, Volume 2, Issue 2, pp 179–196 | Cite as

Sulfate inhibition of molybdate assimilation by planktonic algae and bacteria: some implications for the aquatic nitrogen cycle

  • Jonathan J. Cole
  • Robert W. Howarth
  • Scott S. Nolan
  • Roxanne Marino


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.

Key words

molybdenum molybdate nutrient limitation phytoplankton 


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Copyright information

© Martinus Nijhoff/Dr W. Junk Publishers 1986

Authors and Affiliations

  • Jonathan J. Cole
    • 1
  • Robert W. Howarth
    • 2
  • Scott S. Nolan
    • 1
  • Roxanne Marino
    • 2
  1. 1.Institute of Ecosystem StudiesMillbrookUSA
  2. 2.Section of Ecology and Systematics, Corson HallCornell UniversityIthacaUSA

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