Research ArticleGEOCHEMISTRY

Carotenoids are the likely precursor of a significant fraction of marine dissolved organic matter

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Science Advances  27 Sep 2017:
Vol. 3, no. 9, e1602976
DOI: 10.1126/sciadv.1602976
  • Fig. 1 GC×GC selected-ion chromatograms (95 m/z) of chemically reduced PPL-DOM and CDPs.

    (A and B) Ion abundances are displayed as green (low)→red (high). Individual compounds appear as sharp ellipses. Exact chemical structures are linked to two distinct peaks (black structures), and the mass spectrum is provided with 95 m/z, a prominent hydrocarbon fragmentation ion (C7H11+), identified in red. Chemical formulas shown in the figure were validated with hydrocarbon standards. Gray structures were inferred on the basis of their similarity to confirmed products. The inset in (B) shows a generic carbon backbone of carotenes/carotenoids and identifies positions where carbon bond cleavage can form alicyclic compounds of sizes C10 to C15 denoted in the chromatogram.

  • Fig. 2 Identification of CDP through 1H-13C NMR correlations.

    Simulated one-bond (A) and multiple-bond (D) modeled 1H-13C correlations for an intact carotene. Colors highlighted on the structure correspond to the same colored regions in each spectrum. Linewidth in the simulations (A and D) matches that of the main spectral features seen in DOM. This is to demonstrate the spectral area that may be occupied in complex mixtures of similar overlapping chemical shifts. Exact chemical shifts from the simulated structure were superimposed upon the simulated 2D NMR spectrum and appear as darker crosses (spikes in the 1D 1H projection). One-bond (B and C) and multiple-bond (E and F) 1H-13C correlations for BC degradation products (that is, CDP) and terpenoid-rich PPL-DOM, respectively, with overlaid regions corresponding to key one-bond correlations observed in the model compound. Blue regions in HMBC spectra correspond to long-range correlations. The black dashed rectangle (E and F) identifies the region where correlations that would shift methylene protons further downfield are expected but do not appear. This is discussed in greater detail in the main text.

  • Fig. 3 Simulation of oxidized carotenoids through 1H-13C NMR correlations.

    (A) Simulated long-range 1H-13C correlations for BC. See Fig. 2 for the description of regions. (B) Simulated long-range 1H-13C correlations for oxidized carotene. (C) Observed long-range 1H-13C correlations for CDP with those correlations simulated in (A) and (B) superimposed. Linewidth in the simulations (A and B) matched that of the main spectral features seen in CDP. This was used to demonstrate the spectral area that may be occupied in complex mixtures of similar overlapping chemical shifts. The exact chemical shifts from the simulated structure were superimposed upon the simulated NMR and appear as darker crosses.

  • Fig. 4 Proposed transformation of carotenoids to CDP through oxidation.

    Biotic (bacterial oxidation/reduction) and abiotic (for example, photochemistry) processes are depicted as leading mechanisms of post-biosynthesis modification, resulting in smaller (carbon chain) compounds, such as those identified in Fig. 1. Oxygen-containing functional groups must be abundant in native CDP. Although OH and COOH are expected to be the most prominent groups, their positions along the carbon backbone are unknown. Not shown are heterocyclic compounds that may also be generated, but ring opening during reduction would produce compounds similar to those shown here. Laboratory chemical reduction of PPL-DOM produced alicyclic compounds that resembled those produced in the reduction of experimental CDP. These reduction products were used to link to the proposed biosynthetic precursor.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/9/e1602976/DC1

    Supplementary Text

    fig. S1. 1H NMR spectra of five PPL-DOM time series samples collected at the SIO Pier.

    fig. S2. Detection of compounds A and B in DOM (DOMA and DOMB) with structures identical to ReducedA and ReducedB.

    fig. S3. 1H NMR spectra of PPL-extracted, water-soluble CDP produced from the photooxidation of BC in filtered seawater (blue) or Milli-Q (red).

    fig. S4. Known BC degradation products identified in CDP via GC-MS.

    fig. S5. Similar reduction products are observed in environmental PPL-DOM and PPL-extracted CDP.

    fig. S6. Comparison of bulk PPL-DOM to terpenoid-rich PPL-DOM by 2D NMR.

    fig. S7. 1H NMR spectra of PPL-DOM (red), terpenoid-rich PPL-DOM (green; retained on the second PPL column following acid hydrolysis; see text), and the sugar- and amino acid–rich fraction (blue; not retained by the second PPL column following acid hydrolysis).

    fig. S8. Simulated HMBC NMR correlations for oxidized carotenoids.

    table S1. Isotope and elemental data for PPL-DOM samples collected from the SIO Pier (32.87°N, 117.26°W).

    scheme S1. A single-step reduction of diverse oxygen-containing functional groups to their corresponding alkane.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • fig. S1. 1H NMR spectra of five PPL-DOM time series samples collected at the SIO Pier.
    • fig. S2. Detection of compounds A and B in DOM (DOMA and DOMB) with structures identical to ReducedA and ReducedB.
    • fig. S3. 1H NMR spectra of PPL-extracted, water-soluble CDP produced from the photooxidation of BC in filtered seawater (blue) or Milli-Q (red).
    • fig. S4. Known BC degradation products identified in CDP via GC-MS.
    • fig. S5. Similar reduction products are observed in environmental PPL-DOM and PPL-extracted CDP.
    • fig. S6. Comparison of bulk PPL-DOM to terpenoid-rich PPL-DOM by 2D NMR.
    • fig. S7. 1H NMR spectra of PPL-DOM (red), terpenoid-rich PPL-DOM (green; retained on the second PPL column following acid hydrolysis; see text), and the sugar- and amino acid–rich fraction (blue; not retained by the second PPL column following acid hydrolysis).
    • fig. S8. Simulated HMBC NMR correlations for oxidized carotenoids.
    • table S1. Isotope and elemental data for PPL-DOM samples collected from the SIO Pier (32.87°N, 117.26°W).
    • scheme S1. A single-step reduction of diverse oxygen-containing functional groups to their corresponding alkane.

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