A new approach to quantify system efficiency with dissolved oxygen isotopes during engineered growth of Galdieria sulphuraria

The microalgae Galdieria sulphuraria was grown in a first technical approach in a closed system experiment for 29 days. In this time period, 6 separate flasks were sampled for oxygen concentration and their stable isotope ratios (18O/16O) in dissolved and headspace phases. The oxygen isotope composition of the water was also analysed as an input for the transfer of its 18O/16O ratio to molecular oxygen via production by algae. This photosynthetic transfer of the isotope composition of water was counterbalanced by oxygen consumption that enriched both phases in 18O. For this reason, neither dissolved oxygen nor oxygen in the headspace reached the 18O-depleted oxygen isotope ratio of H2O despite excessive photosynthesis. Oxygen that was produced by photosynthesis accumulated with a yield of 13.96 mmol L− 1 in the headspace and with 0.5 mmol L− 1 in the fluid phase. This difference was due to rapid degassing of the solution. It was further amplified by preferential consumption of the dissolved O2 phase. In order to quantify oxygen production and its consumption we determined photosynthesis/respiration (P/R) ratios with a formula that combined O2 concentrations and its isotope ratios. It revealed a P/R ratio of 7.7 after 11 days. After this it decreased again and moved towards dominance of respiration. With this work our results introduce a new method to monitor the growth and efficiency of algae in controlled experiments.