Keynote: frontiers and challenges in isotopic characterisation of DOM
Professor Tim Eglinton FRS, ETH Zurich, Switzerland
Dissolved organic matter (DOM) in natural waters is comprised of highly complex mixtures of components that reflect myriad biotic and abiotic production, modification, translocation and decomposition processes. This complexity presents formidable challenges in our ability to characterise DOM, understand its interactions with other carbon pools, and predict its response to natural and anthropogenic forcing. We currently lack a comprehensive understanding of the dynamics and pathways of DOM production, transformation and transport, as well as links between dissolved and particulate carbon phases. Recent analytical and instrumental advances are yielding detailed information on the chemical composition of DOM, however bridging information gleaned at the molecular level with dynamics observed in bulk DOM remains an elusive challenge. In this context, determination of isotopic characteristics of specific DOM constituents holds potential to provide key information on the sources and cycling of DOM. While diverse source-specific “biomarker” compounds are routinely targeted for stable isotopic and radiocarbon characterisation of particulate organic matter (POM), such approaches are much less well developed for DOM. This presentation will provide examples of insights derived from current molecular 14C investigations of POM in aquatic systems and how they may inform DOM isotopic studies. Some glimpses into the isotopic variability within DOM in a range of environments spanning soils to fluvial systems will be provided. Finally, examples that show how isotopic (14C) gradients in space and time may be exploited to derive novel information on DOM sources and cycling will be presented, with the goal of motivating further research in this area.
Exploring DOM in large lakes: Successes and challenges
Professor Elizabeth Minor, University of Minnesota Duluth, USA
The world’s largest five freshwater lakes include both tropical meromictic and temperate holomictic lakes ranging in age from twenty-five million to ~10,000 years old. They contain >50% of earth’s surface liquid freshwater. Three of the lakes support extensive human populations in their watersheds. Despite their demonstrated importance in economic, cultural, and environmental milieus, large lakes are understudied in terms of carbon cycling, including the roles of dissolved organic matter (DOM)--as dissolved organic carbon (DOC); as chromophoric dissolved organic matter (CDOM); and at the compound class or molecular level. Existing data shows that large lakes have lower DOC and CDOM concentrations and clearer water than the global lake median. However, available CDOM/chlorophyll data (for Lake Superior) shows a ratio higher than mean ocean surface water and much higher than would be predicted by a temperate-lake-based relationship between lake area and CDOM/chlorophyll. Qualitative characterisation indicates that DOM in Lakes Baikal, Superior, and Michigan contains a large proportion of terrestrially-derived but reworked organic matter. No such data exists for Lakes Malawi and Tanganyika. Large lakes present unique challenges for DOM studies as they have differing inorganic matrices (in terms of major ions & oxidation levels), have variable levels of non-chromophoric DOM, and require oceanographic-style efforts to sample at sufficient temporal and spatial resolution. As these lakes are critical resources for humans and key environmental systems on a global scale, and as they are subject to climate change, land-use and lake-use pressures, such efforts should be an upcoming focus for aquatic scientists.
Novel methods for compound specific determination of DOM composition and their relation to landscape character using examples from the NERC DOMAINE platform
Dr Charlotte Lloyd, University of Bristol, UK
The flux of dissolved organic matter (DOM) into rivers is rising due to a range of factors, including inputs of organic wastes from livestock production, discharge of sewage effluent, as well as the mobilisation of soil organic matter stores. There is a growing body of research showing that many DOM compounds are bioavailable and can be rapidly assimilated by stream biota, which may have important implications for nutrient cycling and riverine health. With this in mind, it is vital to gain a more comprehensive understanding of the composition of riverine DOM at a molecular level. DOM is an extremely complex mixture of individual compounds and therefore poses some analytical challenges. Most often, DOM is quantified as a bulk nutrient fraction then characterised by parameters such as hydrophobicity, molecular weight, aromaticity and functional group. However, recent advancements in analytical approaches can help allowing more detailed characterisation to compound level. Typically, molecular-scale analysis of DOM has been carried out in a targeted way, where compounds of interest are analysed. However, here, we propose a hierarchical approach to DOM characterisation using a suite of cutting-edge analytical techniques in order to obtain a wide analytical window spanning different size fractions and polarities. A number of UK-based case studies are used to illustrate the power of using this combination of analytical techniques to provide a truly untargeted approach to DOM characterisation.
Molecular diversity of dissolved organic matter in freshwater and marine systems
Professor Thorsten Dittmar, University of Oldenburg, Germany
Organic remnants of aquatic organisms have accumulated over thousands of years in dissolved form to one of the largest organic carbon pools on our planet’s surface. Dissolved organic matter (DOM) contains more carbon than the entire vegetation on Earth combined. The reasons behind its persistence and its potential for carbon storage in the future are unknown. During growth and upon death, cells release a myriad of organic compounds on which microorganisms grow. A minor fraction of this organic matter decomposes so slowly that it has persisted in the global ocean for millennia. The resulting mixture of largely unknown compounds has reached an extraordinarily high level of molecular diversity. The current paradigm is that microbes cannot decompose this mixture because suitable metabolic pathways have not evolved. Here, Professor Dittmar presents an emerging, alternative concept that assumes the existence of enzymatic machineries that continuously transform any form of DOM, but due to extreme dilution encounters of cells and substrate units are rare events in the ocean. According to this concept, marine microbes appear most powerful in decomposing any form of organic matter, but rates slow down as molecular diversity increases.