Molybdenum assimilation by cyanobacteria and phytoplankton in freshwater and salt water

We measured the uptake rate of molybdatc and related kinetic parameters for nine taxa of cyanobacteria and for the natural phytoplankton communities of six freshwater lakes containing planktonic Nz-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,) were in the same concentration range (lo-30 nM) as the half-saturation constants for molybdate uptake (K,,,). Sulfate also inhibited molybdate uptake, but this inhibition was much less specific. The K, for sulfate was in the mM range-three orders of magnitude above the K, for molybdate uptake. Despite these high K, 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 l-5% due to sulfate. In marine and estuarine systems this inhibition would be 15-20% and in some saline lakes could be > 70%. Molybdenum is one of the essential cofactors for the vast majority of known N,-fixation systems and many nitrate reductase systems (Fogg and Wolfe 1954; Manheim and Landergren, 1978; Robson et al. 1986). The function of MO in both nitrogenase and nitrate reductase, as well as in several other enzyme systems such as formate dehydrogenase, is that it assists in the catalysis of the transfer of two, or a multiple of two, electrons to or from the substrate (Pope et al. 1980). The MO requirements for nitrogenase activity exceed those for nitrate reductase (Fogg and Wolfe 1954). In oxic natural waters MO is thought to be present primarily as the oxyanion, MoOd2-, and its concentration ranges from 1 to 100 nM in most environments (Howarth et al. 19883; Marino et al. 1990). For an organism to synthesize an active nitrogenase enzyme, it must first be able to transport MoOd2from I Present address: Hart Road, Conway, Massachusetts