Monitoring Photosynthetic Activity in Microalgal Cells by Raman Spectroscopy with Deuterium Oxide as a Tracking Probe

Microalgae offer great potential for the production of biofuel, but high photosynthetic activity is demanded for the practical realisation of microalgal biofuels. To this end, it is essential to evaluate the photosynthetic activity of single microalgal cells in a heterogeneous population. In this study, we present a method to monitor the photosynthetic activity of microalgae (in particular Euglena gracilis, a microalgal species of unicellular, photosynthetic, flagellate protists as our model organism) at single-cell resolution by Raman spectroscopy with deuterium from deuterium oxide (D₂O) as a tracking probe. Specifically, we replaced H₂O in culture media with D₂O up to a concentration of 20 % without disturbing the growth rate of E. gracilis cells and evaluated C−D bond formation as a consequence of photosynthetic reactions by Raman spectroscopy. We used the probe to monitor the kinetics of the C−D bond formation in E. gracilis cells by incubating them in D₂O media under light irradiation. Furthermore, we demonstrated Raman microscopy imaging of each single E. gracilis cell to discriminate deuterated cells from normal cells. Our results hold great promise for Raman-based screening of E. gracilis and potentially other microalgae with high photosynthetic activity by using D₂O as a tracking probe.