| Summary: | Microalgae have a growing recognition of generating biomass and capturing carbon in the form of CO<sub>2</sub>. The genus <i>Chlorella</i> has especially attracted scientists’ attention due to its versatility in algal mass cultivation systems and its potential in mitigating CO<sub>2</sub>. However, some aspects of how these green microorganisms respond to increasing concentrations of CO<sub>2</sub> remain unclear. In this work, we analyzed <i>Chlorella sorokiniana</i> and <i>Chlorella vulgaris</i> cells under low and high CO<sub>2</sub> levels. We monitored different processes related to carbon flux from photosynthetic capacity to carbon sinks. Our data indicate that high concentration of CO<sub>2</sub> favors growth and photosynthetic capacity of the two <i>Chlorella</i> strains. Different metabolites related to the tricarboxylic acid cycle and ATP levels also increased under high CO<sub>2</sub> concentrations in <i>Chlorella sorokiniana</i>, reaching up to two-fold compared to low CO<sub>2</sub> conditions. The signaling molecules, inositol polyphosphates, that regulate photosynthetic capacity in green microalgae were also affected by the CO<sub>2</sub> levels, showing a deep profile modification of the inositol polyphosphates that over-accumulated by up to 50% in high CO<sub>2</sub> versus low CO<sub>2</sub> conditions. InsP<sub>4</sub> and InsP<sub>6</sub> increased 3- and 0.8-fold, respectively, in <i>Chlorella sorokiniana</i> after being subjected to 5% CO<sub>2</sub> condition. These data indicate that the availability of CO<sub>2</sub> could control carbon flux from photosynthesis to carbon storage and impact cell signaling integration and energy levels in these green cells. The presented results support the importance of further investigating the connections between carbon assimilation and cell signaling by polyphosphate inositols in microalgae to optimize their biotechnological applications.
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