Biodiesel is a kind of renewable and environment-friendly energy, which is limited by the shortage of raw materials. Microalgae, with the merits of fast growth, high lipid content and without competing for arable land, are thus considered to be ideal raw materials for biodiesel production. However, usually microalgae growth and lipid accumulation do not run in parallel throughout cultivation, which necessarily lowers overall lipid productivity. Hence, the realizing of simultaneous microalgal growth and lipid production is of great importance for commercial production of biodiesel from microalgae biomass.  

PhD student WEN Xiaobin, supervised by Prof. LI Yeguang from Wuhan Botanical Garden, realized the one-step lipid production with continuous cultivation of oleaginous microalgae by properly manipulating specific nitrate input rate (SNI). This method provided an alternative way to produce lipids by using simultaneous microalgae growth and lipid accumulation, thus leading to higher lipid productivity.    

Batch and feed-batch cultures of Chlorella pyrenoidosa XQ-20044 were carried out at different nitrate concentrations. Results showed the existence of certain conditions under which high lipid content could be achieved without sacrificing cell growth. Steady-state studies of C. pyrenoidosa XQ-20044 were then carried out using a chemostat system with constant medium composition, illumination and air flow rate, but with dilution rates that varied between 0.24 and 2.4 d^-1. Steady states were achieved with all the tested dilution rates. When the dilution rate was in the range of 0.48-1.44d^-1, all the supplied nitrate was assimilated by cells and the lipid productivity was significantly improved.    

Proper SNI was the key for simultaneous algal growth and lipid accumulation. The optimal SNI rate for the concurrent growth and lipid accumulation was in the range of 0.78–4.56 mmol g^-1d^-1 for C. pyrenoidosa XQ-20044 under experimental conditions.  

Enhanced lipid productivity (144.93 mg L^-1d^-1) demonstrates the feasibility of one-step production of microalgal lipids by continuous culture, which provides a new strategy in development of microalgal biodiesel. Also, to the best of our knowledge, this is the highest lipid productivity so far recorded in chemostat study for microalgal lipid production. 

This work was supported by Ministry of Science and Technology of China and National Natural Science Foundation of China. The paper entitled “Enhanced lipid production in Chlorella pyrenoidosa by continuous culture” was published in Bioresource Technology (doi:10.1016/j.biortech.2014.03.077).