Organic matter in aquatic environments includes dissolved and particulate compounds with varying lability, ranging from labile compounds that turn over rapidly, such as amino acids and carbohydrates, to refractory components that persist for thousands to millions of years. The lability of organic matter influences its fate, either incorporation into microbial biomass, respiration by heterotrophic microbes, or export to various regions. Organic matter lability is influenced by intrinsic molecular properties, as well as biotic factors, such as microbial abundance, enzymes, and community composition. In addition, abiotic factors, such as photochemical reactions, oxygen stress, sorption/desorption, and physical aggregation/disintegration, also influence the transformation and fate of organic matter. While recent advances in chemical analysis and microbial studies have significantly enhanced our understanding of these processes, several big questions remain regarding the fate of organic matter. When and where do biotic versus abiotic factors dominate organic matter transformations? Can we identify common principles that determine the fate of organic matter across all water bodies? In this session, we invite presentations on the lability and fate of organic matter in aquatic environments, including but not limited to chemical characterization, ecosystem-specific studies on organic matter source and fate, microbial production and uptake, and pathways of degradation and transformation. We welcome both field observation and lab experimental studies, aiming to synthesize research across disciplines and better understand the factors shaping organic matter lability and its role in biogeochemical processes in aquatic environments.
Lead Organizer: Shuting Liu, Kean University (liushut@kean.edu)
Co-organizers:
Yuan Shen, Xiamen University (yuanshen@xmu.edu.cn)
John Paul Balmonte, Lehigh University (jpb422@lehigh.edu)
Presentations
02:30 PM
Testing the priming effect as a mechanism explaining DOC drawdown in the deep ocean (9338)
Primary Presenter: Richard LaBrie, McGill University (richard.labrie@mcgill.ca)
The deep ocean dissolved organic carbon (DOC) pool represents one of the largest reservoirs of fixed carbon (C) on the planet. With an average radiocarbon age of several thousand years, the DOC pool appears to be mostly inert. Yet, there is a concentration gradient across ocean basins that cannot be explained with a slow and steady removal process, suggesting a localized drawdown of DOC by microbial populations. Here, we used Antarctic circumpolar deepwater to test the hypothesis that episodic addition of labile DOC “primes” microbial community uptake, resulting in deep sea DOC drawdown. We conducted bottle incubations using an array of different organic compounds, including organic N- or P-containing substrates, to determine if the presence of nitrogen or phosphorus could further stimulate DOC consumption. We monitored DOC, dissolved organic N and dissolved organic P concentrations over 2 months to assess if a priming effect could be detected and how it would impact dissolved organic matter stoichiometry. Simultaneously, we tracked cell density and taxonomic composition to detect whether addition of priming substrates induced shifts in microbial community structure. We observed differential responses in the microbial communities and DOC degradation dynamics, but no clear indication of a priming effect. Our results suggest that deep ocean DOC degradation is unlikely to be primed by labile compounds and that other mechanisms are required to explain DOC drawdown.
02:45 PM
CARBOHYDRATE COMPOSITION OF PARTICULATE ORGANIC MATTER IN THE NORTHWEST SARGASSO SEA AND IMPLICATIONS FOR MICROBIAL UTILIZATION (9381)
Primary Presenter: Lillian Henderson, University of California, Santa Barbara (lillianhenderson05@gmail.com)
The chemical composition of particulate organic matter (POM) in the ocean partially controls its lability/ stability, affecting whether it is exported to the deep ocean and/or consumed or altered by microbial and metazoan food web processing. We used the carbohydrate monomer composition to identify signatures of labile and diagenetically altered POM within the upper 500 m of the water column during three seasons in the northwest Sargasso Sea. Examining size-fractionated POM, we find a high glucose content and high C:N ratios within the upper euphotic zone, especially in large (>20 μm) particles especially during stratified periods when nitrate concentrations were below detection. As depth increased, into the lower euphotic zone and mesopelagic, there was a decrease in glucose contributions and an increase in contributions from monomers deriving from recalcitrant polysaccharides such as peptidoglycan and fucose-based structures. To assess the role of phytoplankton and prokaryotes in shaping the carbohydrate composition of POM, we investigated size-fractionated community composition and abundance and diversity of carbohydrate-active enzymes (CAZymes) within the metagenomes. We observe distinct variations in phytoplankton and heterotrophic prokaryotic communities in both small and large particles over depth. Additionally, the abundance and diversity of CAZymes are distinct between upper euphotic and mesopelagic samples. We will discuss potential linkages between microbial metabolic capabilities and the composition of POM.
03:00 PM
Diel cycle in lipids and carbohydrates is central to global marine metabolism (9389)
Primary Presenter: Daniel Lowenstein, MIT/WHOI Joint Program in Chemical Oceanography (patton.lowenstein@gmail.com)
Every day, phytoplankton in the ocean fix carbon dioxide into energy storage molecules like carbohydrates and lipids, then respire most of that carbon back to carbon dioxide at night, making up the central metabolism of the global ocean. However, the magnitude of the oscillation of these energy storage pools is not well constrained. Using a combination of mass spectrometry and ion chromatography, we quantified the contribution of lipids and carbohydrates to total particulate organic matter over a diel cycle in the euphotic zone in four diverse oceanographic locations—the oligotrophic North Pacific Subtropical Gyre and Sargasso Sea, the coastal Pacific near Oregon, and the Gulf of Maine. Microbial communities in these locations exhibit a dynamic daily cycle in both lipids and carbohydrates—both reservoirs reach maxima at sunset and decline to minima by sunrise—and the magnitude of this oscillation accounts for as much as 75% of daily primary production. Differences in this cycle emerge between sampling location, influenced by light level, photic zone depth, day length, and microbial community composition. The timing and magnitude of the oscillation has implications for the structure and lability of marine organic matter at global scales, as the size of these pools relative to total particulate organic matter fundamentally changes the macromolecular composition of carbon in the surface ocean throughout each day.
