The presence of liquid water makes our planet habitable. Water in soils, sediments, lakes, rivers and the ocean forms the largest habitat for life on Earth. During life and upon death, all organisms release dissolved organic matter (DOM) to their environment. These molecular traces of life travel with water through land- and seascapes. DOM in the ocean and freshwaters contains more carbon than all living biomass on Earth combined. An in-depth knowledge of the molecular composition of the DOM pool is crucial to understand its role in the global carbon cycle. DOM is one of the most diverse mixtures known. So far, only the structure of a few components has been elucidated, thus, its molecular composition remains largely unknown. NMR spectroscopy is a promising tool for the molecular-level characterization of complex mixtures such as DOM. Major drawbacks of this spectroscopic technique in the past were the lack of sensitivity and insufficient spectral resolution. Large amounts of DOM were required and overlapping signals of the manifold DOM constituents resulted in broad unresolved spectral features. Recent technical and methodical improvements, the application of multivariate statistical analyses and the development of new chemical derivatization strategies may overcome these limitations. Here, we review the application of NMR spectroscopy within the quickly emerging field of the structural characterization of marine DOM. In the first section, this review covers fundamental aspects of NMR spectroscopy and its application to the analysis of DOM. The major points in the following are (1) a comprehensive overview of the current state of NMR spectroscopy for the analysis of marine DOM, (2) a discussion of the most important technical and methodical improvements and (3) suggestions for future implementations of NMR for the characterization of DOM. This review provides an overview for experts but also serves as a starting point for beginners.

In our continued search for biologically active metabolites from cultures of rare Basidiomycota species, we found eight previously undescribed cyathane-xylosides from submerged cultures of Dentipellis fragilis, which were named dentifragilins A–H. In addition, the known cyathane derivatives striatal D and laxitextine A were isolated. All compounds were characterized by high-resolution electrospray ionization mass spectrometry (HR-ESIMS) as well as by 1D and 2D nuclear magnetic resonance (NMR) spectroscopy. Several of the compounds exhibited significant activities in standardized cell-based assays for the determination of antimicrobial and cytotoxic effects. The discovery of cyathanes in the genus Dentipellis has chemotaxonomic implications, as this class of diterpenoids has already been shown to be characteristic for mycelial cultures of the related genera Hericium and Laxitextum, which are classified as Dentipellis in the family Hericiaceae.

The carbonyl functionality of natural organic matter (NOM) is poorly constrained. Here, we treated Suwannee River NOM (SRNOM) with ammonium acetate and sodium cyanoborohydride to convert ketone-containing compounds by reductive amination to their corresponding primary amines. The total dissolved nitrogen content increased by up to 275% after amination. Up to 30% of the molecular formulas of SRNOM contained isomers with ketone functionalities as detected by ultrahigh-resolution mass spectrometry. Most of these isomers contained one or two keto groups. At least 3.5% of the oxygen in SRNOM was bound in ketone moieties. The conversion of reacted compounds increased linearly with O/H values of molecular formulas and was predictable from the elemental composition. The mean conversion rate of reacted compounds nearly followed a log-normal distribution. This distribution and the predictability of the proportion of ketone-containing isomers solely based on the molecular formula indicated a stochastic distribution of ketones across SRNOM compounds. We obtained isotopically labeled amines by using ¹⁵N-labeled ammonium acetate, facilitating the identification of reaction products and enabling NMR spectroscopic analysis. ¹H,¹⁵N HSQC NMR experiments of derivatized samples containing less than 20 μg of nitrogen confirmed the predominant formation of primary amines, as expected from the reaction pathway.

In this work, we introduce a novel method for compound identification in mixtures based on nuclear magnetic resonance spectra. Contrary to many other methods, our approach can be used without peak-picking the mixture spectrum and simultaneously optimizes the fit of all individual compound spectra in a given library. At the core of the method, a minimum cost flow problem is solved on a network consisting of nodes that represent spectral peaks of the library compounds and the mixture. We show that our approach can outperform other popular algorithms by applying it to a standard compound identification task for 2D ¹H,¹³C HSQC spectra of artificial mixtures and a natural sample using a library of 501 compounds. Moreover, our method retrieves individual compound concentrations with at least semiquantitative accuracy for artificial mixtures with up to 34 compounds. A software implementation of the minimum cost flow method is available on GitHub (https://github.com/GeoMetabolomics-ICBM/mcfNMR).

Dissolved organic matter (DOM) cycling across the land-ocean continuum is highly complex, and our limited understanding of DOM molecular transformations hinders a full assessment of land-ocean connectivity in the global carbon cycle. Here, we applied one- and two-dimensional high-field ¹H nuclear magnetic resonance (NMR) spectroscopy and ultrahigh-resolution mass spectrometry (FT-ICR-MS) to investigate sources and transformations of solid-phase extractable DOM along an Amazonian mangrove-fringed river-to-ocean transect. Relative abundances of aromatic compounds decreased from the river to the coastal ocean, whereas aliphatic compounds increased. NMR spectroscopic features, commonly associated with carbohydrates, are probably related to flavonoid- and lignin-derived structural motifs. These structural features were more readily detected by ¹H NMR spectroscopy, whereas aromatics were more effectively detected by FT-ICR-MS. We tentatively identified polycyclic aromatic sulfur-containing compounds as being predominantly derived from urban areas, whereas sulfurized aliphatic compounds originated from sulfidic mangrove sediments. Surprisingly, while the number of DOM molecular formulas decreased along the river-to-coastal continuum, coastal marine DOM exhibited greater structural diversity than terrigenous DOM. Here, we showed that the interplay of distinct molecular pathways, particularly (photo)oxidation processes and sulfur incorporation, structurally diversifies DOM in coastal marine environments.

Four previously undescribed drimane sesquiterpenoids were isolated from cultures of the wood-inhabiting basidiomycete Dentipellis fragilis along with two other drimanes that were previously only known as synthetic or biotransformation products.