Cultivation of Microalgae

According to the different nutrient modes of microalgae, the culture modes can be divided into photoautotrophic culture, mixed culture and heterotrophic culture.

Photoautotrophic culture

Photoautotrophy is a natural way of nutrition for microalgae. Photosynthetic autotrophic growth of microalgae is affected by many environmental factors, including nutrient conditions, light, temperature, pH and aeration conditions. At present, the main methods for achieving high-density photoautotrophic culture of microalgae include optimizing microalgae culture medium and culture conditions, using different dilution ratios of fed to avoid substrate inhibition, and developing photobioreactor with high light energy transfer efficiency. By adding feed to the culture system, the concentration of nutrients in the culture solution can be kept within a certain range for a long time, which can ensure the growth of microalgae without the inhibition of the substrate, thereby improving the yield of microalgae.

Mixed culture and heterotrophic culture

Most microalgae are photoautotrophic organisms, but some microalgae have the ability to utilize organic carbon sources. With the continuous discovery of the growth of microalgae using organic matter, people gradually know that microalgae have certain heterotrophic ability. In fact, many natural environments in which microalgae live, such as soil, water, and especially wastewater, contain a large amount of organic matter. From the physiological point of view, most microalgae should have the ability to grow heterotrophically. The growth process of microalgae using organic matter mainly has two forms: one is that microalgae can use organic matter to carry out heterotrophic growth under completely dark conditions, and there is no difference between the general chemical heterotrophic organisms; another form is that microalgae using organic matter to carry out heterotrophic growth under certain lighting conditions, and this method of utilizing both light energy and organic matter is called mixed heterotrophic culture. Compared with photoautotrophic culture, heterotrophic culture and mixed heterotrophic culture have the advantages of fast growth, weak dependence on light or no need of illumination, which is beneficial to high-density culture of microalgae.

According to the different culture methods, the culture of microalgae can be divided into batch culture, fed-batch culture, semi-continuous culture and continuous culture.

Batch culture

The batch culture of microalgae has the advantages of simple operation and low cost, and is a commonly used cultivation method in the laboratory. In recent years, the use of microalgae to treat sewage is also one of the research hotspots on microalgae cultivation. The main purpose of treating sewage with microalgae is to remove pollutants such as nitrogen, phosphorus or heavy metals from the sewage. It is possible to reduce the pollutants in the sewage to a very low level by culturing the microalgae. In this case, microalgae in batch culture can effectively remove contaminants, and its ability to remove divalent metal ions is also unmatched by microalgae in other culture modes.

Fed-batch culture

The fed-batch culture refers to a culture method in which one or more nutrients are added to the reactor intermittently or continuously during the culture process. The basic feature of this method is to control the concentration of nutrients in the reactor by changing the rate of nutrient addition. In microalgae culture, carbon, nitrogen and phosphorus are essential nutrients, but for some algae species, excessive concentration of CO2, ammonia or phosphorus will inhibit or toxic the growth of microalgae. The fed-batch culture mode can effectively relieve the inhibitory effect of nutrients and maintain the suitable culture environment for the growth of microalgae. Compared with batch culture, fed-batch culture can better control the concentration of nutrients in the culture solution, significantly increase the biomass of microalgae and the yield of the final target product, and can effectively improve the utilization rate of nutrients.

Semi-continuous culture

Semi-continuous culture refers to a method of harvesting a certain amount of algae liquid after the algae cells reach a certain concentration, and then adding an equal amount of the culture solution to continue the cultivation. Semi-continuous culture is not only widely used for large-scale culture, but also a culture mode commonly used in the laboratory research of microalgae. In the semi-continuous culture process, the fresh culture medium is used to replace the same amount of the original culture liquid, so that the nutrient content in the culture liquid is increased, the biological density is decreased, and the light transmittance is increased, and therefore the photosynthesis efficiency of the algae is enhanced, and the growth rate of algae is increased. The semi-continuous culture mode has proven to be one of the best culture methods for producing biodiesel on a scale by using microalgae.

Continuous culture

Continuous culture refers to a culture method in which fresh culture solution is continuously added to a culture system at a constant flow rate while flowing out of the culture solution at the same rate so that the growth environment of the cells in the reactor is at a constant state. This culture method makes the growth rate and metabolic activity of the cells relatively constant, thereby achieving the purpose of stably and rapidly culturing the microalgae or producing a large amount of metabolites. Therefore, continuous culture plays a role that cannot be replaced by other models, both in the high-volume production of microalgae powder and in the stable production of certain important metabolites.