"What makes a racehorse fast?" Exploring the role of photosynthetic and metabolic performance in growth of photosynthetic cells

为快速增长的世界人口提供粮食将需要在不久的将来增加粮食产量。提高作物的光合作用性能在这里被认为是一个很有前途的方法，因为它远离它的生物极限。特别高的光合作用输出通常发生在必须应对特殊胁迫条件的生物体中。例如，小球藻是最近从沙漠生物土壤结皮中分离出来的一种绿色微藻，它表现出了迄今为止报道的光合作用有机体中最快的生长速度和每叶绿素的最高光合作用速率。问题来了，什么定义了光合作用细胞生长的上限？我们已经证明，华支睾吸虫的高增长率是由于其高度的代谢灵活性所致。因此，该项目的主要目标是破译限制藻类和植物生长的光合作用细胞代谢途径中的瓶颈。这一目标将通过以下方法实现：首先，我们想要比较研究非常快(C.ohadii)和较慢(C.reinhardtii)生长的微藻在不同条件下中央和光合代谢的代谢率(通量)，以确定可能导致不同生长速度的反应或反应组合(“潜在瓶颈”)。其次，我们的目标是确定在中心和光合代谢途径中催化最重要反应的关键酶的绝对浓度。这将使我们能够确定它们在体内的kcat水平，以更好地基于代谢模型评估代谢物通量。第三，我们将使用转基因方法来测试已确定的反应是否以及哪些反应限制了生长较慢的、可遗传的莱茵哈蒂藻的生长。为此，我们希望用来自快速生长的藻类C.ohadii的同源基因来取代C.reinhardtii基因，或者改变天然C.reinhardtii基因的表达。最后，我们的目标是研究这些代谢干预对工程菌生长和光合作用速率以及代谢网络中代谢产物通量和碳分配模式的影响。从长远来看，从微藻系统中获得的知识将被用于改良作物以提高产量。

Supplying the rapidly growing world population will require an increase in food production in the near future. Increasing the photosynthetic performance of crop plants is considered a promising approach here, as it is far from its biological limits. Particularly high photosynthetic outputs often occur in organisms that have to cope with special stress conditions. For example, Chlorella ohadii, a green microalga recently isolated from a desert biological soil crust, exhibits the fastest growth rate and highest photosynthetic rate per chlorophyll ever reported for a photosynthetic organism. The question arises, what defines the upper limit for the growth of a photosynthetic cell? We have already shown that the high growth rates of C. ohadii result from its high metabolic flexibility. Accordingly, the main goal of this project is to decipher bottlenecks in the metabolic pathways of photosynthetic cells that limit the growth of algae and plants. This goal will be achieved by the following approaches: first, we want to comparatively investigate the metabolic rates (fluxes) in the central and photosynthetic metabolism of very fast (C. ohadii) and slower (C. reinhardtii) growing microalgae under different conditions to identify reactions or combinations of reactions that could be causative for the different growth rate ("potential bottlenecks"). Second, we aim to determine the absolute concentrations of key enzymes that catalyze the most important reactions in the central and photosynthetic metabolic pathways. This will allow us to determine their in vivo kcat levels to better assess metabolite fluxes based on metabolic models. Third, we will use transgenic approaches to test whether and which of the identified reactions limit the growth of the slower-growing, genetically accessible alga C. reinhardtii. For this, we want to replace C. reinhardtii genes with orthologs from the fast-growing alga C. ohadii or alter the expression of native C. reinhardtii genes. Finally, we aim to investigate the effects of these metabolic interventions on growth and photosynthetic rates, as well as metabolite fluxes and carbon allocation patterns in the metabolic network of the engineered strains. In the long term, the knowledge gained from the microalgal systems will be used to modify crops to produce higher yields.