Although certain trace metals are essential micronutrients required for growth, elevated concentrations of some of the same metals exert deleterious effects on marine phytoplankton populations. Laboratory studies have indicated that metal toxicity depends on metal ion concentrations rather than total dissolved metals. Although it is believed that bioavailability is largely controlled by the degree to which dissolved trace metals are organically chelated, it has not been definitively established which organic compounds chelate trace metals in natural seawater. In an effort to define ecologically significant interactions between dissolved trace metals and naturally occurring organic matter we selected marine humus (humic and fulvic acids) as being likely to interact with trace metals in seawater. These compounds, derived from plant and animal sources, are being widely studied for their role in the transport and toxicity of metal ions in terrestrial, aquatic, and marine ecosystems and are known to comprise up to half of the total dissolved organic matter in seawater. Guided by a recent hypothesis explaining the structure of marine humus and its geochemical diagenesis a laboratory synthesis of marine fulvic acid was accomplished. The resulting material was physically, spectroscopically and chemically identical to one or more natural marine fulvic acids isolated from the Gulf of Mexico. We confirm here that isolated natural marine fulvics and marine fulvics synthesized in the laboratory affect the bioavailability of trace metals to marine phytoplankton.
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