School of Natural Resources, University of Nebraska-Lincoln, Lincoln, NE 68588, USA.
Phytochemistry. 2012 Mar;75:50-9. doi: 10.1016/j.phytochem.2011.12.007. Epub 2012 Jan 5.
Microalgae are emerging as suitable feedstocks for renewable biofuel production. Characterizing the metabolic pathways involved in the biosynthesis of energy-rich compounds, such as lipids and carbohydrates, and the environmental factors influencing their accumulation is necessary to realize the full potential of these organisms as energy resources. The model green alga Chlamydomonas reinhardtii accumulates significant amounts of triacylglycerols (TAGs) under nitrogen starvation or salt stress in medium containing acetate. However, since cultivation of microalgae for biofuel production may need to rely on sunlight as the main source of energy for biomass synthesis, metabolic and gene expression changes occurring in Chlamydomonas and Coccomyxa subjected to nitrogen deprivation were examined under strictly photoautotrophic conditions. Interestingly, nutrient depletion triggered a similar pattern of early synthesis of starch followed by substantial TAG accumulation in both of these fairly divergent green microalgae. A marked decrease in chlorophyll and protein contents was also observed, including reduction in ribosomal polypeptides and some key enzymes for CO₂ assimilation like ribulose-1,5-bisphosphate carboxylase/oxygenase. These results suggest that turnover of nitrogen-rich compounds such as proteins may provide carbon/energy for TAG biosynthesis in the nutrient deprived cells. In Chlamydomonas, several genes coding for diacylglycerol:acyl-CoA acyltransferases, catalyzing the acylation of diacylglycerol to TAG, displayed increased transcript abundance under nitrogen depletion but, counterintuitively, genes encoding enzymes for de novo fatty acid synthesis, such as 3-ketoacyl-ACP synthase I, were down-regulated. Understanding the interdependence of these anabolic and catabolic processes and their regulation may allow the engineering of algal strains with improved capacity to convert their biomass into useful biofuel precursors.
微藻作为可再生生物燃料生产的合适原料正在兴起。为了充分发挥这些生物作为能源的潜力，需要对参与生物合成富含能量化合物（如脂类和碳水化合物）的代谢途径以及影响它们积累的环境因素进行表征。模式绿藻莱茵衣藻在含有乙酸盐的培养基中，在氮饥饿或盐胁迫下会积累大量的三酰基甘油(TAGs)。然而，由于微藻的生物燃料生产可能需要依赖阳光作为生物量合成的主要能源，因此在严格的自养条件下，研究了氮饥饿对莱茵衣藻和球等鞭金藻的代谢和基因表达变化。有趣的是，营养物质的耗尽在这两种相当不同的绿色微藻中引发了相似的早期淀粉合成模式，随后大量 TAG 积累。还观察到叶绿素和蛋白质含量明显下降，包括核糖体多肽和一些 CO₂同化的关键酶如核酮糖-1,5-二磷酸羧化酶/加氧酶的减少。这些结果表明，富含氮的化合物如蛋白质的周转可能为营养缺乏细胞中的 TAG 生物合成提供碳/能量。在莱茵衣藻中，编码二酰基甘油：酰基辅酶 A 酰基转移酶的几个基因，催化二酰基甘油向 TAG 的酰化，在氮饥饿下转录丰度增加，但令人惊讶的是，编码从头脂肪酸合成酶的基因，如 3-酮酰-ACP 合酶 I，下调。了解这些合成代谢和分解代谢过程的相互依赖性及其调控机制，可能允许对藻类菌株进行工程改造，以提高其将生物质转化为有用生物燃料前体的能力。