03:15 PM
OXYGEN-LIMITED METABOLISM AND THE PERSISTENCE OF OXYGEN DEPENDENT ORGANIC MATTER (ODDOM) IN HYPOXIC MARINE ENVIRONMENTS (9489)
Primary Presenter: Benjamin Daniels, Oregon State University (danieben@oregonstate.edu)
In oxygen minimum zones (OMZs), the transition to hypoxia plays a pivotal role in determining the fate of organic matter. The Hypoxic Barrier Hypothesis predicts that as oxygen declines, oxygenase enzymes—which are crucial for the degradation of many organic compounds—become oxygen-limited long before respiration slows. This limitation may partly explain why oxygen declines tend to stall in the hypoxic range without progressing to anoxia. We conducted mesocosm experiments to investigate how controlled hypoxia affects organic matter lability and microbial community metabolism over an extended period. We hypothesized that compounds requiring oxygenase-mediated degradation would accumulate under hypoxic conditions and could be classified as oxygen-dependent dissolved organic matter (ODDOM). The data reveal a shift toward recalcitrant dissolved organic matter (RDOM) in the hypoxic treatment, where compounds resembling ODDOM accumulated. In other experiments, microbial respiration rates were lower in hypoxic conditions when communities were provided with model ODDOM compounds, relative to control experiments with oxygenase-independent carbon sources. Our findings highlight the relationship between oxygen availability, microbial metabolism, and organic matter lability. This research contributes to a better understanding of how OMZs and other low-oxygen environments influence the biogeochemical cycling of carbon. These results offer critical insights into the roles of both biotic and abiotic factors in regulating the transformation and fate of organic matter in aquatic systems.
03:30 PM
Oxidation state of bioavailable dissolved organic matter influences bacterioplankton respiration and growth efficiency (9083)
Primary Presenter: Brandon Stephens, National Taiwan University (bstephens@ntu.edu.tw)
Oxygen consumption by oceanic microbes can predict respiration (CO2 production) but requires an assumed respiratory quotient (RQ; ΔO2 /ΔCO2). Measured apparent RQs (ARQs) can be impacted by various processes, including nitrification and changes in dissolved organic matter (DOM) composition, leading to discrepancies between ARQ and actual RQ. In DOM remineralization experiments conducted in the eastern North Atlantic Ocean, ARQs averaged 1.39 ± 0.14, similar to predictions for complete consumption of plankton biomass. DOM removed with an elevated nominal oxidation state (i.e., more oxidized DOM), detected by liquid chromatography-tandem mass spectrometry, coincided with increased hydrolyzable amino acid removal, increased ARQs and bacterioplankton respiration (BR), and a decreased bacterioplankton growth efficiency (BGE). Across experiments, evidence emerged that DOM partial oxidation and nitrification elevate BR relative to bacterial consumption of plankton-derived carbon. These rare synoptic measurements of interrelated variables reveal complex biochemical and cellular processes underlying variability in large-scale CO2 production estimates.
03:45 PM
Relationships between organic matter content, sediment nutrient pools, and oxygen demand in mixed seagrass beds in Jobos Bay, Puerto Rico (9284)
Primary Presenter: Troy Mutchler, Kennesaw State University (tmutchle@kennesaw.edu)
The invasive seagrass, Halophila stipulacea (Forsskål and Niebuhr) Ascherson, spread widely throughout Puerto Rico in the last decade, particularly after Hurricane Maria in 2017. The seagrass now forms large monospecific beds in Jobos Bay and also co-occurs with the native seagrass, Thalassia testudinum Banks ex König. Because of its different morphology and life cycle, contributions of the invasive seagrass to detrital pools in sediments may differ in quantity and quality, potentially altering carbon (C) and nitrogen (N) cycling within the estuary. We measured sediment oxygen demand (SOD), organic matter and its elemental content in surface sediments, and sediment ammonium pools to explore potential relationships between microbial activity and C and N pools within the sediments of four sites with differing seagrass composition. Initial results from March 2023 indicate average organic matter content (±s.e.) of the upper three cm of sediments was 5±1 kg m-2 in beds where T. testudinum was abundant and 4±1 kg m-2 in beds with abundant H. stipulacea; whereas, average porewater ammonium concentrations varied across sites (85±28 to 213±202 µM). SOD ranged from 749±22 to 1239±142 at sites containing seagrass. Data were also collected in July and October 2023 and March 2024. Potential changes in sediment conditions associated with the spread of H. stipulacea will generate important insights into C and N budgets within the bay.
SS11B - Biotic and abiotic influences on the lability and fate of organic matter
Description
Time: 2:30 PM
Date: 27/3/2025
Room: W207AB