Abstract
Through a combination of selective extractions and molecular characterization techniques including Isoelectric Focusing Chromatography and Electrospray Ionization Fourier-Transform Ion Cyclotron Resonance Mass spectrometry, molecular structures of diatom (Phaeodactylum tricornutum) and coccolithophore (Emiliania huxleyi)-associated biopolymers that are responsible for the distinct partitioning behavior between 210Pb and 210Po were determined. Our results show that diatom-derived biopolymers have distinctive elemental grouping distributions as compared to those excreted by the coccolithophore, with the former consisting of more heterogeneous elements (i.e., nitrogen, sulfur and phosphorus-containing organic compounds). For the coccolithophore culture, two 210Pb-enriched biopolymers (non-attached exopolymeric substances and coccosphere shell-associated biopolymers) have a higher abundance of CHO-type compounds, suggesting CHO-only-type compounds as the main binding moieties for 210Pb. In contrast, such association was not evident in the diatom culture. Different with 210Pb, 210Po enrichment in coccolithophore-derived attached exopolymeric substances and Fe-Mn-associated metabolites coincided with the higher abundance of nitrogen/sulfur-containing organic compounds in these two biopolymer fractions, suggesting the strong parallel of Po with the production of nitrogen-rich organic matter as well as sulfur-containing amino acids. These different associations between 210Pb/210Po and organic functional groups were further explored by separating 210Pb or 210Po-labeled coccolithophore-derived biopolymers via isoelectric focusing. This technique suggests that phosphate group-containing molecules but not the other molecules that contain heterogeneous elements (e.g., CHONS, CHON, and CHOS) as the strongest binding agents for 210Pb, while the more hydrophobic (high protein to carbohydrate ratio) nitrogen/sulfur-enriched organic moieties acted as the main 210Po-binding ligands. It is concluded that the deficiency of 210Po with respect to 210Pb can be influenced by the relative abundance of nitrogen/sulfur-enriched organic moieties to the nitrogen/sulfur-depleted organic compounds in the water column. This behavior constrains the application of 210Po-210Pb approach to quantify the particulate organic carbon (POC) export flux in the ocean. It also explains that differences in chemical binding of the 210Po as compared to those of other radionuclides (e.g., thorium-234) as the main factor. That suggests that differences in decay half-lives or physical factors are less important when these nuclides are applied to estimate the POC flux in the ocean.
Highlights
Occurring radionuclides with strong particle-reactivity, such as lead (Pb), polonium (Po), and thorium (Th), are widely used in estimating various biogeochemical and physical processes associated with marine particles in the ocean, mostly based on their strong particle reactivity and making use of disequilibria between parent and daughter radionuclides (e.g., Santschi et al, 1980; Anderson et al, 1983; Kumar et al, 1995; Murray et al, 2005; Stewart et al, 2010; Yang et al, 2011)
Diatom and coccolithophores serve as the most important contributors to global oceanic primary production and to carbon flux (Nelson et al, 1995; Armbrust, 2009), whereby in-situ production and excretion of biogenic materials account for the majority of natural organic matter in the water column
To fill this knowledge gap, the diatom P. tricornutum and coccolithophore Emiliania huxleyi were incubated for later separation of different biopolymer fractions, including extracellular fractions, frustule/coccosphere-associated biopolymers and intracellular biopolymers from cell lysis (Chuang et al, 2015b; Lin et al, 2017)
Summary
Occurring radionuclides with strong particle-reactivity, such as lead (Pb), polonium (Po), and thorium (Th), are widely used in estimating various biogeochemical and physical processes associated with marine particles in the ocean, mostly based on their strong particle reactivity and making use of disequilibria between parent and daughter radionuclides (e.g., Santschi et al, 1980; Anderson et al, 1983; Kumar et al, 1995; Murray et al, 2005; Stewart et al, 2010; Yang et al, 2011) Most of these tracer applications are generally resting on inferences from correlations of isotopic ratios with bulk particle properties, experimental evidence reveals organic compound-specific scavenging and partitioning for different radionuclides, either in particulate or in colloidal phases (Hayes et al, 2015a,b; Lam et al, 2015; Lin et al, 2015). Considering the significant contribution of diatoms and coccolithophores to the carbon flux in the ocean, a better understanding on the association between radionuclides and organic components of diatom and coccolithophore-associated biopolymers can provide molecular level insights into the processes that control the oceanographic applications of natural radionuclides, such as the 210Po-210Pb pair, in predicting the organic carbon flux
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