Organic matter is a dynamic entity turning over in aquatic systems at highly variable time scales. In the search for controls on organic matter degradation, a multitude of interacting environmental factors have been proposed, including biotic and abiotic conditions, the molecular composition of DOM itself and the low concentrations of the different DOM components. Recent advances in analytical chemistry and biotechnology have provided researchers with an unprecedented ability to characterize the specific organic compounds that participate in biotic and abiotic transformations as well as the enzymes involved in metabolic processes and the genes encoding them. Furthermore, abiotic factors including photolytic degradation and adsorption processes interact with biotic factors in complicated fashion. In this session, we encourage presentations based on studies examining controls and mechanisms of OM turnover, as well as comparative studies and modelling efforts describing a framework for predicting OM turnover. In this session, we will explore how measuring the activities of extracellular and cytoplasmic enzymes in aquatic ecosystems, the concentrations and fluxes of metabolites, the microbes, genes, transcripts, and proteins present in those ecosystems, and the compounds present in dissolved and particulate organic matter all influence each other and the broader ecosystem. Consistent with this year's meeting theme, we are particularly interested in how microbial metabolic processes and microbe-organic matter interactions influence the resilience and recovery of aquatic ecosystems.
Lead Organizer: Andrew Steen, University of Tennessee - Knoxville (asteen1@utk.edu)
Co-organizers:
Maria Montserrat Sala, Institut de Ciències del Mar (CSIC) (msala@icm.csic.es)
Jesus M. Arrieta, Spanish Institute of Oceanography (jesus.arrieta@ieo.csic.es)
Jutta Niggemann, ICBM, University of Oldenburg (jutta.niggemann@uol.de)
Presentations
03:00 PM
EXPLORING THE REGULATIONS OF THE RESPIRATORY QUOTIENT ACROSS ECOSYSTEMS (6711)
Primary Presenter: Konstantinos-Marios Vaziourakis, Uppsala University (konstantinos.vaziourakis@ebc.uu.se)
There is substantial variation in estimates of the respiratory quotient (RQ) which is defined as the molar ratio of produced CO2 and consumed O2 during microbial mineralization of organic matter (OM). The resulting RQ value of microbial metabolism is usually dependant upon the composition of the organic matter and the properties of the microbial degrading communities. While several studies have examined the controlling factors and the magnitude of RQ, in either terrestrial or in aquatic ecosystems, broader cross-ecosystem comparisons are missing, and universal insights into the extrinsic (environmental) and intrinsic (organic matter composition) controls on RQ are lacking. In this study, we calculated the RQ across a broad range of environmental samples, including soils, aquatic sediments, peat, lake and marine water by measuring CO2 production and O2 consumption in the headspace of incubation vials using membrane inlet mass spectrometry (MIMS). We also assessed the microbial metabolic profiles using BIOLOG Ecoplates and determined the energy content of the natural OM with bomb calorimetry. Our findings show that 1) RQ values differ significantly between the ecosystem types, 2) the extent of microbial respiration across the different studied ecosystems is correlated with the bulk energy content of the OM (kJ per g organic carbon) and 3) RQ is regulated by other environmental parameters which constrain the energetic controls on OM decomposition. We argue that cross-system comparison of a wider range of ecosystems will help us understand the persistence and decay of OM.
03:15 PM
Large carbon export, but short residence times, of transparent exopolymer particles in the global ocean (4874)
Primary Presenter: Isabel Reche, Universidad de Granada (ireche@ugr.es)
Acidic polysaccharides released by phytoplankton and prokaryotic heterotrophs promote the formation of gel-like transparent exopolymer particles (TEPs). TEPs can have a relevant contribution to the biological carbon pump due to their carbon-rich composition and their ability to coagulate and sink towards the deep ocean. However, little is known about TEPs distribution, carbon export, and residence times below the export (200 m) and sequestration (1000 m) depths. We provide the first comprehensive inventory of TEP from the ocean surface to a depth of 4000 meters in the tropical and subtropical Atlantic, Indian, and Pacific Oceans, evaluating its contribution to carbon export and sequestration into the deep ocean. Results indicate that TEP concentration is primarily determined by primary production, with higher concentrations located above the deep chlorophyll maxima. In the deep ocean, TEP concentrations are lower yet mirror the concentrations in the surface, demonstrating the significance of TEP sinking below both the export compartment (2.8 Pg C yr-1; 27% of total POC flux at 200 m) and the sequestration compartment (0.8 Pg C yr-1; 36% of total POC flux at 1000 m). In situ incubation experiments conducted across ocean basins indicate short TEP residence times, averaging 27 and 333 days in the export and sequestration compartments, respectively. These findings reveal that the export and subsequent sequestration of carbon by TEP sinking into the deep ocean diverts it from the long times observed for the dissolved carbon fraction (i.e. centuries) in the global carbon cycle.
03:30 PM
Linking microbial enzymatic activities and carbohydrate structures in particulate organic matter in the western North Atlantic (5382)
Primary Presenter: Chad Lloyd, University of North Carolina at Chapel Hill (cchadlloyd@gmail.com)
Heterotrophic bacteria process nearly half of the organic matter produced by phytoplankton in the surface ocean. Much of this organic matter consists of structurally complex polysaccharides, which must initially be hydrolyzed by specific extracellular enzymes to smaller sizes prior to remineralization. Here, we hypothesize that bacterial communities and their enzyme activities reflect available target substrates, which we expect to vary across marine provinces. To test this hypothesis, we concurrently assessed bacterial community composition and activity of polysaccharide hydrolases, as well as carbohydrate abundance and structural complexity at multiple depths and stations in the western North Atlantic. Congruence in monosaccharide composition of particulate organic matter at different stations and depths contrasted with clear structural differences suggested by polysaccharide-specific antibody probing. Bacterial community composition and polysaccharide hydrolase activities also varied by depth and station, suggesting that the structure and function of bacterial communities—as well as the structural complexity of their target substrates—could be interlinked in a complex manner. Our findings reflect the likely underestimation of polysaccharide variation across depth and location due to the low structural resolution of previous carbohydrate analyses.
