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| Titel |
High-field NMR spectroscopy and FTICR mass spectrometry: powerful discovery tools for the molecular level characterization of marine dissolved organic matter |
| VerfasserIn |
N. Hertkorn, M. Harir, B. P. Koch, B. Michalke, P. Schmitt-Kopplin |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 10, no. 3 ; Nr. 10, no. 3 (2013-03-08), S.1583-1624 |
| Datensatznummer |
250018143
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| Publikation (Nr.) |
 copernicus.org/bg-10-1583-2013.pdf |
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| Zusammenfassung |
High-performance, non-target, high-resolution organic structural spectroscopy
was applied to solid phase extracted marine dissolved organic matter
(SPE-DOM) isolated from four different depths in the open South Atlantic
Ocean off the Angola coast (3° E, 18° S; Angola Basin) and
provided molecular level information with extraordinary coverage and
resolution. Sampling was performed at depths of 5 m (Angola Current;
near-surface photic zone), 48 m (Angola Current; fluorescence maximum),
200 m (still above Antarctic Intermediate Water, AAIW; upper mesopelagic
zone) and 5446 m (North Atlantic Deep Water, NADW; abyssopelagic,
~30 m above seafloor) and produced SPE-DOM with near 40% carbon
yield and beneficial nuclear magnetic resonance (NMR) relaxation properties,
a crucial prerequisite for the acquisition of NMR spectra with excellent
resolution. 1H and 13C NMR spectra of all four marine SPE-DOM
showed smooth bulk envelopes, reflecting intrinsic averaging from massive
signal overlap, with a few percent of visibly resolved signatures and
variable abundances for all major chemical environments. The abundance of
singly oxygenated aliphatics and acetate derivatives in 1H NMR spectra
declined from surface to deep marine SPE-DOM, whereas C-based aliphatics and
carboxyl-rich alicyclic molecules (CRAM) increased in abundance. Surface
SPE-DOM contained fewer methyl esters than all other samples, likely a
consequence of direct exposure to sunlight. Integration of 13C NMR
spectra revealed continual increase of carboxylic acids and ketones from
surface to depth, reflecting a progressive oxygenation, with concomitant
decline of carbohydrate-related substructures. Aliphatic branching increased
with depth, whereas the fraction of oxygenated aliphatics declined for
methine, methylene and methyl carbon. Lipids in the oldest SPE-DOM at 5446 m
showed a larger share of ethyl groups and methylene carbon than observed in
the other samples.
Two-dimensional NMR spectra showed exceptional resolution and depicted
resolved molecular signatures in excess of a certain minimum abundance.
Classical methyl groups terminating aliphatic chains represented
~15% of total methyl in all samples investigated. A
noticeable fraction of methyl (~2%) was bound to olefinic
carbon. Methyl ethers were abundant in surface marine SPE-DOM, and the
chemical diversity of carbohydrates was larger than that of freshwater and
soil DOM.
In all samples, we identified sp2-hybridized carbon chemical
environments with discrimination of isolated and conjugated olefins and
α,β-unsaturated double bonds. Olefinic proton and carbon atoms
were more abundant than aromatic ones; olefinic unsaturation in marine
SPE-DOM will be more directly traceable to ultimate biogenic precursors than
aromatic unsaturation. The abundance of furan, pyrrol and thiophene
derivatives was marginal, whereas benzene derivatives, phenols and
six-membered nitrogen heterocycles were prominent; a yet unassigned set of
six-membered N-heterocycles with likely more than one single nitrogen
occurred in all samples. Various key polycyclic aromatic hydrocarbon
substructures suggested the presence of thermogenic organic matter at all
water depths.
Progressive NMR cross-peak attenuation from surface to deep marine SPE-DOM
was particularly strong in COSY NMR spectra and indicated a continual
disappearance of biosignatures as well as entropy gain from an ever
increased molecular diversity. Nevertheless, a specific near-seafloor
SPE-DOM signature of unsaturated molecules recognized in both NMR and
Fourier transform ion cyclotron mass spectrometry (FTICR/MS) possibly
originated from sediment leaching. The conformity of key NMR and FTICR/MS
signatures suggested the presence of a large set of identical molecules
throughout the entire ocean column even though the investigated water masses
belonged to different oceanic regimes and currents.
FTICR/MS showed abundant CHO, CHNO, CHOS and CHNOS molecular series with
slightly increasing numbers of mass peaks and average mass from surface to
bottom SPE-DOM. The proportion of CHO and CHNO negative ions increased from
surface to depth, whereas CHOS and especially CHNOS molecular series markedly
declined. While certain rather aliphatic CHOS and CHNOS ions were observed
solely in the surface, deep marine SPE-DOM was enriched in unique unsaturated
and rather oxygenated CHO and CHNO molecular series. With the exception of
abyssopelagic SPE-DOM at 5446 m, which showed a peculiar CHOS chemistry of
unsaturated carbon and reduced sulphur (black sulphur), CHO and CHNO
molecular series contributed ~87% to total positive electrospray
ionization FTICR mass peak integral, with a near constant ratio of
CHNO / CHO molecular compositions near 1.13 ± 0.05. In case of all
four marine SPE-DOM, remarkably disparate average elemental compositions as
determined from either MS and NMR spectra were observed, caused by a
pronounced ionization selectivity in electrospray ionization FTICR/MS.
The study demonstrates that the exhaustive characterization of complex
unknowns in marine DOM will enable a meaningful classification of individual
marine biogeosignatures. Future in-depth functional biodiversity studies with
a clear understanding of DOM structure and function might eventually lead to
a novel, unified perception of biodiversity and biogeochemistry. |
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