Permeability of methylamine across the membrane of a cyanobacterial cell

• Ammonia (NH3) is the preferred nitrogen source for many eukaryotic algae and the cyanobacteria, Synechococcus R-2 (PCC 7942) and Synechocystis (PCC 6803). • Ammonia in solution is a weak base with a pKa of 9.25; hence, under environmental conditions it is present in solution as NH3 and NH4+. The uncharged form is a readily diffusible small molecule but NH4+ has an ionic radius similar to K+ and behaves like an alkali metal cation. Accumulation and retention of NH3 + NH4+ is therefore a complicated function of the permeabilities of two very different chemical species. • NH3 is usually thought to be highly permeable across cell membranes although it is difficult to find actual measurements primarily because there is no convenient radioactive tracer for N. Many studies have therefore used 14C-methylamine (14CH3-NH2) as a convenient analogue tracer. • Permeability of 14CH3-NH2 was measured in Synechocystis over a range of pH values; permeability of uncharged amines was shown to be governed by pH. The permeability of CH3NH2 is c. 50 µm s−1 at pHo 10 but increases very rapidly to c. 300 µm s−1 at pHo 7. This finding has serious implications for attempts to develop stochastic models of amine uptake and retention by cells.