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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 facilitate the CO 2  mitigation couple with its starch-based biorefinery.    Electronic supplementary material  The online version of this article (10.1186/s13068-019-1523-7) contains supplementary material, which is available to authorized users.

biotechnology for biofuels

Application of an in situ CO2–bicarbonate system under nitrogen depletion to improve photosynthetic biomass and starch production and regulate amylose accumulation in a marine green microalga Tetraselmis subcordiformis