Heterotrophic cultivation of microalgae has been proposed as a viable alternative method for novel high-value biomolecules, enriched biomass, and biofuel production because of their allowance of high cell density levels, as well as simple production technology. Tetradesmus bernardii, a newly isolated high-yielding oleaginous microalga under photoautotrophic conditions, is able to grow heterotrophically, meaning that it can consume organic carbon sources in dark condition. We investigated the effect of different carbon/nitrogen (C/N) ratios on the growth and lipid accumulation of T. bernardii in heterotrophic batch culture under two nitrogen sources (NaNO3 and CO(NH2)2). In addition, we conducted time-resolved transcriptome analysis to reveal the metabolic mechanism of T. bernardii in heterotrophic culture. T. bernardii can accumulate high biomass concentrations in heterotrophic batch culture where the highest biomass of 46.09 g/L was achieved at 100 g/L glucose concentration. The rate of glucose to biomass exceeded 55% when the glucose concentration was less than 80 g/L, and the C/N ratio was 44 at urea treatment. The culture was beneficial to lipid accumulation at a C/N ratio between 110 and 130. NaNO3 used as a nitrogen source enhanced the lipid content more than urea, and the highest lipid content was 45% of dry weight. We performed RNA-seq to analyze the time-resolved transcriptome of T. bernardii. As the nitrogen was consumed in the medium, nitrogen metabolism-related genes were significantly up-regulated to speed up the N metabolic cycle. As chloroplasts were destroyed in the dark, the metabolism of cells was transferred from chloroplasts to cytoplasm. However, storage of carbohydrate in chloroplast remained active, mainly the synthesis of starch, and the precursor of starch synthesis in heterotrophic culture may largely come from the absorption of organic carbon source (glucose). With regard to lipid metabolism, the related genes of fatty acid synthesis in low nitrogen concentration increased gradually with the extension of cultivation time. T. bernardii exhibited rapid growth and high lipid accumulation in heterotrophic culture. It may be a potential candidate for biomass and biofuel production. Transcriptome analysis showed that multilevel regulation ensured the conversion from carbon to the synthesis of carbohydrate and lipid.

中文翻译：

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不同碳源和氮源下伯氏异养油性微藻Tetradesmus bernardii的生物量，脂质积累动力学和转录组

由于其允许高细胞密度水平以及简单的生产技术，已经提出将异养微藻培养作为新型高价值生物分子，丰富的生物质和生物燃料生产的可行替代方法。Tetradesmus bernardii是一种在光自养条件下新分离的高产含油微藻，能够异养生长，这意味着它可以在黑暗条件下消耗有机碳源。我们调查了在两个氮源（NaNO3和CO（NH2）2）下，异养分批培养中不同碳/氮（C / N）比对伯氏疟原虫生长和脂质积累的影响。此外，我们进行了时间分辨的转录组分析，以揭示伯氏嗜热菌在异养培养中的代谢机制。T. 伯纳地氏菌可以在异养分批培养中积累高生物量浓度，在100 g / L葡萄糖浓度下，最高生物量为46.09 g / L。当葡萄糖浓度小于80g / L时，葡萄糖对生物质的比率超过55％，并且在尿素处理下C / N比为44。该培养物对C / N比在110和130之间的脂质积累是有益的。用作氮源的NaNO3比尿素的脂质含量增加更多，最高脂质含量为干重的45％。我们进行了RNA测序，以分析伯氏疟原虫的时间分辨转录组。随着培养基中氮的消耗，氮代谢相关基因被显着上调，以加快氮代谢周期。由于叶绿体在黑暗中被破坏，细胞的代谢从叶绿体转移到细胞质。然而，碳水化合物在叶绿体中的存储仍然是活跃的，主要是淀粉的合成，并且异养培养中淀粉合成的前体可能主要来自有机碳源（葡萄糖）的吸收。关于脂质代谢，低氮浓度下脂肪酸合成的相关基因随着培养时间的延长而逐渐增加。伯氏疟原虫在异养培养中显示出快速生长和高脂质积累的特点。它可能是生物质和生物燃料生产的潜在候选者。转录组分析表明，多级调节确保了碳从碳到碳水化合物和脂质合成的转化。主要是淀粉的合成，异养培养中淀粉合成的前体可能主要来自有机碳源（葡萄糖）的吸收。关于脂质代谢，低氮浓度下脂肪酸合成的相关基因随着培养时间的延长而逐渐增加。伯氏疟原虫在异养培养中显示出快速生长和高脂质积累的特点。它可能是生物质和生物燃料生产的潜在候选人。转录组分析表明，多级调节确保了碳从碳到碳水化合物和脂质合成的转化。主要是淀粉的合成，异养培养中淀粉合成的前体可能主要来自有机碳源（葡萄糖）的吸收。关于脂质代谢，低氮浓度下脂肪酸合成的相关基因随着培养时间的延长而逐渐增加。伯氏疟原虫在异养培养中显示出快速生长和高脂质积累的特点。它可能是生物质和生物燃料生产的潜在候选人。转录组分析表明，多级调节确保了碳从碳到碳水化合物和脂质合成的转化。低氮浓度下脂肪酸合成的相关基因随着培养时间的延长而逐渐增加。伯氏疟原虫在异养培养中显示出快速生长和高脂质积累的特点。它可能是生物质和生物燃料生产的潜在候选人。转录组分析表明，多级调节确保了碳从碳到碳水化合物和脂质合成的转化。低氮浓度下脂肪酸合成的相关基因随着培养时间的延长而逐渐增加。伯氏疟原虫在异养培养中显示出快速生长和高脂质积累的特点。它可能是生物质和生物燃料生产的潜在候选者。转录组分析表明，多级调节确保了碳从碳到碳水化合物和脂质合成的转化。