The photoautotrophic, unicellular N₂-fixer, Cyanothece, is a model organism that has been widely used to study photosynthesis regulation, the structure of photosystems, and the temporal segregation of carbon (C) and nitrogen (N) fixation in light and dark phases of the diel cycle. Here, we present a simple quantitative model and experimental data that together, suggest external dissolved inorganic carbon (DIC) concentration as major limiting factors for Cyanothece growth, due to its high C-storage requirement. Using experimental data from a parallel laboratory study as a basis, we show that after the onset of the light period, DIC was rapidly consumed by photosynthesis, leading to a sharp drop in the rate of photosynthesis and cellular growth. In N₂-fixing cultures, high rates of photosynthesis in the morning enabled rapid conversion of DIC to intracellular C storage, hastening DIC consumption to levels that limited further uptake. N₂-fixing condition allows only a small fraction of fixed C for cellular growth since a large fraction was reserved in storage to fuel night-time N₂ fixation. Our model provides a framework for resolving DIC limitation in aquatic ecosystem simulations, where DIC as a growth-limiting factor has rarely been considered, and importantly emphasizes the effect of intracellular C allocation on growth rate that varies depending on the growth environment.

光合自养的单细胞 N ₂固定剂Cyanothece是一种模式生物，已广泛用于研究光合作用调节、光系统结构以及光和暗相中碳 (C) 和氮 (N) 固定的时间分离的周期。在这里，我们提出了一个简单的定量模型和实验数据，表明外部溶解无机碳 (DIC) 浓度是Cyanothece生长的主要限制因素，因为它的 C 储存要求很高。以平行实验室研究的实验数据为基础，我们表明在光照期开始后，DIC迅速被光合作用消耗，导致光合作用和细胞生长速率急剧下降。客栈_2-固定培养物，早晨的高光合作用使 DIC 快速转化为细胞内 C 储存，将 DIC 消耗加速到限制进一步吸收的水平。N ₂固定条件仅允许一小部分固定C用于细胞生长，因为大部分被保留在储存中以促进夜间N ₂固定。我们的模型为解决水生生态系统模拟中的 DIC 限制提供了一个框架，其中 DIC 作为生长限制因素很少被考虑，重要的是强调了细胞内 C 分配对取决于生长环境的生长速率的影响。