The growth of cyanobacteria in natural waters may be dependent on an efficient ammonium transport system. Such a system may also play a role in the retention of internally generated ammonium from N2fixation, nitrate reduction, and photorespiration. Methylammonium is commonly used as an ammonium analogue to study transport. The kinetics of methylammonium uptake by cyanobacteria are biphasic with a first rapid phase representing transport into the cells and a second slower phase dependent on metabolism by the primary ammonia assimilating enzyme glutamine synthetase. At high external pH values (pH > 9) diffusion of the uncharged species becomes increasingly dominant while at neutral pH values the uptake is thought to be an active process dependent on membrane potential.