03:45 PM
Dissolved organic phosphorus molecular weight distribution and bioavailability in the surface ocean: from genes to geochemistry. (6977)
Primary Presenter: Sonya Dyhrman, Columbia University (sdyhrman@ldeo.columbia.edu)
Dissolved organic phosphorus (DOP) is critical for supporting primary production in key regions such as the western North Atlantic, but the production, composition, and bioavailability of DOP remain poorly constrained. Leveraging a new method (Hull and Ruttenberg 2022), samples from the North Atlantic and North Pacific were screened for DOP molecular weight distribution and subjected to enzyme hydrolysis to determine the fraction of bioavailable phosphomonoester (PME) and phosphodiester (PDE). The distribution of total DOP into molecular weight size fractions of <500Da, 500-1000Da, 1-100kDa, and >100 kDa varied across surface environments. Differences in molecular weight distribution were most striking for DOP recovered in the smallest fraction (<500Da), which was dominated by bioavailable PME and PDE compounds. Recovery of DOP in the <500Da fraction ranged from non-detectable in the lowest phosphate (P) environment to the dominant size fraction (>60% of total DOP) in comparatively higher P environments. These patterns may be driven by changes in phytoplankton DOP production and microbial DOP hydrolysis as a function of P physiology. Moving forward we are examining these dynamics with experimental work to track the expression of P metabolism pathways and production and consumption processes. Together, this combination of approaches is providing new insight into the heterogeneity of DOP and the processes that control DOP cycling.
04:00 PM
Iron acquisition in marine heterotrophic bacteria and the associated impact of iron bioavailability on microbial turnover of organic matter (7174)
Primary Presenter: Adit Chaudhary, University of California San Diego (adchaudhary@ucsd.edu)
Iron is an essential micronutrient for heterotrophic marine bacteria due to its role as a cofactor in enzymes involved in key metabolic processes such as respiration. However, iron is present at extremely low concentrations in the open ocean and most of it is bound in organic ligand complexes. The concentration and bioavailability of this ligand-bound iron is an important abiotic factor that can regulate the ability of marine bacteria to remineralize organic matter. In this study, we utilized a combination of lab model-based and field-based approaches to investigate the importance of different ligand substrates, primarily siderophores, as sources of bioavailable iron for marine bacteria and their impact on bacterial growth and turnover of organic matter. In the lab, we utilized functional genomics techniques to elucidate the iron-acquisition mechanisms associated with common marine siderophores (desferrioxamine B and petrobactin) for the marine copiotroph Alteromonas macleodii. The impact of Alteromonas macleodii’s ability to utilize these iron-siderophore complexes on its growth rate and biomass production was tested in artificial and natural seawater-based conditions. In addition, field-based incubations of microbial communities artificially enriched with iron in different productivity zones of the California current ecosystem were analyzed using metatranscriptomics. Overall, the results from these different approaches will highlight the role of various microbial genes/transcripts involved in iron-acquisition from the marine ligand pool, and the subsequent impact of these iron-acquisition strategies on the microbial turnover of organic matter in marine systems.
04:15 PM
VERTICAL VARIATIONS IN THE ELEMENTAL AND BIOCHEMICAL COMPOSITION OF PARTICULATE ORGANIC MATTER IN THE EUPHOTIC ZONE AND IMPLICATIONS FOR MICROBIAL UTILIZATION (6118)
Primary Presenter: Lillian Henderson, University of Miami (lch39@miami.edu)
Marine particles are a critical contributor to carbon export, food webs, and the sedimentary record, but uncertainties remain in the origins and composition of this material. Variations over depth in both &delta<sup>13</sup>C values of particulate organic carbon (POC) and ratios of POC to particulate nitrogen (PN) have previously been interpreted as a degradative signal. However, we have found that photosynthesis is responsible for relatively low &delta<sup>13</sup>C values in the deeper, low-light portions of the euphotic zone during stratified conditions at the Bermuda Atlantic Time-series Study site. Here, we additionally describe differences in POC:PN between the upper and lower euphotic zones in POM size fractions ranging between 0.3 to >20 µm, suggesting that variations in the biochemical composition of particulate organic matter (POM) may coincide with differences in photosynthetic &delta<sup>13</sup>C values. We will discuss how variations in POC:PN and the relative proportions of amino acids and carbohydrates relate to differences in heterotrophic bacterial activity and community composition over the same depths. The composition of POM may also influence the labile pool of dissolved organic matter via exudation, viral lysis, and extracellular hydrolysis. Lastly, because the lower euphotic zone is an important contributor to integrated euphotic zone POC, variations in organic matter composition over depth may affect the &delta<sup>13</sup>C values and chemical composition of material passed up marine food webs and exported to the deep ocean.
SS052A Microbial and Abiotic Factors Influencing the Turnover and Fate of Organic Matter in Aquatic Systems
Description
Time: 3:00 PM
Date: 6/6/2023
Room: Sala Ibiza A