| Microalgae are a group of lower plants with the capacity of photosynthesis, widely distributed in marine, freshwater, soil and other environments. They have higher light energy conversion efficiency, shorter growth cycle, do not occupy arable land and also can synthesize and accumulate large amounts of triacylglycerol(TAG), so the development of microalgae biomass as energy raw material, have an unparalleled advantage. In this paper, Pseudochlorococcum polymorphum UTEX 1791 and Pseudochlorococcum typicum UTEX 1792 are choosed as experiment materials, using BG-11 medium, This paper discusses the characteristics and laws of two algae's growth and lipid accumulation under different sodium nitrate concentrations and light conditions. The results showed:(1) The sodium nitrate concentrations of 0.3 to 1 g/L are suitable for the growth of Pseudochlorococcum polymorphum UTEX 1791, the final dry weights are about 3.7 g/L; Pseudochlorococcum typicum UTEX 1792 culturing in 1 g/L sodium nitrate concentration is growing best, gets the dry weight 4.35 g/L. The chlorophyll a and total lipid content have a similar representations. In the initial, the chlorophyll a increased, but the no total lipid content has no significant growth, in culture-medium, with the decline of chlorophyll a, the total lipid content increased in differet degrees, the 0.3 g/L group have a significantly increase, UTEX 1791 increased by 11.65%, UTEX 1792 increased by 24.99%. The two green algae in 0.3 g/L low nitrogen condition get the highest total lipid content, UTEX 1791 is 26.28%, UTEX 1792 is 44.35%, the largest maximum lipid productivity in this condition were 0.98g/L and 1.61 g/L.(2) Grown in 3 cm and 6 cm light path reactors 140μmol photons/m2·s light intensity, the chlorophyll a concentration of the two green algae increased first and then decreased, the biomass and total lipid content of Pseudochlorococcum polymorphum UTEX 1791 are both lower than Pseudochlorococcum typicum UTEX 1792; the biomass and lipid content of the two green algae obtained under 3 cm light path reactors are both higher than the 6 cm light path group. the total lipid content of Pseudochlorococcum polymorphum UTEX 1791 is 35.69%, Pseudochlorococcum typicum UTEX 1792 is 42.86%. The maximum lipid productivity are 1.63 g/L, and 1.94 g/L; at 0.3 g/L low N conditions, the total lipid content of Pseudochlorococcum polymorphum UTEX 1791 is 46.49%, Pseudochlorococcum typicum UTEX 1792 is 52.03%, The maximum lipid productivity are 2.01 g/L and 1.81 g/L.(3) The light intensity of 350μmol photons/m2·s has a facilitation on the growth and total fat accumulation of the two algae, the final biomass and total lipid content was higher than 140μmol photons/m2·s group; the final total lipid content of Pseudochlorococcum polymorphum UTEX 1791 under both light conditions is lower than Pseudochlorococcum typicum UTEX 1792. In 350μmol photons/m2·s conditions, the final total lipid content of UTEX 1791 under normal nitrogen concentration is 24.60%, UTEX 1792 is 32.82%, the maximum lipid productivity is 0.97 g/L and 1.42 g/L;, the total fat content of UTEX 1791 under low nitrogen conditions is 32.14%, UTEX 1792 is 46.70%, the maximum lipid productivity are 1.25 g/L and 2.04 g/L.(4) Under different light path and light intensity conditions, the two soil algae are both get a higher C18:1 fatty acids, and the low nitrogen group of Pseudochlorococcum polymorphum UTEX 1791 under 350μmol photons/m2-s light intensity gets C18:1 fatty acids 48.43%, the low nitrogen group of Pseudochlorococcum typicum UTEX 1792 in 6 cm light path reactor obtained C18:1 fatty acids 50.26%. The normal nitrogen concentration group of UTEX 1791 gets more unsaturated fatty acids than low nitrogen group. Under the same conditions, the changes of lighting conditions have little effection on the fatty acid composition of UTEX 1792. Higher sodium nitrate concentration promotes to synthesize unsaturated fatty acids of algal cells, lower nitrate concentration promotes the chain extension of fatty acids.
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