Molecular Nature of Marine Particulate Organic Iron-Carrying Moieties Revealed by Electrospray Ionization Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (ESI-FTICRMS)

Abstract

Marine sinking particulate organic matter (POM), acting as a link between surface primary production and burial of organic matter in marine sediments, undergoes a variety of physical and biochemical alterations on its way to the deep ocean, resulting in an increase in its un-characterizable proportion with diagenesis. Further, the binding ligands in POM for iron, an essential nutrient to marine life and tightly coupled with organic matter, has rarely been studied. In the current study, we employed an approach combining sequential extraction with ultrahigh resolution mass spectrometry (ESI-FTICRMS), in order to explore and unravel the chemical characteristics of organic matter compounds relevant to marine particle flux within the mesopelagic and deep ocean, with a focus on the potential iron-carrying molecules. With increasing depth, POM increases in aliphaticity, and decreases in intensity-normalized O/C ratios, aromatics, and carboxylic-rich alicyclic molecules (CRAM)-like compounds. The potential iron-carrying molecules account for ∼14% of total identified molecules, and appear to have been incorporated into the marine particles via ion complexation, hydrophobic interaction, and/or interlayered "occlusion." The relative abundance of iron-binding organic molecules in these three operationally-defined categories changes with depth: "surficially-complexed" fraction decreases with depth, the "interlayeredoccluded" fraction increases to a comparable extent and "hydrophobic interaction" fraction occurs at all depths. Collectively, the potential iron-carrying organic molecules exhibit a set of unique molecular characteristics: a relatively lower average H/C ratio and a higher O/C ratio compared to bulk POM, a dominance of aromatics, black carbon-like compounds and CRAM-like compounds, and minor amounts of aliphatics. These molecules exhibit partial similar molecular features as precursors formed from photochemical reactions in the surface ocean, but they have been greatly modified by flux processes. Noticeably, a minor fraction of these iron-carrying molecules (<1%)

Incorporation of Hydroxamate Siderophore and Associated Fe Into Marine Particles in Natural Seawater

“…On the other hand, since the siderophore compound has a strong association with Fe, with the Fe-siderophore complex remaining intact without microbial processes, this may imply that the siderophore compounds can serve as one of the carriers for the Fe being removed into the deeper ocean, corresponding with the field observation in the deep North Atlantic Ocean (e.g., 3200 m) showing considerable fractions of the Fe-carrying organic molecules that were identified to contain hydroxamate siderophore functionalities (Xu et al, 2020).…”

“…Our modified Csaky method has a detection limit of 1.5 µg-AHA equivalent/g-particles, with a precision better than 10% (Xu et al, 2015). It should be mentioned that we noticed a "browning" effect of this method when analyzing natural particle samples, likely caused by interactions with H 2 SO 4 (Xu et al, 2020). This "browning" effect can then be corrected by subtracting the absorbance of a control sample aliquot without the addition of the coloring reagent.…”

“…A microbially derived protein encapsulating mechanism can also serve as an alternative pathway (Xu et al, 2020) for the incorporation of Fe-siderophore complex into suspended particles/aggregates in the present study. For example, ferritin proteins, which are produced by marine microbes for Fe storage (Marchetti et al, 2009;Shire and Kustka, 2015), have been demonstrated to encapsulate siderophores into cavity-like structures (Dominguez-Vera, 2004), probably facilitating the incorporation of Fe-siderophore complex into the microbiallyderived marine particles in our experiments.…”

“…The non-radiolabeled culture was also filtered and ultrafiltered for the measurement of size-fractionated DFO concentrations, using a modified "Csaky" method (Gillam et al, 1981;Xu et al, 2015Xu et al, , 2020. For the measurement of DFO in particulate phases, the suspended particles/aggregates were pretreated overnight with 5 mL of 10% HF on an orbital shaker at room temperature.…”

“…Most importantly, siderophore compounds are commonly detected only at low-molecular-weight (LMW, typically <1 kDa or 3 kDa) range in the ocean (Macrellis et al, 2001;Mawji et al, 2011), and the presence of siderophores in high-molecular-weight (HMW) range [i.e., marine particles (>0.4 µm)/colloids (1 kDa/3 kDa-0.4 µm)] are usually not considered or not measured in field work, due to the difficulty and high requirement in experimental condition to directly measure the particulate siderophore. Nevertheless, few studies (Chuang et al, 2013;Velasquez et al, 2016;Xu et al, 2020) have detected the presence of siderophore compounds in concentrated marine particles (e.g., sinking particles in sediment trap and marine colloids after natural seawater being concentrated by >100 times with ultrafiltration technique), implying the potential incorporation of siderophore into marine particles in the water column. However, direct evidence for this incorporation is still insufficient, while it can provide new insights into the transport and cycling of Fe in the ocean.…”