Abstract:

Cyanobacteria evolved oxygenic photosynthesis approximately 2.5 billion years ago, which has gradually changed the composition of the atmosphere since then. In order to cope with rising greenhouse gas emission and severe environmental pollution, microalgae, important in carbon sequestration via photosynthesis in the ecosystem, has attracted growing attention. Although microalgae can outcompete terrestrial higher plants in terms of photosynthetic rate and solar energy conversion efficiency, the potentials of microalgal photosynthesis have not been materialized yet. In this paper, we reviewed the progress in modifications of microalgal photosynthesis and its related pathways. The employed approaches include modifications of light-harvesting antenna, manipulations of the expression levels of the key enzymes in Calvin-Benson-Bassham cycle, construction of photorespiratory bypass, as well as the engineering of carbon-concentrating mechanism to increase the photosynthetic efficiency in microalgae. The bottlenecks in the employed approaches have also been discussed. To further improve the photosynthetic capacity of microalgae, it is necessary to screen new components with higher photosynthetic performance from other species, especially the ones adapted to extreme conditions. Transcription factors and microRNAs may also play important roles in regulating photosynthetic efficiency and biomass accumulation of microalgae. While the construction of alternative carbon-fixation pathways and photorespiratory bypass can accelerate carbon fixation, the introduction of synthetic pathways, by which photosynthetic end-products can be consumed at a higher rate, can mitigate the sink limitation on photosynthesis. The development of synthetic biology provides unprecedented opportunities to generate microalgae species with higher energy conversion and carbon fixation rate, more resistance to photodamage but less production of reactive oxygen species. This paper proposes to construct high-efficiency engineering strains for carbon-sequestration by selection of optimal chassis, elucidation of regulatory mechanisms of carbon fixation, introduction of exogenous metabolic pathways and modifications of endogenous metabolic network. It can be expected that the further improvement of the carbon-sequestration ability of microalgae will effectively reduce carbon emissions and make substantial contributions to the achievement of China's goals for carbon sequestration.

Key words: microalgae, biological carbon sequestration, synthetic biology, photosynthesis, light-harvesting antenna, Rubisco, carbon-concentrating mechanism

摘要：

约25亿年前蓝藻进化出现了生氧光合作用，逐渐改变了空气组分；如今，为了应对不断增长的温室气体排放和日益严峻的环境问题，在地球生态系统中发挥重要光合固碳功能的微藻再次获得人们的极大关注。微藻拥有陆地高等植物无法比拟的光合作用速率和太阳能转化效率，其光合潜能还远未发挥。本文首先回顾了目前针对微藻光合作用各个阶段以及光合作用相关途径改造的策略和研究进展，并着重分析不同策略中的瓶颈问题。其次，结合高等植物光合作用的改造方法，讨论进一步提升微藻光合能力的可行方案。最后，本文根据合成生物学方法和概念，提出以微藻作为光合固碳底盘生物，通过外源代谢途径的导入和背景代谢网络的改造，设计构建微藻高效固碳工程株的技术流程。可以预见，微藻固碳能力的进一步提升，将有效降低碳排放，为我国固碳目标的实现做出实质性贡献。

关键词: 微藻, 生物固碳, 合成生物学, 光合作用, 捕光天线, 核酮糖-1，5-二磷酸羧化酶/加氧酶, 碳浓缩机制