**2. Attributes of microalgae**

#### **2.1 Cultivation of microalgae**

Cultivation of microalgae for biodiesel production aims at maximizing the lipid productivity along with the growth rate of the microalgae. In the batch cultivation system, microalgae are exponentially grown in the log phase to increase their biomass and then they subjected to a starvation phase by omitting or limiting the nutrient supply towards the end of the stationary phase of growth. As, algal oil is a secondary metabolite, so nutrient deprivation can lead to a higher yield.

The two most common methods of microalgae cultivation are open cultivation systems and controlled closed cultivation systems.

Open-air cultivation systems comprise natural or artificial ponds, raceway ponds, and the inclined surface systems driven by paddle wheels, usually operating at water depths of 15–30 cm [19]. They represent the classical processes used for production

## *Microalgae: An Exquisite Oil Producer DOI: http://dx.doi.org/10.5772/intechopen.104895*

of algal biomass. Although different types of open reactors have been studied since last few decades by different research groups, but the most commonly used systems are shallow big ponds, tanks, circular ponds, and raceway ponds. Some of the major advantages of an open cultivation system are minimal capital, operating costs, and lower energy requirement for culture mixing. The disadvantages are open systems require large areas to scale up, susceptibility to contamination (by birds, small insects and rotifers), adverse weather conditions, difficulty to regulate growth parameters viz. evaporation rate, culture temperature, etc. A scientific investigation reported the damaging effects of the occurrence of rotifers to the cultures of *Tetraselmis, Chlorella, Nannochloropsis*, *Scenedesmus*, and the damage caused by amoeba to diatoms.

On the other hand, closed and controlled cultivation systems employ photobioreactors to attain axenic single-species culture of microalgae. Photobioreactors are successfully used for producing large quantities of microalgal biomass [20]. The different types of photobioreactor (PBR) include horizontal or serpentine tube, flat-plate, bubble column, airlift column and stirred tank. PBRs can be designed and calibrated according to the research need and the experimental organism. This closed system utilizes relatively little space, while increasing the light availability and minimum contamination issues. However, the limitations of PBRs include bio-fouling, overheating, benthic algal growth, cleaning issues, growth limitation due to high build-up of dissolved oxygen and costlier operation [21, 22].
