N₂ fixation constitutes an important new nitrogen source in the open sea. One group of filamentous N₂ fixing cyanobacteria (Richelia intracellularis, hereafter Richelia) form symbiosis with a few genera of diatoms. High rates of N₂ fixation and carbon (C) fixation have been measured in the presence of diatom-Richelia symbioses. However, it is unknown how partners coordinate C fixation and how the symbiont sustains high rates of N₂ fixation. Here, both the N₂ and C fixation in wild diatom-Richelia populations are reported. Inhibitor experiments designed to inhibit host photosynthesis, resulted in lower estimated growth and depressed C and N₂ fixation, suggesting that despite the symbionts ability to fix their own C, they must still rely on their respective hosts for C. Single cell analysis indicated that up to 22% of assimilated C in the symbiont is derived from the host, whereas 78–91% of the host N is supplied from their symbionts. A size-dependent relationship is identified where larger cells have higher N₂ and C fixation, and only N₂ fixation was light dependent. Using the single cell measures, the N-rich phycosphere surrounding these symbioses was estimated and contributes directly and rapidly to the surface ocean rather than the mesopelagic, even at high estimated sinking velocities (<10 m d⁻¹). Several eco-physiological parameters necessary for incorporating symbiotic N₂ fixing populations into larger basin scale biogeochemical models (i.e., N and C cycles) are provided.

N ₂固定是公海重要的新氮源。一组丝状N ₂固定蓝细菌（胞内黎氏菌，以下简称黎氏菌）与一些硅藻属形成共生。在硅藻-黎其利亚共生体存在的情况下，测量到了较高的 N ₂固定率和碳 (C) 固定率。然而，尚不清楚伴侣如何协调 C 固定以及共生体如何维持高 N ₂固定率。这里，报告了野生硅藻-Richelia种群中的N ₂和C 固定。旨在抑制宿主光合作用的抑制剂实验导致估计生长较低并抑制 C 和 N ₂固定，这表明尽管共生体有能力固定自己的 C，但它们仍然必须依赖各自的宿主来获得 C。单细胞分析表明，共生体中 22% 的同化 C 来自宿主，而 78-91% 的宿主 N 来自共生体。确定了大小依赖性关系，其中较大的细胞具有较高的 N ₂和 C 固定，并且只有 N ₂固定是光依赖性的。使用单细胞测量，估计了这些共生体周围的富氮藻圈，并直接快速地对表层海洋而不是中层海洋做出了贡献，即使在估计的高下沉速度（<10 md ^-1 ）下也是如此。提供了将共生N ₂固定群体纳入更大盆地规模的生物地球化学模型（即N和C循环）所需的几个生态生理参数。