Exploring the Metabolic Flexibility of the Desert Alga Chlorella Ohadii Using O2 Exchange Patterns
Chlorella ohadii is a microalga isolated from the biological sand crusts in the southern deserts of Israel, one of the harshest environments on earth. This alga exhibits a multiphasic growth pattern under both extreme 3000 µmol photons m−2 s−1 (EIL) and low 100 µmol photons m−2 s−1 (LL) illumination levels. Previous studies suggest that shifts in metabolic processes drive the transitions between the growth phases described here. However, the precise mechanisms distinguishing these phases, especially in terms of photosynthetic fluxes, remain poorly characterized. We therefore aimed to identify differences in in vivo photosynthetic rates between the phases using a highly sensitive system.
Methods
O2 evolution rate was measured usinga FireSting optical oxygen meter. Each measurement was conducted under both in vivo light, i.e. under incident light, considering the shadowing in the vessel and saturated maximum light. For each measurement, 5 mL of the culture was sampled from a bioreactor, aerated by shaking in a 50 mL tube, and then transferred to a 5 mL quartz cuvette connected to an O₂ probe. To account for light respiration, O2 exchange was also measured using Membrane Inlet Mass Spectrometer (MIMS). The O2 metabolic chamber was fully controlled for temperature and illumination (Fig 1).
Fig 1. Growth curves of C. ohadii cell cultures grown in photobioreactors. Cell density (O.D.720nm, green), pH (purple) and Oxygen level (blue) are shown along the different growth phases
Fig 3: O2 metabolic chamber used for this study
Using the FireSting optical oxygen meter with robust oxygen probe, we were able to extract in vivo O2 evolution rate from C. ohadii different growth phases. Different light intensities and growth phases influenced oxygen dynamics in this alga. At lower light intensities, respiration exceeds photosynthesis, resulting in a net consumption of oxygen, whereas higher light intensities tend to promote positive net photosynthesis (FIG 2). The transition between growth phases also plays a role, with cultures adjusting their metabolic activity over time. Notably, in phase 1, both cultures grown at LL and EIL show significant variations in consumption rates, with higher consumption rate than in phase 2. Additionally, in phase 2, algae grown in LL exhibit oxygen consumption, while those grown EIL in P2 demonstrate net oxygen production.
Fig 2: Oxygen concentration of C. ohadii under various growth and lighting conditions and phases (n=3). The Y-axis represents oxygen concentration (O₂ [µM]), and the X-axis shows time (minutes). A and B display data for cultures grown at 100 and 3000 µmol photons m⁻² s⁻¹ during phase 1 (P1), respectively, while C and D show corresponding data for phase 2 (P2). Each condition involved two cycles: 4 minutes of darkness followed by 5 minutes of in vivo light exposure in the first cycle, and 4 minutes of darkness followed by 5 minutes of maximum light in the second cycle (n=3)
Conclusion
This study provides important insights into the metabolic responses of C. ohadii under varying light intensities and growth phases, particularly in relation to oxygen dynamics. Furthermore, O2 exchange rates measured with the FireSting optical oxygen meter were in qualitative agreement with MIMS data, yet the former provided a far more sensitive and robust readout
Ayla Atila and Haim Treves
Plant Metabolism Department, Rheinland-Pfälzische Technische Universität, Kaiserslautern-Landau
02/12/25