Abstract
In this work, we investigated the molecular basis of autotrophic vs. mixotrophic growth of Chlorella sorokiniana, one of the most productive microalgae species with high potential to produce biofuels, food and high value compounds. To increase biomass accumulation, photosynthetic microalgae are commonly cultivated in mixotrophic conditions, adding reduced carbon sources to the growth media. In the case of C. sorokiniana, the presence of acetate enhanced biomass, proteins, lipids and starch productivity when compared to autotrophic conditions. Despite decreased chlorophyll content, photosynthetic properties were essentially unaffected while differential gene expression profile revealed transcriptional regulation of several genes mainly involved in control of carbon flux. Interestingly, acetate assimilation caused upregulation of phosphoenolpyruvate carboxylase enzyme, enabling potential recovery of carbon atoms lost by acetate oxidation. The obtained results allowed to associate the increased productivity observed in mixotrophy in C. sorokiniana with a different gene regulation leading to a fine regulation of cell metabolism.
Highlights
Photosynthetic conversion of light provides the energy necessary for biomass formation in living organisms
2 Kcal/L increase of energy stored as biomass was obtained as a consequence of the 3.48 Kcal/L added to the growth medium as acetate, indicating a metabolic energetic efficiency of acetate utilization of at least ~57%, close to the energetic yield of acetyl-CoA oxidized by the Krebs cycle previously reported in the case of C. reinhardtii[5]
Photosynthetic traits were not significantly affected in mixotrophy compared to autotrophy: upregulation of genes coding for electron acceptors downhill plastoquinones pool as plastocianine, ferrodoxin and Ferredoxin-NADP+ reductase (FNR), and downregulation of PGR5-like subunit suggests increased electron transport from plastoquinones to NADPH and reduced cyclic electron transport across PSI
Summary
Photosynthetic conversion of light provides the energy necessary for biomass formation in living organisms. Even in the case of non-model species as Chlorella spp. or Scenedesmus, mixotrophic growth is effective to increase the biomass and lipid productivity[3,6,7]. This is not a general feature of microalgae, since some species, as the marine algae Nannochloropsis gaditana, exhibited similar growth in autotrophy and in presence of different reduced carbon source, due to a reduced photosynthetic efficiency in mixotrophy[8]. In this work autotrophic growth of the thermotolerant high productive strain Chlorella sorokiniana was compared to its mixotrophic growth in the presence of acetate as reduced carbon source in the medium. To investigate the influence of mixotrophic growth in presence of acetate in C. sorokiniana, photosynthetic properties and differential gene expression of autotrophic vs. mixotrophic cultures were analyzed in order to identify strategy to further foster the metabolism and improve biomass production for industrial applications
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