Spatiotemporal monitoring of intracellular metabolic dynamics by resonance Raman microscopy with isotope labeling.

Cellular metabolites are valuable in a diverse range of applications. For example, the unicellular green alga Haematococcus lacustris produces as a secondary metabolite the carotenoid pigment astaxanthin (AXT), which is widely used in nutraceutical, cosmetic, and food industries due to its strong antioxidant activity. In order to enhance the productivity of H. lacustris, spatial and temporal understanding of its metabolic dynamics is essential. Here we show spatiotemporal monitoring of AXT production in H. lacustris cells by resonance Raman microscopy combined with stable isotope labeling. Specifically, we incorporated carbon dioxide (13CO2) labeled with a stable isotope (13C) into H. lacustris cells through carbon fixation and traced its conversion to 13C-AXT using our resonance Raman microscope. We incubated H. lacustris cells under various conditions by switching, pulsing, and replacing 13CO2 and 12CO2. By measurement of these cells we determined the fixation time of 13C-carbon, visualized the intracellular localization of 13C- and 12C-AXTs, and revealed the dynamic consumption-production equilibrium of the accumulated AXT. This work is a valuable step in the development of effective screening criteria for high AXT-producing H. lacustris cells.