, 2007), as well as land to ocean carbon transfer (e.g., Drenzek et al, 2009; 14C Blanks in Algal CulturesKusch et al, 2010b; Feng et al, 2013)
We show that blank C of unknown origin contributed significantly to the dissolved inorganic carbon (DIC)
DIC utilization by algae ranged from ∼0.8–1.2 mmol/kg in the E. huxleyi cultures and ∼0.4–0.8 mmol/kg in the T. pseudonana cultures (Supplementary Table S2)
Summary
Compound-specific radiocarbon analysis (CSRA) has revolutionized our understanding of carbon cycling in the ocean including both sedimentary and metabolic processes in the water column and sediments (e.g., Pearson et al, 2005; Ingalls et al, 2006; Mollenhauer and Eglinton, 2007; Mollenhauer et al, 2007), as well as land to ocean carbon transfer For a range of compounds or even compound classes typically found in marine particulate organic matter, authentic standards are not readily available Either, they are not produced commercially or are manufactured solely from either plant biomass or petroleum sources limiting our ability to characterize either the fossil or modern blank sources, respectively. Authentic standards can be obtained when culturing marine algae in the laboratory under controlled dissolved inorganic carbon (DIC) 14C levels (Mollenhauer et al, 2005) While this approach is laborious and time-consuming, it allows for the determination of blank C contribution associated with the isolation of target compounds from different compound classes and with various 14C endmembers. 14C isotopic composition during the experiments, impairing a valuable assessment of blank C associated with the purification of chlorophyll a as well as alkenones (E. huxleyi) and low molecular weight (LMW) alkanoic acids (T. pseudonana)
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