Isotope tools have been widely used in aquatic science, however, over the last decade multiple new techniques have been introduced or refined, and are now being applied to solving broad questions in biogeochemistry, marine ecology, and paleoceanography. These include compound-specific stable- (2H, 13C, 15N) and radio-isotopes (14C) measured on various types of individual organic molecules (e.g. lipids, amino compounds, and carbohydrates), as well as the use of isotopomers, and isotope ratios of heavy metals (e.g. Sr, Pb, Mg, Fe). This symposium seeks to bring together different perspectives and analytical approaches to help solve emerging questions in biogeochemical cycles, ecology, and environmental sciences. We invite submissions regarding development or application of any novel stable isotope approaches for studying aquatic systems. This could include single compound isotopic measurements, allowing investigation of carbon and nitrogen sources and flux in present or past on a new level of detail and specificity, but could also encompass novel analytical, modeling, or applied approaches using bulk stable isotope measurements.
Lead Organizer: Matthew McCarthy, UC Santa Cruz (mdmccar@ucsc.edu)
Co-organizers:
Carsten Schubert, Eawag, Biogeochemistry (carsten.schubert@eawag.ch)
Natasha Vokhshoori, Smithsonian, WA. DC (VokhshooriN@si.edu)
Presentations
10:30 AM
Progress and frontiers in ocean applications of compound-specific isotope geochemistry (6139)
Primary Presenter: Hilary Close, University of Miami (hclose@miami.edu)
The naturally occurring isotope ratios of individual organic compounds were first measured more than 60 years ago, yet the application of compound-specific isotope analysis (CSIA) within oceanic settings is still limited. In this overview, I will describe the types of compounds and isotopes currently accessible to CSIA, some examples of diverse applications in the study of marine organic matter, and practical considerations that currently limit the application of these techniques for studies of marine organic materials such as dissolved organic matter, animal growth bands, and microbial cells. I will present possible outlooks for the future of the field and invite discussion. While the most common instrumental techniques for CSIA have been in use for more than 30 years, the field currently lies at a frontier for possible revolution using high resolution mass spectrometry. The oceanic applications of CSIA also lie at a frontier, with the potential for global-scale data sets within reach and analysis of microscale materials and difficult-to-access environments becoming more achievable.
10:45 AM
Anaerobic vs. aerobic metabolisms in organisms: a view from compound-specific isotope analysis of amino acids (5134)
Primary Presenter: Yuko Takizawa, Hokkaido University (takizaway@lowtem.hokudai.ac.jp)
The stable nitrogen isotope ratios of amino acids in organisms have been employed to understand the diet-resource utilization and trophic energy transfer among organisms in food webs. This method is proposed by empirical observations that a universal isotopic enrichment in 15N is found in glutamic acid for diverse species. In general, glutamic acid is deaminated with enzymatic reaction, incorporated directly into the tricarboxylic acid cycle, and extracted energy under aerobic metabolism. However, little knowledge is available for change in the ratio associated with ‘anaerobic metabolism’ such as glycolysis and fermentation in organisms. Therefore, we investigated laboratory-cultured samples to see the change in the ratio under anaerobic metabolisms. The eukaryote Saccharomyces cerevisiae and the bacteria Lactococcus lactis were incubated with organic substrates and were compared the ratios between before (i.e., t=0, non-fermented) and after incubation (i.e., t=f, fermented). We found negligible isotopic fractionation for phenylalanine, as well as for isoleucine, proline, and glutamic acid, but variable for glycine, alanine, valine, and leucine in the samples. Our results suggest that activity of anaerobic metabolism can be recorded in the latter amino acids, and that comparison of the isotope ratios between two metabolically-different amino acids (e.g., alanine and glutamic acid) will be useful to evaluate the contribution of anaerobic metabolism in biological and environmental samples.
11:00 AM
EVIDENCE OF DISTINCT MICROBIAL 15N-AA FRACTIONATION PATHWAYS IN PARTICULATE ORGANIC MATERIAL FROM WASTEWATER TREATMENT PLANTS (7234)
Primary Presenter: Owen Sherwood, Dalhousie University (owen.sherwood@dal.ca)
Compound-specific nitrogen isotope analysis of amino acids (15N-AA) is a potentially powerful technique for investigating mechanistic pathways of microbial degradation of organic material (OM) in aquatic environments. The technique leverages differences in isotopic fractionation among individual amino acids during metabolic processing. Four general microbial metabolic patterns of 15N-AA fractionation have been proposed: 1) algal-like de novo synthesis; 2) animal-like heterotrophy, 3) selective microbial resynthesis, and 4) extracellular hydrolysis. However, only a few of these patterns have been clearly demonstrated outside of the laboratory. We investigated 15N-AA fractionation in biogeochemically contrasting wastewater treatment plants (WWTPs) employing enhanced primary (physical-chemical flocculation) and secondary (sedimentation plus conversion of ammonia to nitrate) treatment. Patterns of 15N-AA were similar in the influent particulate OM at both WWTPs, but diverged with successive treatment, becoming lighter by an average -3‰ across all AAs in the primary WWTP (pattern 1) and heavier by +6‰ in the secondary WWTP (pattern 4). The concentrations of total hydrolysable AAs, D/L ratios, and the “sum-V” parameter for pattern (3) remained relatively unchanged among treatment stages, suggesting that metabolisms involving de novo synthesis or extracellular hydrolysis may be invisible to these degradation proxies in nutrient-rich environments. These results add new empirical evidence for metabolically distinct 15N-AA fractionation patterns in detrital organic material.
