Quantitative Partitioning of Photoautotrophic and Photoheterotrophic Contributions During Mixotrophic Growth of Scenedesmus obliquus

ABSTRACT

Mixotrophic growth of microalgae involves the simultaneous assimilation of inorganic and organic carbon sources, but the quantitative contributions of photoautotrophic and photoheterotrophic metabolisms remain poorly resolved. In this study, a stoichiometric and data reconciliation framework was developed to partition mixotrophic metabolism of the microalga Scenedesmus obliquus into photoautotrophic and photoheterotrophic fluxes using macroscopic measurements of carbon, nitrogen and oxygen exchanges.

Continuous cultures were conducted under photoautotrophic, photoheterotrophic and mixo‐trophic conditions in a controlled photobioreactor. Elemental (C, N) and redox balances were applied to reconcile the experimental measurements before estimating metabolically consistent flux distributions.

Under the specific experimental conditions investigated, mixotrophic biomass productivity was consistent with the sum of the photoautotrophic and photoheterotrophic productivities. This observation did not provide evidence of a positive macroscopic synergistic effect, although run‐to‐run biological variability could not be assessed because each trophic mode was investigated in a single independent steady‐state experiment. Based on external fluxes, photoautotrophy was the dominant contributor to biomass formation under these conditions, accounting for approximately 73% of carbon assimilation. Under photoheterotrophic conditions, near‐zero net and fluxes were observed, suggesting efficient intracellular coupling between respiratory metabolism and photosynthetic carbon fixation.

Light availability emerged as a major determinant of biomass productivity in both photoautotrophic and mixotrophic cultures. Although differences in light attenuation between trophic modes affected the comparison of biomass productivities, the radiative‐transfer analysis showed that these differences alone could not explain the approximately additive relationship observed between mixotrophic, photoautotrophic and photoheterotrophic productivities. Overall, this work provides a quantitative framework for analyzing mixotrophic metabolism from macroscopic fluxes and offers new insights into carbon and oxygen partitioning in microalgal cultures, with implications for wastewater treatment and the optimization of photobioprocesses.