Reconciling cyanobacterial fixed-nitrogen distributions and transport experiments with quantitative modelling

TL;DR

This study uses a unified quantitative model to reconcile conflicting experimental observations of fixed-nitrogen distribution in cyanobacterial filaments, highlighting the roles of transport mechanisms and cell growth variability.

Contribution

The paper develops an integrated model that explains both peaked and dip-shaped fixed-nitrogen distributions observed in experiments, accounting for transport, immobile nitrogen, and growth differences.

Findings

The model reproduces both Wolk et al. and Popa et al. results.

Cell growth variability explains distribution differences.

Periplasmic transport remains a plausible mechanism.

Abstract

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.