  When deprived of fixed nitrogen (fN), certain filamentous cyanobacteria
differentiate nitrogen-fixing heterocysts. There is a large and dynamic
fraction of stored fN in cyanobacterial cells, but its role in directing
heterocyst commitment has not been identified. We present an integrated
computational model of fN transport, cellular growth, and heterocyst commitment
for filamentous cyanobacteria. By including fN storage proportional to cell
length, but without any explicit cell-cycle effect, we are able to recover a
broad and late range of heterocyst commitment times and we observe a strong
indirect cell-cycle effect. We propose that fN storage is an important
component of heterocyst commitment and patterning in filamentous cyanobacteria.
The model allows us to explore both initial and steady-state heterocyst
patterns. The developmental model is hierarchical after initial commitment: our
only source of stochasticity is observed growth rate variability. Explicit
lateral inhibition allows us to examine $\Delta$patS, $\Delta$hetN, and
$\Delta$patN phenotypes. We find that $\Delta$patS leads to adjacent
heterocysts of the same generation, while $\Delta$hetN leads to adjacent
heterocysts only of different generations. With a shortened inhibition range,
heterocyst spacing distributions are similar to those in experimental
$\Delta$patN systems. Step-down to non-zero external fixed nitrogen
concentrations is also investigated.
