Short CommunicationDetoxification of ammonium to Nannochloropsis oculata and enhancement of lipid production by mixotrophic growth with acetate
Introduction
Nitrogen is one of the most important nutrients for microalgae. Though various forms of nitrogen (ammonium, nitrite, nitrate, and urea) are available for algal growth, it is widely assumed that ammonium is the preferential nitrogen source due to fewer biochemical steps and lower energetic requirement for ammonium assimilation (Escobar et al., 2006, Gutierrez et al., 2016). However, ammonium is not a universal nitrogen source either in lab-scale or in industrial-scale algal cultivation. Although some microalgae species selected from ammonium-rich wastewaters show strong tolerance to high ammonium (173 mg NH4+-N L−1 in Jiang et al., 2016; 893 mg NH4+-N L−1 in Nwoba et al., 2016), wastewater is often maintained at or diluted to low ammonium concentrations (<30 mg NH4+-N L−1) before feeding to microalgae (Cai et al., 2013, Ma et al., 2016). The limitation of ammonium to algal cultivation is likely due to the inhibition of photosynthesis by damaging photosystem or uncoupling photophosphorylation with electron transport (Gutierrez et al., 2016).
Ammonium inhibition has been reported in many algal species (Collos and Harrison, 2014). However, the lipid content of some microalgae was enhanced when ammonium was used as a nitrogen source, especially in the presence of acetate (Chandra et al., 2016, Gutierrez et al., 2016). Though there was no direct evidence of the enhancement effect of ammonium toxicity to microalgal lipid content, there is a strong correlation. In this study, we investigated the ammonium inhibition of the oleaginous microalga Nannochloropsis oculata, as well as the enhancement of both growth and lipid production by using acetate as carbon source and ammonium as a nitrogen source in mixotrophic cultivation. This work directly tests the assumption in microalgae derived biofuel production that using ammonium as nitrogen source enhances biomass and lipid production.
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Microalga strain and culture conditions
The microalga Nannochloropsis oculata GY-H14 was obtained from the Shanghai Guangyu Biological Technology Company (SGBT, http://www.leadingtec.cn). Microalgal cells were purified according to the following procedure: the algal culture was spread on agar plates containing 100 μg mL−1 ampicillin, colonies were picked and streaked on fresh ampicillin-agar plates, and the pick and streak procedure was repeated for three times. The purity of algal cells was assessed using microscopy and Luria-Bertani
Effects of organic substrates and nitrogen sources on algal growth and lipid production
The effects of different organic substrates and nitrogen sources on the growth and lipid production of N. oculata after 12 days cultivation are shown in Fig. 1. Growth inhibition was observed under mixotrophic cultivation with xylose and arabinose, and no growth was observed under heterotrophic cultivation with glucose, xylose, arabinose, galactose, and glycerol. Only acetate enhanced algal growth in either mixotrophic or in heterotrophic cultivations (Fig. 1a). In addition, acetate could
Conclusions
In this study, the toxicity of ammonium in N. oculata was reduced by using acetate as the carbon and energy source. Based on the ammonium toxicity, algal lipid production was significantly enhanced in mixotrophic condition. According to the results, we supposed that the acetate supplies carbon and energy sources for algal growth and lipid synthesis, while the ammonium probably induces the production of extra NADPH for lipid synthesis by uncoupling photophosphorylation with electron transport.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 21276099; No. 41301318; No. 41473072), the Specialized Research Found for the Doctoral Program of Higher Education of China (No. 20120172120045), and the Fundamental Research Funds for the Central Universities (No. 2015ZM171).
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