Global variability in dissolved organic matter: implications for metal bioavailability and risk assessment

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Dissolved organic matter (DOM) is a ubiquitous substance in natural waters on the earth's surface. Dissolved organic matter is crucial in global carbon balance with implications/controls on nutrient cycling as well as climate change. Carbon is obviously a significant element in life processes and DOM represents a potentially dynamic component of carbon on earth. In addition to DOM's role in influencing productivity and climate, a secondary role of DOM is to bind pollutants and influence their bioavailablity and transport. Toxicity prediction models such as the Biotic Ligand Model (BLM) implicitly take DOM into account due to its potential to complex metals and mitigate their toxicity.******Dissolved organic matter is often represented quantitatively as dissolved organic carbon (DOC) and reported in milligrams carbon per litre. DOC is a useful proxy because it can be readily measured, using a carbon analyzer, but it remains an open question if molecular differences in DOM need to be taken into account in understanding carbon/nutrient cycling, climate change, as well as pollutant fate and transport? DOM is known to be a mixture of small low molecular weight carboxylic acids along with macromolecular components. The source of DOM influences the composition. It has been shown by my lab, and others, that all DOM is not created equal in terms of optical properties, or in terms of reactivity towards metal and protons (acid-base and metal titrations), or even in terms of toxicity mitigation (darker organic matter tends to be more protective than lighter coloured organic matter). The significance of these differences on a wide geographic scale has not been assessed nor has the specific implications for DOM dependent modelling and risk assessment.******Work in my lab has focused on samples from Southern Ontario Canada (previous NSERC Discovery Grant) and marine/estuarine organic matter from coastal North America (previous NSERC CRD grant). It is necessary to expand this earlier work to a much wider range of samples and also using increasingly sophisticated analytical techniques. Using Discovery funds a North-South set of samples will be collected from North America and using collaborators and complementary funding sources, samples will also be collected from the Northwest Territories in Canada, the Amazon Basin in Brazil and the three gorges region of Chongqing province in China.******Molecular spectroscopy will be used as a "fingerprint'' technique applied on all samples. To assess metal binding titrimetry will be utilized with an emphasis on free ion measurement. Characterization of DOM binding to a suite of metals will allow for unified models to account for speciation. Globally, as regulators move towards the BLM-like approaches developed in North America and Europe it is important to make sure that the DOC that "drives" these models in the same at a molecular level.