Fig. 4.

Nitrogenase activity after a pre-incubation under air or an argon atmosphere.

Microbial mats were pre-incubated at approximately 50°C for 10 h under air or an argon atmosphere. After pre-cultivation, acetylene-reducing activity was measured under argon gas phase with the addition of 0% or 5% O₂ without (A) or with (B) molybdate (20 mM). Three experimental results under each condition are shown in individual bars in a 20-mL vial. Note that the vertical scale differs between Figs. 4A and 4B.