Sequential phototrophic–mixotrophic cultivation of oleaginous microalga Graesiella sp. WBG-1 in a 1000 m(2) open raceway pond

BACKGROUND: Microalgae are an important feedstock in industries. Currently, efforts are being made in the non-phototrophic cultivation of microalgae for biomass production. Studies have shown that mixotrophy is a more efficient process for producing algal biomass in comparison to phototrophic and heterotrophic cultures. However, cultivation of microalgae in pilot-scale open ponds in the presence of organic carbon substrates has not yet been developed. The problems are heterotrophic bacterial contamination and inefficient conversion of organic carbon. RESULTS: Laboratory investigation was combined with outdoor cultivation to find a culture condition that favors the growth of alga, but inhibits bacteria. A window period for mixotrophic cultivation of the alga Graesiella sp. WBG-1 was identified. Using this period, a new sequential phototrophic–mixotrophic cultivation (SPMC) method that enhances algal biomass productivity and limits bacteria contamination at the same time was established for microalgae cultivation in open raceway ponds. Graesiella sp. WBG-1 maximally produced 12.5 g biomass and 4.1 g lipids m(−2) day(−1) in SPMC in a 1000 m(2) raceway pond, which was an over 50% increase compared to phototrophic cultivation. The bacterial number in SPMC (2.97 × 10(5) CFU ml(−1)) is comparable to that of the phototrophic cultivations. CONCLUSIONS: SPMC is an effective and feasible method to cultivate lipid-rich microalgae in open raceway ponds. Successful scale-up of SPMC in a commercial raceway pond (1000 m(2) culture area) was demonstrated for the first time. This method is attractive for global producers of not only lipid-rich microalgae biomass, but also astaxanthin and β-carotene. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s13068-019-1367-1) contains supplementary material, which is available to authorized users.