Figure 2. Incubation setup for Experiment 2.

Water in the incubation bottle was initially labeled with ^18-18O₂. The labeled water was pumped with a flow rate of 0.73 mL min⁻¹ out of the bottle and across the membrane inlet of the mass-spectrometer. The water was replaced by degassed unlabeled water from a gas tight bag that was positioned at the upper edge of the pressure port to keep a slight overpressure. Helium was used as carrier gas for stable and fast mass detection.