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Microalgae cultivation for being a biological feedstock of high-value products has long depended on culture conditions and various important factors. The objective of this research was to investigate the effects of nitrogen to phosphorus molar ratios on the growth and fatty acid accumulation of a freshwater microalga Chlorococcum humicola (TISTR 8551). The microalgae having an initial density of 2.00x106 cell mL-1 was cultivated in a bubble column photobioreactor with a working volume of 10 L under batch condition for 14 days. The growth broth employed was modified BG-11 medium with the initial molar ratios of nitrogen to phosphorus (N:P) of 50:1 (GM-100%N), 5:1 (GM-10%N), and 2.5:1 (GM-5%N). It was found that the growth and fatty acid of the microalgae had different responses to a variation of the initial nitrogen concentration of the growth medium. The microalgae cultivated in the GM-100%N provided maximum biomass productivity of 0.77 ± 0.14 g L-1 d-1 and a specific growth rate () of 0.139 d-1. The figure of obtained was higher than those cultivated in the GM-10%N and GM-5%N with 69.51 and 98.57%, respectively, because the GM-100%N had enough concentrations of nitrogen for the microalgae to synthesize protein and chlorophyll. Then, these compounds were used by the microalgae for photosynthesis and cell division resulted in lower activities of fatty acid synthesis and accumulation than those cultivated in the growth medium having relatively lower initial N:P molar ratios. On the other hand, the microalgae cultivated in the GM-5%N gave the highest cumulative fatty acid content of 30.6 ± 1.2 %DCW. This was because the GM-5%N provided a higher degree of nitrogen starvation. Therefore, the microalgae reduced protein and chlorophyll synthesis activities and turned to increase fatty acid synthesis and accumulation in cells to prevent cell damage and degradation from light exposure during the cultivation period.
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