The increased demand for microalgae in various industries, from bio-based to healthcare products, has attracted interest in creating a new hybrid cultivation system. Current cultivation systems apply bubbling or waterflow to create fluid turbulence and improve microalgae growth. It is understood that a specific rate of stirring motion exerts stress on microalgal cells, increasing the reactive oxygen species (ROS) levels, which could damage cells. However, some positive hormetic responses due to microalgae’s defensive mechanisms have promoted growth. Combining both mechanisms within a single cultivation system may help to double the growth-promoting effect on microalgae. Moreover, including stirring mechanisms within the cultivation system enhances the microalgae's growth. It is worthwhile to study the threshold value of the stirring speed and bubbling rate without damaging the microalgal cell. The biomass, protein, and carbohydrate contents are optimum (4.335 g/L, 0.575mg/mg, and 0.117 mg/mg, respectively) when the stirring speed is 360 rpm under a constant bubbling rate. On the other hand, the optimum bubbling rate was determined when the bubbling rate was 15 v/v% of the cultivation system (1L), which was 150 cc/min. The biomass, protein and carbohydrate content concentrations are 5.229 g/L, 0.577 mg/mg, and 0.087 mg/mg, respectively. A mathematical model is synthesised better to represent the relationship between both mechanisms and microalgae growth. SEM imaging of microalgal cells at different stirring speeds showed that increased stirring speed from 0 to 420 rpm contributed to cell disruption. The cell damage was most severe at a stirring speed of 420 rpm, indicating that a higher mechanical stress acted on the cell. The upscaling of the microalgae cultivation system from 1 L of a laboratory-scale photobioreactor to 5 L of an upscaled photobioreactor showed eminent success, where the biomass was able to reach approximately 5 times the biomass weight of a laboratory-scale photobioreactor (5.283 g/L to 27.860 g/L). The upscaled cultivation stirring and bubbling rates were set at a rounded value of 350 and 400 rpm, and 15 % of the total volume of the cultivation system, respectively, as determined from previous experiments. A study on the possible inclusion of machine learning (ML) was conducted using different methods to address high-level problems such as salinity, temperature, and nutrient concentrations. The usefulness and feasibility of ML in microalgae cultivation were discussed and elaborated on, along with its potential utilisation in the industry. A discussion on the use of the optimal stirring rate and bubbling rate in a closed system was presented, together with the importance of a closed system in an automated system with machine learning. The final goal of these experiments was to study the feasibility of a closed system with stirring and bubbling automated by machine learning.
Lau Zhi Lin (Sat,) studied this question.