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

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.