MOLYBDENUM ASSIMILATION BY CYANOBACTERIA AND PHYTOPLANKTON IN FRESH-WATER AND SALT-WATER

We measured the uptake rate of molybdate and related kinetic parameters for nine taxa of cyanobacteria and for the natural phytoplankton communities of six freshwater lakes containing planktonic N2-fixing cyanobacteria. Molybdate uptake followed saturation kinetics and was competitively inhibited by both tungstate and sulfate. Tungstate inhibited molybdate uptake in a nearly mole-for-mole fashion; inhibition constants (K(i)) were in the same concentration range (10-30 nM) as the half-saturation constants for molybdate uptake (K(m)). Sulfate also inhibited molybdate uptake, but this inhibition was much less specific. The K(i) for sulfate was in the mM range-three orders of magnitude above the K. for molybdate uptake. Despite these high K(i) values, however, sulfate can be an important inhibitor of molybdate uptake in many natural waters as sulfate concentrations are usually some 4-6 orders of magnitude greater than molybdate concentrations. At ambient sulfate and molybdate concentrations in most freshwater lakes, molybdate uptake would be inhibited by 1-5% due to sulfate. In marine and estuarine systems this inhibition would be 15-20% and in some saline lakes could be >70%.