Abstract
BackgroundMicroalgae 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.ResultsLaboratory 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 m2 raceway pond, which was an over 50% increase compared to phototrophic cultivation. The bacterial number in SPMC (2.97 × 105 CFU ml−1) is comparable to that of the phototrophic cultivations.ConclusionsSPMC 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 m2 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.
Highlights
Microalgae are an important feedstock in industries
We showed a successful scale-up of the sequential phototrophic–mixotrophic cultivation (SPMC) in a 1000 m2 raceway pond, where an over 50% increase in biomass productivity was achieved
Further increases in acetate concentration inhibited the growth of Graesiella sp
Summary
Microalgae are an important feedstock in industries. Currently, efforts are being made in the non-phototrophic cultivation of microalgae for biomass production. Cultivation of microalgae in pilot-scale open ponds in the presence of organic carbon substrates has not yet been developed. The phototrophic growth of microalgae is technically feasible with open ponds and photo-bioreactors for commercial use; it has not yet been established to give satisfactory cell productivity in practice [6, 7]. To improve biomass and lipid productivity, phototrophic culture conditions have been modified in favor of cell growth and lipid accumulation. These conditions include illumination intensity [8], type and concentration of certain nutrition [9,10,11], type of reactor and mixing [4, 12].
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