Dissolved organic matter plays a critical role in aquatic biogeochemical cycles, fuels microbial metabolism, and stores as much carbon as the atmospheric carbon reservoir. Because the reactivity and fate of dissolved organic matter is largely controlled by its chemical composition, a wide range of analytical approaches including ultra-high resolution mass spectrometry, nuclear magnetic resonance spectroscopy, natural abundance isotopic analyses, and fluorescence spectroscopy, have been employed to trace its sources and transformations along the land–ocean continuum. Applying these approaches to environmental samples requires overcoming significant logistical challenges, including high salinity, sample volatility and degradation, and low organic carbon concentrations. In addition, each method captures only a subset of the total dissolved organic matter pool. As a result of these challenges and others, our understanding of the environmental role of dissolved organic matter within aquatic biogeochemistry remains limited.
In this session, we welcome studies that integrate various analytical approaches to characterize the composition and/or fate of dissolved organic matter along the land–ocean continuum. We also welcome contributions focused on novel techniques for isolating, concentrating, and purifying organic matter prior to analysis. We hope that by highlighting analytical advances made throughout the aquatic continuum across salinity and organic carbon concentration gradients, the insights gained may inform future efforts as a community to characterize dissolved organic matter in a changing environment. Submissions by students and early career researchers, and researchers from BIPOC, LGBTQIA+, and other underrepresented identities are highly encouraged.
Lead Organizer: Jennifer Bowen, Stanford University (bowen.jenniferc@gmail.com)
Co-organizers:
Benjamin Granzow, Scripps Institution of Oceanography (bgranzow@ucsd.edu)
Margot White, ETH Zurich (margot.white@eaps.ethz.ch)
Presentations
02:30 PM
RELATIONSHIPS BETWEEN THE ORGANIC COMPOSITION OF PARTICULATE AND DISSOLVED ORGANIC MATTER IN AN OLIGOTROPHIC WATER COLUMN (8796)
Tutorial/Invited: Invited
Primary Presenter: Hilary Close, University of Miami (hclose@miami.edu)
The production and consumption of particulate organic matter (POM) in the epipelagic ocean control the quantity of organic matter available for export and sequestration in the deep ocean via the passive flux of particles. Historically, models have considered respiration of POM as the main process attenuating sinking flux, and recent models also better account for disaggregation processes. However, there has been less focus on POM flux attenuation via solubilization of POM to dissolved organic matter (DOM). Largely through the hydrolytic enzymatic activity of bacteria, solubilization decreases POM inventories while potentially both contributing to a long-lived pool of DOM and providing substrates for short-term microbial uptake. Recently, compound-specific isotopic analysis of amino acids (CSIA-AA) has been used to identify POM size fractions and water column depths where solubilization of POM leaves isotopic imprints on the remaining particles. Examining multiple seasons and diel patterns at the Bermuda Atlantic Time-series Study site, we compare size-fractionated POM and DOM composition (individual amino acids and carbohydrates), along with CSIA-AA results from particles that identify solubilization mechanisms linking the particulate and dissolved amino acid pools. We also evaluate the community composition of particle-attached bacteria likely involved in solubilization processes. We discuss how the observed composition of DOM and POM results from the dynamics of cell exudation, lysis, and particle solubilization and suggest new directions for resolving these dynamics.
02:45 PM
CONVERGENCE AND DIVERGENCE IN THE NORTH PACIFIC METABOLOME (9314)
Primary Presenter: Joshua Sacks, University Of Washington (jssacks@uw.edu)
Metabolites are small, organic, biomolecules that are the product of microbial metabolism and interactions and are major substrates in global elemental cycles of carbon and other elements. At the ecosystem level, microbial metabolite composition is controlled by a combination of taxonomy and physiology but the direct links between microbial groups and metabolites remains poorly understood. Here we focus on mapping the distribution of osmolytes, metabolites used by organisms to acclimate to their environment that dominate quantifiable marine metabolomes. We measure these compounds in both dissolved and particulate phases across multiple cruises in the North Pacific and Puget Sound, spanning large gradients in nutrients and productivity. We find osmolyte concentrations in dissolved and particulate phases are correlated across our dataset with most of the total metabolite pool existing outside of cells. We combine our metabolite datasets with co-located datasets of microbial community composition and nutrients and ask how these factors relate to marine metabolite distributions. Some compounds display sharp latitudinal gradients in relative abundance while others, such as betaines, are remarkably consistent despite large changes in biomass, community composition, and nutrients. Time of day also emerged as a major driver of metabolome composition with diel processes driving the abundance of compounds such as sucrose at basin scales. These results reveal the combined impact of community composition and physiology on marine metabolomes.
03:00 PM
Using thermal slicing ramped pyrolysis gas chromatography mass spectrometry to investigate composition and structure of natural organic matter (8965)
Primary Presenter: Kaijun Lu, Coastal Carolina University (klu@coastal.edu)
Natural organic matter (NOM) in aquatic systems represents one of Earth’s largest reservoirs of reduced carbon, yet its molecular structure remains poorly understood. We introduced a novel technique, thermal slicing ramped pyrolysis gas chromatography mass spectrometry (TSRP-GC-MS), coupled with computing, to analyze the thermal lability and molecular structure of NOM. This method identifies and categorizes pyrolyzates produced at low temperatures (<370°C) based on their functional groups. The release patterns of pyrolyzates are integrated and transformed into activation energy based on distributed activation energy model (DAEM) to offer insights into how organic compounds are assembled within the structure. For instance, we quantified the bonding energy of n-alkanes in asphaltenes from light sweet crude oil, showing an increase from ~108 kJ/mol in native asphaltenes to ~140 kJ/mol in photo-irradiated samples, indicating tighter binding in the latter, which may partially explain their resistance to biodegradation. This approach can be extended to create energy matrices for different chemical classes in any NOM samples. For marine sinking particles, the release energy ranges from ca. 120 kJ/mol for acetic-, aldehyde-, and sterol-like structures, to ca. 160 kJ/mol or higher for alkane- and alkene-like classes. These energies shift as NOM lability changes. Overall, TSRP-GC-MS offers a powerful tool to quantify the binding strength of different chemical classes in NOM, providing novel insights into its structure and stability.