11:15 AM
PAST METHANE AVAILBILITY IN ALASKAN LAKES 2-5 TIMES HIGHER DURING THE HOLOCENE THERMAL MAXIMUM (6373)
Primary Presenter: Roseanna Mayfield, Newcastle University (roseanna.mayfield@newcastle.ac.uk)
A large proportion of the world’s freshwater lakes and ponds are in northern high-latitude regions, which are warming almost twice as fast as the global average. As temperatures increase, it is anticipated that lake methane production will also increase, resulting in a positive feedback to global warming. Palaeoenvironmental proxy data have shown substantial climate variation during the Holocene in Alaska, including a warmer-than-present Holocene Thermal Maximum (HTM) at c. 11 - 6 cal ky BP. By quantifying lake methane production under earlier warm periods, we can provide empirical data to validate future methane production scenarios. Past methane production can be estimated from changes in the composition of stable carbon isotopes (δ13C values) within different components of a lake ecosystem. As methane typically has characteristically highly negative δ13C values (-85 to -65‰), a methane signal can be traced through the ecosystem. We use δ13C values of aquatic invertebrate remains and specific biomarker proxies for methane-generating bacteria. The δ13C values measured on sedimentary remains of these proxies are lower when methane-derived carbon is an important part of the lake carbon cycle. Our proxy-based estimates of methane production from two lakes from boreal/ interior Alaska suggest that during the HTM, methane availability could have been 2-5 times higher than at present.
11:30 AM
SHORT-TERM VARIATIONS IN TROPHIC DYNAMICS OF ZOOPLANKTON IN RESPONSE TO FRESHWATER DISCHARGE IN A HIGHLY ALTERED ESTUARY (6238)
Primary Presenter: Hyuntae Choi, Hanyang University (dmb122@hanyang.ac.kr)
In the estuarine environment, the freshwater discharge regulated by the sea dike influences the planktonic ecosystem as an anthropogenic stressor. Particularly, the freshwater discharge events are mostly concentrated in summer rainy season in the Northeast Asia countries, rapidly altering phytoplankton composition in estuary. In this study, we aimed to understand the variations in diet utilization of zooplankton under rapid changes in phytoplankton community triggered by freshwater discharge event using nitrogen isotope ratio of amino acids in the Yeongsan River estuary (Korea), which has a dike to control freshwater use. In result, freshwater discharge events increased concentrations of inorganic nitrogen and altered size-fractionated chlorophyll-a composition. In addition, nitrogen isotope ratios of both POM and particle-feeding zooplankton near the sluice gate increased close to those in downstream freshwater within two days, implying the nitrogen source in discharged freshwater is rapidly reflected to organisms in estuarine planktonic ecosystem. The trophic position of particle-feeding zooplanktons did not show any significant difference during episodic freshwater discharge event except for Acartia after two days near sea dike, likely due to their wide availability on particle size and origin. This study results suggest particle-feeding zooplankton have a high adaptability on diet utilization in rapidly changing environment by the regulated freshwater discharge in the estuarine environment.
11:45 AM
DEPICTING THE INFLUENCE OF SEASONAL UPWELLING ON ZOOPLANKTON TROPHIC DYNAMICS BY COMPOUND SPECIFIC ISOTOPE ANALYSIS (6869)
Primary Presenter: Inés Viana, Instituto Español de Oceanografía (ines.viana@ieo.csic.es)
Mesozooplankton, mainly comprised by a high number of copepod species, is influenced by high primary productivity enhanced by seasonal upwelling events. Although many copepod species have been traditionally considered as herbivores, they might feed on different food sources (e.g. microzooplankton or marine snow) when available. This diet plasticity can be tracked by changes in zooplankton isotopic composition, as their signature integrates both the changes in the nutrient sources (baseline) and their trophic position (TP). However, the extent to which changes in trophic dynamics of mesozooplankton are driven by any of these factors is poorly understood. With the aim of investigating whether the upwelling intensity affects the nitrogen baseline and trophic indicators of this group, the taxonomic composition of mesozooplankton samples and their nitrogen stable isotope composition (δ15N and CSIA-AA) were investigated during two years in a station at the NW Iberian Peninsula. As expected, higher phytoplankton availability during upwelling events induced zooplankton herbivory, resulting in lower TPs. However, variations in the microbial contribution index to TP, computed from CSIA-AA, revealed that the optimal coupling between metazoan and microbial food webs (i.e. 25% microbial contribution) was reached at moderate upwelling intensities, and decreased to low contributions (<15%) in both downwelling and strong upwelling situations. Therefore, enhanced phytoplankton productivity during moderate upwelling disruptions leads to a shorter and more efficient food web.
SS030A Novel Stable Isotope Approaches In Aquatic Biogeochemistry and Ecology
Description
Time: 10:30 AM
Date: 9/6/2023
Room: Sala Ibiza B