A growth-boosting synergistic mechanism of Chromochloris zofingiensis under mixotrophy

Abstract

The mixotrophic cultivation approach, consisting of heterotrophy and photoautotrophy, has been suggested as a promising strategy to reach a high growth rate and biomass productivity. However, the underlying growth-boosting mechanisms remain elusive. In this study, Chromochloris zofingiensis, a promising high-value-added astaxanthin producer, was selected to elucidate how mixotrophy benefits cellular growth. Results showed that mixotrophy yielded the highest biomass, which was significantly higher than the sum of photoautotrophic and heterotrophic cultivation, indicating a significant growth-boosting effect. The chlorophyll fluorescence and transcriptomics results confirmed that the highest biomass production under mixotrophy was attributed to a higher photosynthesis performance, decreased light damage and photorespiration, as well as significantly upregulated glucose metabolism compared to photoautotrophy. Compared to heterotrophy, mixotrophy consumed less glucose and had a lower glucose metabolic efficiency, but led to higher biomass yield. Taken together, we propose a synergistic growth-promoting mechanism between photosynthesis and glucose metabolism under mixotrophy, in which cells not only directly benefit from extra energy from exogenous organic carbon metabolism but also the reutilization of the byproduct CO2 for photosynthesis. In return, O2 evolved from photosynthesis was consumed by glucose metabolism as well. Furthermore, the coexistence of photosynthesis and glucose metabolism contributes to higher energy usage efficiency and biomass yield, correspondingly diminishing the waste and decreasing the damage. This study provides insights into the synergistic mechanism under mixotrophy and helps to better select growth strategies for commercially valuable algal species.