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Background Microalgal starch is regarded as a promising alternative to crop-based starch for biorefinery such as the production of biofuels and bio-based chemicals. The single or separate use of inorganic carbon source, e.g., CO 2 and NaHCO 3 , caused aberrant pH, which restricts the biomass and starch production. The present study applied an in situ CO 2 –NaHCO 3 system to regulate photosynthetic biomass and starch production along with starch quality in a marine green microalga Tetraselmis subcordiformis under nitrogen-depletion (−N) and nitrogen-limitation (±N) conditions. Results The CO 2 (2%)–NaHCO 3 (1 g L −1 ) system stabilized the pH at 7.7 in the −N cultivation, under which the optimal biomass and starch accumulation were achieved. The biomass and starch productivity under −N were improved by 2.1-fold and 1.7-fold, respectively, with 1 g L −1 NaHCO 3 addition compared with the one without NaHCO 3 addition. NaHCO 3 addition alleviated the high-dCO 2 inhibition caused by the single CO 2 aeration, and provided sufficient effective carbon source HCO 3 − for the maintenance of adequate photosynthetic efficiency and increase in photoprotection to facilitate the biomass and starch production. The amylose content was also increased by 44% under this CO 2 –bicarbonate system compared to the single use of CO 2 . The highest starch productivity of 0.73 g L −1 day −1 under −N cultivation and highest starch concentration of 4.14 g L −1 under ±N cultivation were both achieved with the addition of 1 g L −1 NaHCO 3 . These levels were comparable to or exceeded the current achievements reported in studies. The addition of 5 g L −1 NaHCO 3 under ±N cultivation led to a production of high-amylose starch (59.3% of total starch), which could be used as a source of functional food. Conclusions The in situ CO 2 –NaHCO 3 system significantly improved the biomass and starch production in T. subcordiformis . It could also regulate the starch quality with varied relative amylose content under different cultivation modes for diverse downstream applications that could promote the economic feasibility of microalgal starch-based biofuel production. Adoption of this system in T. subcordiformis would faci...

Microalgal starch, a potential feedstock for renewable fuels and chemicals, is usually induced under stress, which hinders the simultaneous enhancement of starch content and yield. This study applied γ-aminobutyric acid (GABA) to modulate cell metabolism for improving photosynthetic starch production in microalga Tetraselmis subcordiformis under nitrogen limitation (±N) and depletion (−N). Under ±N, 2.5 mM GABA abated nitrogen metabolism but enhanced starch content by 27% with an uncompromised starch yield due to the moderate inhibition of cell growth and elevated photoprotection. Under −N, 10 mM GABA entitled improved stress tolerance and enhanced starch content by 23.4% with a simultaneous 28.6% promotion of starch yield. The maximum starch content and yield under this condition reached 69.0% DW and 1.84 g L −1 , respectively, which represented the highest starch levels in this microalga and exceeded most of the starch production performance among the microalgae reported. GABA addition was promising to improve microalgal starch production.

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