  Filamentous cyanobacteria growing in media with insufficient fixed nitrogen
differentiate some cells into heterocysts, which fix nitrogen for the remaining
vegetative cells. Transport studies have shown both periplasmic and cytoplasmic
connections between cells that could transport fixed-nitrogen along the
filament. Two experiments have imaged fixed-nitrogen distributions along
filaments. In 1974,Wolk et al found a peaked concentration of fixed-nitrogen at
heterocysts using autoradiographic techniques. In contrast, in 2007, Popa et al
used nanoSIMS to show large dips at the location of heterocysts, with a
variable but approximately level distribution between them. With an integrated
model of fixed-nitrogen transport and cell growth, we recover the results of
both Wolk et al and of Popa et al using the same model parameters. To do this,
we account for immobile incorporated fixed-nitrogen and for the differing
durations of labeled nitrogen fixation that occurred in the two experiments.
The variations seen by Popa et al are consistent with the effects of
cell-by-cell variations of growth rates, and mask diffusive gradients. We are
unable to rule out a significant amount of periplasmic fN transport.
