Optimization of intermittent CO2 supply and flocculation recovery for the cultivation of a haptophyte Isochrysis galbana

創価大学博士（工学）The demand for high fucoxanthin and fatty acids content in Isochrysis galbana is rising year by year, therefor, the low-cost cultivation　and harvesting in large-scale has become an indispensable issue. Microalgal cultivation is influenced by several factors such as solar　irradiance, nutrient availability, agitation, and CO2 supply condition. Continuous aeration for CO2 supply occupies a considerable　portion of total energy consumption for microalgae cultivation. While the intermittently CO2 supply has a potential to reduce aeration cost, it may cause a carbon starvation and inhibit the grow of microalgae. Microalgal harvesting also a challenge due to its small cell size. There is a method of flocculation that reduces the cost of harvesting algae by simply adding flocculant and adjusting the pH. Therefore, firstly, this study investigated the effects of concentration and frequency of CO2 supply on productivity of marine microalgae I. galbana. Semi-continuous cultivation of I. galbana was conducted under different CO2 concentration and frequency (0.04 -10% CO2 and continuous, 1/9 minutes of supply; Study 1). Secondly, the effects of using different pH values (between pH 8 and 10) and PO4 (Ca2+) concentrations (between 0 and 10 mM) on microalgal sedimentation were investigated by evaluating the sedimentation efficiency of microalgae cells (Study 2). As a result, under air supply conditions, the biomass productivity reached 0.17 g L-1 d-1 with continuous supply(control). The 5% continuous condition showed highest biomass productivity (0.35 g L-1 d-1). The productivity was only 0.11 g L-1 d−1 in continuous CO2 supply with its concentration of 10%, indicating that a higher DIC concentration also would inhibit the growth of microalgae. On the other hand, the condition with CO2 concentration of 5% and 10% showed 0.3 and 0.22 g L-1 d-1 of productivity in intermittent CO2 supply of 1/9min/min, respectively. These productivities were higher than control. In addition, under air supply conditions, a higher fucoxanthin content was achieved 0.61 mg/g-DW with continuous supply, than 5% and 10% (0.18 and 0.11 mg/g-DW), respectively. However, the fucoxanthin content reached 0.56 mg/g-DW under 5% continuous supply with 1/9 min/min agitation frequency. This result suggests that continuous supply of high concentration of CO2 inhibited the accumulation of fucoxanthin, but intermittent supply was beneficial to the accumulation of fucoxanthin. On the other hand, Fast and efficient sedimentation occurred (within 30 min) at a high PO4 concentration (5,10 mM) at pH 10, the sedimentation rate was achieved 40%.doctoral thesi