Molybdenum availability, nitrogen limitation, and phytoplankton growth in natural waters.

Sulfate inhibits molybdate assimilation by phytoplankton, making molybdate less available in seawater than it is in freshwater. As a result, nitrogen fixation and nitrate assimilation, both processes that require molybdenum, may require a greater expenditure of energy in seawater than in freshwater. This may explain in part why coastal marine ecosystems are usually nitrogen limited whereas lakes usually are not. Experimentally increasing the ratio of sulfate to molybdate (i) inhibits molybdate uptake, (ii) slows nitrogen fixation rates, and (iii) slows the growth of organisms that use nitrate as their nitrogen source.