微藻光合作用优化升级

About 2/1 of the world’s total annualCO2 emissions are fixed into organic matter by algae through photosynthesis. Compared with terrestrial higher plants, microalgae photosynthetic carbon sequestration has many unique advantages: firstly, microalgae photosynthesis efficiency is higher and the growth rate is faster than that of plants, and secondly, microalgae culture does not occupy arable land and freshwater resources, and can even use wastewater and industrial flue gas cultivation. The carbon content in microalgae is close to 50%, which is mainly derived from immobilizedCO2, and the production of 1 kg of microalgae biomass (dry weight) can fix about 1.8 kg ofCO2. More importantly, the microalgal biomass produced is regarded as a sustainable source of bioenergy and biochemical products, and maximizing the value of microalgae biomass resources through the concept of biorefinery has gradually become a research hotspot worldwide.

微藻生物质含有多种具有药用和营养价值的生物活性物质，如多不饱和脂肪酸、胡萝卜素、维生素等，而且微藻生物质还可以生产多种生物能源作为化石能源的替代品，如生物氢、生物乙醇和生物柴油等。此外，微藻不仅是水产养殖动物的天然饵料，也是人类食品的优质蛋白质来源，缓解了食品安全问题。

但微藻规模化培养及商业化应用仍面临实际产量低、培养成本高等挑战。微藻在光合作用进行生物质合成过程中的理论最大太阳能利用效率为8%~10%，当叶片温度为30℃、CO2浓度为387 mL/m3时，C3和C4植物对太阳能转化率的理论最大值分别为4.6%和6%。然而多数情况下，实验室连续培养的微藻实际光能利用率仅为3%左右，规模化培养的转化率甚至更低。这表明微藻的培养还远未发挥其光合潜力，在光合固碳能力和培养技术潜力方面仍有巨大的优化空间。

 

1.微藻光合作用中光反应阶段的优化

图1为微藻光反应阶段示意图。与高等植物类似，微藻通过不同类型的捕光天线（捕光复合物）捕获光能，并将其传导至光系统Ⅱ的反应中心，氧化水释放氧气，产生氢离子和电子，最终生成ATP和NADPH。光系统Ⅱ与Ⅰ之间存在电子传递链，其中包含质体醌和细胞色素b6f（Cyt b6f）等电子受体。截短捕光天线、提高光能利用效率是改造微藻光反应阶段最常用的方法。

 

2.微藻光合作用暗反应阶段的优化

2.1 1,5-二磷酸核酮糖羧化酶/加氧酶的改造

In the CBB cycle, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is responsible for catalyzing the reaction of 1 moleculeCO2 with 1 molecule Ribulose-1,5-bisphosphate (RuBP) to generate 2 molecules of 3-phosphoglycerate, which is considered to be the rate-limiting step of the entire cycle. Rubisco is the most abundant protein in the biosphere and forms a bridge between inorganic carbon and living organisms. But at the same time, Rubisco is also considered to be a very inefficient catalyst. Therefore, in higher plants, Rubisco has also been regarded as the primary target of photosynthetic modification, which has received great attention and research. The structure of Rubisco in cyanobacteria, diatoms, and green algae is very similar, consisting of 3 large subunits and 8 small subunits. The large and small subunit genes of the green algae Rubisco are present in the chloroplast and nuclear genomes, respectively, while the genes encoding the large and small subunits of diatoms are located in the chloroplast genomes. Microalgae have always been regarded as excellent “donors” of Rubisco, and the use of Rubisco as a source of microalgae to replace Rubisco in higher plants is regarded as a potential means to improve the photosynthesis efficiency and biomass of higher plants. However, due to the complex assembly mechanism of Rubisco, the heterologous expression of the microalgae Rubisco in plants did not achieve satisfactory results

2.2 构建光呼吸分支

Rubisco’s oxygenation produces the toxic metabolite 2-phosphoglycolic acid (2-PG), which needs to be recycled through the photorespiration pathway, but the photorespiration process requires the consumption of ATP and NADPH, and the release of immobilizedCO2 andNH3; 2 molecules of 2-PG are converted to 1 molecule of 3-phosphate glyceric acid (3-PGA) through the photorespiration pathway, and the whole process consists of distributed in chloroplasts, peroxisomes, Nine enzymatic steps in the mitochondria and cytoplasm are completed, and it is estimated that photorespiration can release up to 9/1 of the immobilizedCO4, resulting in a huge waste of C and N. However, the photorespiration process recovers 2% of C for cellular metabolism from 2 molecules 2-PG, which plays a very important role in cellular metabolism, therefore, theoretically, constructing a new photorespiration branch to avoid the waste ofCO75 andNH2 or increase the recovery capacity of releasingCO3 can improveCO2to increase biomass production rate. The feasibility of this scheme has been successfully demonstrated in the model organism Arabidopsis thaliana and the oil crop camelina, and the newly constructed photorespiratory branch can help the transformed strain significantly increase the biomass production rate.

2.3 碳富集机理的转化

与C3植物不同，大多数微藻和C4植物都具有碳浓缩机制（CCM）来应对水环境中较低的CO2浓度。在蓝藻中，Rubisco被封存在羧酶体中，而真核微藻Rubisco主要存在于叶绿体蛋白细胞核中，通过CCM，Rubisco活性位点周围的CO2浓度可达周围环境的1000倍以上。碳酸酐酶（CA）是参与CCM的关键酶，负责催化HCO3−与CO2之间的可逆反应。微藻中的CCM通常是一种诱导性机制，通过感知周围的CO2浓度来调节CCM的表达水平。大规模培养微藻时往往采用远高于空气的CO2浓度，这会导致CCM的关闭。

综上所述，通过截短捕光天线、提高捕光能力和光能利用率、改造CBB循环和CCM、构建光呼吸分支等手段改造优化微藻光合系统已取得许多重要的开创性进展，但总体进展落后于高等植物特别是经济作物光合作用途径的改造和优化，导致微藻碳汇潜力未能充分发挥。通过挖掘优良光合元件、调控因子和构建新途径，可以有效缓解瓶颈步骤的限制，实现光暗反应的能量平衡。光合作用除了受光调控外，还受最终产物的消耗效率调控，这表明需要在细胞水平上同步进行光合作用相关代谢途径的改造。合成生物学的快速发展，为以微藻作为光合碳封存底盘生物，设计构建高效碳封存微藻工程菌株奠定了基础。利用合成生物学的方法和理念可以不受物种限制，设计或合成光能效率和碳封存效率更高、抗光损伤、产生活性氧更少的光合系统。

 

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百伦 Biotechnology Co., Ltd. stands as a leading supplier and premier technical service provider, specializing in the provision of comprehensive bioreactor systems and advanced control solutions. Our extensive product line encompasses a wide array of offerings, ranging from bioreactors (fermenters) to animal cell bioreactors, biological shakers, and control systems tailored for bioprocessing applications. With a capacity spanning from 0.1L to 1000KL, we are committed to fostering the growth of China’s bioreactor industry on a global scale.

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