Flocculation of Nannochloropsis sp induced by sodium hydroxide: flocculation efficiency, viability effect on microalgae and their use as food for rotifers

Aquaculture laboratories highly dependent of Nannochloropsis sp. develop mechanisms for mass production, with the challenge of be able to extract high cell concentrations from the culture medium. Flocculation by alkaline agents is an alternative that promises greater advantages, and sodium hydroxide may be the most convenient. However, to achieve maximum efficiency of flocculation are important the amount of flocculent, exposure time, cell concentration and type of microalgae. Therefore, the amount of 0.5N NaOH and the exposure time required are investigated to Nannochloropsis sp., harvests at high density at 50x10(6) cells mL(-1). The results show that with 17 mL NaOH 0.5N per liter of culture of microalgae can be achieved 94.9% flocculation efficiency at pH 9.7 in less than 60 min. Supernatant cells (byproduct of the process) were viable as inoculum of a new culture of microalgae. Microalgae concentrate (355.7x10(6) cells mL(-1), dry weight 2.03 g L-1) produced was adequate as food for rotifers. Therefore, employment of 0.5N NaOH is recommended as a simple, convenient and inexpensive method for producing Nannochloropsis sp. concentrates for aquaculture purposes.