Net oxygen production rates

In order to calculate the net oxygen production rates under different atmospheric conditions, the dissolved oxygen evolution over time was measured by a retractable fibre oxygen minisensor (OXR430, PyroScience, Germany) connected to a MINI-PAM-II (Walz, Germany) inside a constantly stirred measurement cuvette and recorded using the WALZ Wincontrol 3 software V3.28. Cultures were set up as for the open-culture setup in T₂₅ ventilated suspension flasks in buffered ASN-III with and without the addition of 120 µM Fe(II). The oxic cultures were then exposed to normal air with present-day atmospheric levels (PAL) of CO₂ or supplemented with 0.2% CO₂. The anoxic cultures in the open-culture system were exposed to an anoxic atmosphere with 0.2% CO₂. Measurements were conducted on 8-day old, mid-exponential phase cultures that were shaken to equilibrate to the growth atmosphere, before 400 µl was transferred into the suspension cuvette (KS-2500, Walz, Germany) of the MINI-PAM-II, which was also situated in the corresponding atmosphere. The Chl a content of 1 ml of culture was determined in parallel. Oxygen measurements were taken for 5 min using the standard growth light irradiance of 25 PPFD under their experimental atmospheric conditions. The difference between the start and end point measurements/readings of dissolved oxygen and the Chl a concentration was used to calculate the net oxygen production rates per mg Chl a per hour. The net oxygen production rate in the closed anaerobic growth vessels was determined from the oxygen measurements taken on day 5 as described in the section ‘Comparison of growth systems’.