Growth medium recycling in Nannochloropsis sp. mass cultivation.

During cell division Nannochloropsis releases the thick and multilayered parent cell wall [Phycologia 35 (1996) 253]. The excretion of autoinhibitory substances in Nannochloropsis cultures has been also reported [J. Appl. Phycol. 11 (1999) 123]. Both wall remains and autoinhibitors may negatively affect culture growth and limit the recycling of the exhaust culture medium, a necessity in commercial microalgae plants to reduce production costs. The effect of medium recycling on growth and productivity of Nannochloropsis sp. cultures grown in 120 l annular reactors was investigated. The use of exhaust medium replenished with nutrients decreased significantly culture productivity. The partial removal of the cell walls alleviated, but did not solve the problem. In addition, medium recycling caused a massive formation of cell aggregates accompanied by a progressive deterioration of the culture. The structure of these aggregates was investigated by transmission electron microscopy. The images showed that the aggregates were held together by cell wall remains, which entrapped cells, bacteria and debris resulting from cell decay. Thus, in high density Nannochloropsis cultures, cell walls might play a key role in reducing productivity, favoring contamination and making the biomass unsuitable as aquaculture feed.