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AOSC Departmental Seminar
March 26, 2015

The role of strong organic ligands in the marine biogeochemistry of trace metals

Johan Schijf
UMCES, Chesapeake Biological Laboratory

The ocean is generally considered to be at steady state and fairly well mixed on a global scale, hence the concentrations of dissolved constituents might be expected to display limited spatial variation. Instead, the vertical distributions of most trace metals are characterized by low concentrations near the surface, increasing gradually with depth to high concentrations in the abyssal ocean. This ‘nutrient-like’ behavior suggests strong microbial uptake in the euphotic zone and release upon remineralization of settling biogenic particles. Yet whereas certain trace metals show prominent correlations with particular macronutrients, these are not easily explained by biological demand. We know that microbes have specific requirements for many trace metals that in some cases can result in widespread growth limitation, for example in High-Nutrient/Low-Chlorophyll (HNLC) zones. It appears that many microbes have a remarkable ability to alleviate such metal biolimitation or, conversely, metal biotoxicity, by means of strong and ostensibly metal-specific organic ligands. Sensitive electrochemical methods have revealed their total concentrations and conditional constants for metal binding, but their identities and chemical properties are otherwise mysterious. I am investigating the possibility that some of them are siderophores, a class of small organic ligands with extreme affinities for Fe(III). Marine siderophores are not commercially sold, chemically intricate, photo-labile, hidden at picomolar concentrations among thousands of other organic compounds, possibly membrane-bound, and therefore difficult to extract and purify, presenting a host of daunting analytical challenges.