A low temperature and high CO2 concentration mediated transition of Trebouxia corticola from growth phase to lipid accumulation phase in an airlift photobioreactor

An indigenous phycobiont, Trebouxia corticola, was evaluated for its response to different temperatures and
carbon dioxide (CO2) concentrations in an airlift photobioreactor. Exposure to 10% (v/v) CO₂ significantly
increased the specific growth rate, biomass concentration, and productivity by 1.79-, 1.20-, and 2.0-fold,
respectively, relative to the control. Low temperature (7◦C) and elevated CO₂ (30% v/v) independently
increased lipid content by 1.94 (51.46% w/w) and 1.74-fold (46.11% w/w), respectively, compared to the
control, although biomass yield decreased. To reconcile biomass and lipid production, a two-phase cultivation
strategy was employed: Phase I (days 0–5; 10% CO₂) for biomass, followed by Phase II (days 6–9, 7◦C with 30%
CO₂) for lipid production. This approach increased lipid content by 1.69-fold (54% w/w) and doubled relative
α-linolenic acid and eicosapentaenoic acid content in biomass compared to Phase I, without compromising
biomass concentration achieved in Phase I. Decreased carbohydrates and photosynthetic efficiency, alongside
elevated reactive oxygen species (ROS) and malondialdehyde in Phase II biomass, suggested ROS-mediated
carbon flux diversion towards lipid synthesis. Metabolomic analysis suggested reprogramming of central carbon
metabolism and redox homeostasis, including activation of the tricarboxylic acid cycle, pantothenate/CoA,
branched-chain amino acid, and the γ-aminobutyric acid shunt. Overall, the two-phase cultivation approach
facilitated a seamless transition of T. corticola from growth to lipid accumulation, supporting its potential for
microalgal lipid production.