Sulfate inhibition of molybdate assimilation by planktonic algae and bacteria: some implications for the aquatic nitrogen cycle |
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Authors: | Jonathan J Cole Robert W Howarth Scott S Nolan Roxanne Marino |
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Institution: | (1) Institute of Ecosystem Studies, New York Botanical Garden, Mary Flagler Cary Arboretum, 12545 Millbrook, NY, USA;(2) Section of Ecology and Systematics, Corson Hall, Cornell University, 14853 Ithaca, NY, USA |
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Abstract: | Molybdenum is required for both dinitrogen fixation and nitrate assimilation. In oxic waters the primary form of molybdenum is the molybdate anion. Using radioactive 99Mol Na2MoO4, we have shown that the transport of molybdate by a natural assemblage of freshwater phytoplankton is light-dependent and follows typical saturation kinetics. The molybdate anion is strikingly similar to sulfate and we present data to show that sulfate is a competitive inhibitor of molybdate assimilation by planktonic algae and bacteria. The ability of freshwater phytoplankton to transport molybdate is inhibited at sulfate concentrations as low as 5% of those in seawater and at sulfate: molybdate ratios as low as 50 to 100 times lower than those found in seawater, Similarly, the growth of both a freshwater bacterium and a saltwater diatom was inhibited at sulfate: molybdate ratios lower than those in seawater.The ratio of sulfate to molybdate is 10 to 100 times greater in seawater than in fresh water. This unfavorable sulfate: molybdate ratio may make molybdate less biologically available in the sea. The sulfate: molybdate ratio may explain, in part, the low rates of nitrogen fixation in N-limited salt waters. |
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Keywords: | molybdenum molybdate nutrient limitation phytoplankton |
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