Question

In oligotrophic open-ocean surface waters, primary productivity is frequently constrained by the scarcity of bioavailable nitrogen. Marine cyanobacteria capable of dinitrogen ($N_2$) fixation—known as diazotrophs—play a pivotal role in mitigating this limitation by converting dissolved nitrogen gas into bioavailable ammonium. While unicellular diazotrophs such as *Crocosphaera watsonii* operate autonomously, certain filamentous diazotrophs form intricate endosymbiotic or epiphytic associations with eukaryotic microalgae, particularly diatoms. These diatom-diazotroph associations (DDAs), such as the partnership between the diatom *Hemiaulus hauckii* and the cyanobacterial endosymbiont *Richelia intracellularis*, exhibit specialized physiological adaptations that optimize nutrient exchange under severe nutrient stress.

High-resolution cellular imaging reveals that *Richelia* resides within the periplasmic space of *Hemiaulus*, isolated from the diatom's cytoplasm by a specialized host membrane. Contrary to earlier hypotheses asserting that *Richelia* provides fixed nitrogen solely during periods of complete nitrogen depletion, recent isotopic tracer experiments demonstrate a continuous, constitutive transfer of nitrogenous compounds to the host diatom regardless of ambient nitrate concentrations. In return, *Hemiaulus* synthesizes and translocates specific dicarboxylic acids, primarily malate and succinate, to fuel the metabolic demands of the endosymbiont's nitrogenase enzyme complex. Crucially, this metabolic coupling allows DDAs to achieve carbon fixation rates significantly exceeding those of non-symbiotic diatoms in low-nutrient environments, thereby accelerating carbon export to the mesopelagic zone via rapid cell sinking following bloom senescence.

According to the passage, high-resolution cellular imaging of the partnership between *Richelia intracellularis* and *Hemiaulus hauckii* indicates that *Richelia* is located in which of the following positions relative to its host?