Many cyanobacteria can fix N2. In the filamentous strains that contain heterocysts, N2-fixation occurs in an anaerobic environment, achieved by separating N2-fixation and oxygenic photosynthesis through the inactivation of PSII in the heterocysts. The nitrogenase requires the input of at least two molecules of ATP per pair of electrons, which makes the overall efficiency for H2-production rather low. Cyanobacterial nitrogenases typically contain molybdenum (Mo) and iron (Fe) in the active site, when sufficient amounts of Mo are available. Under Mo-deprived conditions the Monitrogenase is replaced by an alternative, vanadium-containing (V)-nitrogenase, and if V is limited, some microorganisms can synthesize a third alternative, a Fe- nitrogenase. Depending on the type of nitrogenase (Mo, V, or Fe) present, different amounts of reducing equivalents are allocated for N2-fixation and H2-pro- duction, see [38 ]. When the cell uses the V-nitrogenase, for instance, it only expends half as many electrons per H2 evolved, compared to when the Mo enzyme is used. Consequently, the alternative nitrogenases may be better H2 producers compared to the more common Mo-nitrogenases. The evolved H2 is rapidly consumed by an uptake hydrogenase, a NiFe-enzyme present in N2-fixing cyanobacteria. Since any produced H2 is reoxidized by the uptake hydrogenase, no net production is detected.