Delivery of O₂ to bacteroids in soybean nodule cells: Consideration of gradients of concentration of free, dissolved O₂ in and near symbiosomes and beneath intercellular spaces

Summary

Based on a simulation model of the structure of and distribution of O₂ within infected cells of soybean nodules, gradients of concentration of dissolved O₂ ([O₂]) have been calculated within and between symbiosomes embedded in host cytoplasm, through which the flux of O₂ to the symbiosomes is facilitated by leghaemoglobin. As a consequence of facilitation, gradients of [O₂] in cytoplasm between symbiosomes are very small. Within symbiosomes, from which leghaemoglobin is considered to be absent, respiration by bacteroids generates steeper gradients of [O₂], thus restricting respiration and N₂ fixation. However, if bacteroid mass is considered to be randomly distributed within a symbiosome, about 80% of this mass lies within about 0.6 μm of the surface (the peribacteroid membrane). Consequently, respiration within a symbiosome was calculated to be between 65% and 92% of that attained if bacteroids were directly in contact with the cytoplasm. For N₂ fixation, the corresponding values were 44% to 91%. In cytoplasm, near the surface of a symbiosome, there is a boundary layer in which equilibrium between O₂, leghaemoglobin and oxyleghaemoglobin is perturbed by O₂ consumption within. Calculations of the thickness of the boundary layers gave values of only 3.65 to 3.75×10⁻⁹ m, thus they had little effect on calculated gradients of [O₂] in cytoplasm. In contrast, perturbations of the leghaemoglobin oxygenation equilibrium affected layers of cytoplasm beneath intercellular spaces to a depth of 0.15 to 0.3×10⁻⁶ m in the physiological range of volume average [O₂]. This affected gradients of [O₂] in the cytoplasm near intercellular spaces. Revisions have been made to the model cell, incorporating these new calculations. Results suggest that infected nodule cells may be able to withstand 1–2 μM O₂ in the outermost layers of cytoplasm without inhibition of N₂ fixation caused by excessive O₂ within the symbiosomes.