Suppression of electron flow (and its subsequent restoration with 3-10 mM NaHCO3) on the donor side of photosystem II is shown upon either a partial depletion of pea subchloroplast membranes in bicarbonate or the addition of 5-20 microM formate. At higher concentrations (5 mM) formate induces the known 'bicarbonate effect' on the acceptor side of photosystem II. In preparations depleted of manganese the restoration of electron flow with 0.1-0.2 microM MnCl2 (2-4 Mn per photosystem II reaction center) occurs only in the presence of bicarbonate and it is accompanied by an increased functional binding of manganese. Restoration of electron flow with diphenylcarbazide or NH2OH does not require the addition of NaHCO3. It is suggested that bicarbonate participates in the formation of the Mn-cluster capable of water oxidation or serves as a substrate for the water-oxidizing center.