03:15 PM
Chemical composition dictates the size and age of high molecular weight dissolved organic matter (8886)
Primary Presenter: Benjamin Granzow, Scripps Institution of Oceanography (DOCGranzow@gmail.com)
High molecular weight dissolved organic matter (HMWDOM) accounts for a quarter to a third of marine DOM and more than 15% of total marine primary productivity. Yet, many of the characteristics of HMWDOM remain poorly constrained. HMWDOM consists of two major classes of compounds: acylpolysaccharides (APS) and high molecular weight humic substances (HS). The relationship of these compounds, and the influence they exert on the attributes of HMWDOM as a whole, are not well understood. Using diffusion-ordered NMR spectroscopy coupled with mixed-mode chromatography we separated these components and examined their molecular weight (MW) distribution and carbon isotopic properties. The MWs of APS and HS fell within distinct, narrow envelopes between 0.9 and 16 kDa and, for HMWDOM samples collected in the North Pacific Subtropical Gyre (NPSG), the MW of both components decreased with depth. Trends in the MW distribution, coupled with the isotopic signature of the APS endmember suggest that APS has a short lifetime in the ocean and that APS <2 kDa is efficiently removed from the water column. In contrast, the MW distribution of HS narrows significantly with depth due to the removal of HMW components. While the average radiocarbon of HMWDOM in NPSG is ‑30‰ and -238‰, at 15 m and 915 m respectively, our work shows that APS has a modern radiocarbon value, and that the Δ14C of HMWDOM is controlled primarily by the proportion of HS in the sample. From these results, we propose potential pathways for the production and removal of HS and APS.
03:30 PM
Photochemical fractionation of fluvial stable carbon isotopes: A meta-analysis (9702)
Primary Presenter: Alex Goranov, Old Dominion University (aleksandar.i.goranov@gmail.com)
Fluvial systems annually export large amounts of C to the oceans. The stable C isotope ratio (δ13C) of fluvial dissolved organic matter (typical δ13C values around -28 ‰) is vastly different from that of oceanic dissolved organic matter (around -22 ‰). The broadly accepted view for the apparent disappearance of the -28 ‰ signature is that terrestrial organic matter is oxidized by the time it reaches the ocean, with quantitative flux of this C into the atmosphere. Instead, it is replaced by material from algae fixing atmospheric CO2 and producing oceanic C having an algal δ13C value. However, an alternative proposed explanation for the vastly different δ13C values between rivers and ocean is that photochemical irradiation leads to isotopic fractionation. This could conceal much more significant terrestrial C fluxes to the ocean, with photochemically bleached organic C becoming marine-like in its δ13C value. While numerous studies have tested this alternative proposition, the evidence has been inconclusive. Here, we perform a meta-analysis of all studies that are known to us on this subject. We observe a strong relationship between δ13C fractionation (i.e., increase in δ13C) and C loss (r = 0.87), with cumulative δ13C fractionation reaching ~6 ‰ when 80+% of C is photo-bleached. These results indicate that terrestrial fluxes of C to the oceans are likely underestimated, and thus the fluvial and oceanic cycles of organic C are coupled. This requires revisiting the global biogeochemical cycles and quantitative estimates of terrestrial-to-marine transfers of organic matter.
03:45 PM
EVIDENCE FOR THE INHERENT STOCHASTIC NATURE OF FUNCTIONAL GROUP DISTRIBUTION IN DISSOLVED ORGANIC MATTER ACROSS VARYING WATER MASSES (8950)
Primary Presenter: Nico Mitschke, Carl von Ossietzky Universität Oldenburg (nico.mitschke@uni-oldenburg.de)
Dissolved organic matter (DOM) is one of the most complex mixtures known, with only 5% of marine DOM characterized at the molecular level. Ultrahigh-resolution mass spectrometry (UHR-MS) is the primary method for analyzing DOM, but it typically provides limited structural insights. We converted carboxyl groups of DOM recovered from three different water masses (fluvial as well as surface, and deep Pacific Ocean) to their corresponding methyl esters using isotopically labeled diazomethane and characterized the reaction products by UHR-MS and nuclear magnetic resonance spectroscopy. The conversion of reacted compounds closely followed a log-normal distribution. In accordance with the central limit theorem, this indicates a stochastic reactivity of carboxyl groups across all DOM compounds. Additionally, carboxyl groups were stochastically distributed across the molecular space, with their abundances only dependent on the elemental composition of the respective molecular formula and water type. Computational isomer enumerations supported the stochastic distribution. We established a model based on the stochastic nature of carboxyl groups in DOM to assess their abundance in more than 1,000 samples across major oceanic water masses on the globe. By combining this approach with further molecular analyses, we assessed structural differences between DOM from land, the surface and deep ocean. Photochemical processes and the presence of vascular plant-derived phenols are likely the key drivers behind these differences.
SS41A - Advancing the chemical and isotopic characterization of dissolved organic matter across the land–ocean aquatic continuum
Description
Time: 2:30 PM
Date: 31/3/2025
Room: W201CD