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

在不久的将来，供应迅速增长的世界人口将需要增加粮食生产。提高作物的光合作用性能被认为是一种很有前途的方法，因为它远远超出了其生物学极限。特别高的光合作用输出通常发生在必须应对特殊压力条件的生物体中。例如，最近从沙漠生物土壤结皮中分离出的一种绿色微藻——小球藻（Chlorella ohadii），在光合作用生物中表现出最快的生长速度和每叶绿素最高的光合速率。问题来了，光合细胞生长的上限是什么？我们已经证明，C. ohadii的高生长速率源于其高代谢灵活性。因此，该项目的主要目标是破译光合细胞代谢途径中的瓶颈，这些瓶颈限制了藻类和植物的生长。这一目标将通过以下途径实现：首先，我们希望比较研究生长速度非常快（C. ohadii）和生长速度较慢（C. reinhardtii）的微藻在不同条件下的中央代谢率（通量）和光合代谢，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，我们的目标是确定催化中枢和光合代谢途径中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体内的kcat水平，从而更好地评估基于代谢模型的代谢物通量。第三，我们将使用转基因方法来测试是否以及哪些已确定的反应限制了生长较慢的，遗传上可接近的藻类莱茵哈蒂藻的生长。为此，我们想用快速生长的海藻C. ohadii的同源物来替换C. reinhardtii基因，或者改变原生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株生长和光合速率的影响，以及代谢网络中代谢物通量和碳分配模式。从长远来看，从微藻系统中获得的知识将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.