A changed nitrogenase activity in Rhodospirillum rubrum after substitution of tungsten for molybdenum

The nitrogenase activity in Rhodospirillum rubrum was changed when the cells were made either Mo-deficient or when Mo was replaced by tungsten (W) as trace element in the growth medium: In the absence of N2, normal Mo cells evolved H2 (via nitrogenase) from added malate in the light faster than W cells, which in turn evolved H2 faster than Mo-deficient cells. In the presence of N2, on the other hand, nitrogen fixation rate in W cells was very close to the low rate found with Mo-deficient cells.