Abstract
BackgroundAlthough outdoor cultivation systems have been widely used for mass production of microalgae at a relatively low cost, there are still limited efforts on outdoor cultivation of carbohydrate-rich microalgae that were further used as feedstock for fermentative bioethanol production. In particular, the effects of seasonal changes on cell growth, CO2 fixation, and carbohydrate production of the microalgae have not been well investigated.ResultsThis work demonstrates the feasibility of using outdoor tubular photobioreactors (PBR) for whole-year-round cultivation of a carbohydrate-rich microalga Scenedesmus obliquus CNW-N in southern Taiwan. Time-course profile of the carbohydrate content under nitrogen-deficient conditions was monitored to assess the seasonal changes. The optimal CO2 fixation rate and carbohydrate productivity were 430.2 mg L−1 d−1and 111.8 mg L−1d−1, respectively, which were obtained during the summer time. Under nitrogen starvation, the microalgal biomass can accumulate nearly 45–50% of carbohydrates, mainly composed of glucose that accounted for 70–80% of the total carbohydrates in the microalgal cells. This glucose-rich microalgal biomass is apparently a very suitable carbon source for bioethanol fermentation.ConclusionThis work shows the feasibility of combining CO2 fixation and bioethanol production using microalgae grown in outdoor photobioreactors as feedstock. The understanding of the seasonal changes in the carbohydrate productivity makes this approach more practically viable. The novel strategy proposed in this study could be a promising alternative to the existing technology dealing with CO2 mitigation and biofuels production.
Highlights
Outdoor cultivation systems have been widely used for mass production of microalgae at a relatively low cost, there are still limited efforts on outdoor cultivation of carbohydrate-rich microalgae that were further used as feedstock for fermentative bioethanol production
The tolerance of the focal species with regard to the local ambient temperature needs to be considered with regard to the outdoor cultivation process, with seasonal changes taken into account, as this may significantly influence both cell growth and CO2 fixation [25]
The indigenous carbohydrate-rich microalga S. obliquus CNW-N, with the optimal culture conditions determined in a previous work [26], was grown at various temperatures simulating the different seasons in Taiwan
Summary
Outdoor cultivation systems have been widely used for mass production of microalgae at a relatively low cost, there are still limited efforts on outdoor cultivation of carbohydrate-rich microalgae that were further used as feedstock for fermentative bioethanol production. The effects of seasonal changes on cell growth, CO2 fixation, and carbohydrate production of the microalgae have not been well investigated. Microalgae have recently been proposed as a “third generation feedstock” for bioethanol production with the favorable characteristics of a fast growth rate and high CO2 fixation ability, while some species can accumulate large amounts of carbohydrates (mainly, starch and cellulose) that are suitable for bioethanol fermentation [5, 8]. To solve the above risks, some studies have demonstrated that using closed photobioreactor (PBR) for microalgae cultivation is an effective and promising method due to its higher light regime, higher culture stability, higher CO2 fixation ability, and lower contamination risks compared with open systems (e.g., open pond and raceway pond) [11]. There remains limited information regarding the mass production of most microalgae species in outdoor cultivation systems, with regard to long-term outdoor cultivation [14, 15]
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