TitleSulfate inhibition of molybdenum-dependent nitrogen fixation by planktonic cyanobacteria under seawater conditions: a non-reversible effect
Publication TypeJournal Article
Year of Publication2003
AuthorsMarino, R, Howarth, RW, Chan, F, Cole, JJ, Likens, GE
Type of ArticleJournal Article

The trace element molybdenum is a central component of several enzymes essential to bacterial nitrogen metabolism, including nitrogen fixation. Despite reasonably high dissolved concentrations ( for a trace metal) of molybdenum in seawater, evidence suggests that its biological reactivity and availability are lower in seawater than in freshwater. We have previously argued that this difference is related to an inhibition in the uptake of molybdate ( the thermodynamically stable form of molybdenum in oxic natural waters) by sulfate, a stereochemically similar ion. Low molybdenum availability may slow the growth rate of nitrogen-fixing cyanobacteria, and in combination with an ecological control such as grazing by zooplankton, keep fixation rates very low in even strongly nitrogen-limited coastal marine ecosystems. Here we present results from a seawater mesocosm experiment where the molybdenum concentration was increased 10-fold under highly nitrogen-limited conditions. The observed effects on nitrogen-fixing cyanobacterial abundance and nitrogen-fixation inputs were much smaller than expected. A follow-up experiment with sulfate and molybdenum additions to freshwater microcosms showed that sulfate ( at seawater concentrations) greatly reduced nitrogen fixation by cyanobacteria and that additions of molybdenum to the levels present in the seawater mesocosm experiment only slightly reversed this effect. In light of these results, we re-evaluated our previous work on the uptake of radio-labeled molybdenum by lake plankton and by cultures of heterocystic cyanobacteria. Our new interpretation indicates that sulfate at saline estuarine levels (> 8 - 10 mM) up to seawater ( 28 mM) concentrations does inhibit molybdenum assimilation. However, the maximum molybdenum uptake rate (V-max) was a function of the sulfate concentration, with lower Vmax values at higher sulfate levels. This indicates that this inhibition is not fully reversed at some saturating level of molybdenum, as assumed in a simple competitive inhibition model. A multi-enzyme, mixed kinetics model with two or more uptake enzyme systems activated in response to the environmental sulfate and molybdate conditions may better explain the repressive effect of sulfate on Mo-mediated processes such as nitrogen fixation.

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