Heterocyst placement strategies to maximize growth of cyanobacterial filaments

TL;DR

This study models heterocyst placement in cyanobacterial filaments, revealing optimal strategies that depend on external fixed-nitrogen and leakage, with implications for growth efficiency and pattern formation.

Contribution

It introduces a quantitative model comparing heterocyst placement strategies and identifies optimal frequencies for filament growth under varying environmental conditions.

Findings

Optimal heterocyst frequency decreases with more external fixed-nitrogen.

Filaments with local heterocyst placement grow faster than with random placement.

Modelled heterocyst spacing matches experimental patterns in Anabaena sp. PCC 7120.

Abstract

Under conditions of limited fixed-nitrogen, some filamentous cyanobacteria develop a regular pattern of heterocyst cells that fix nitrogen for the remaining vegetative cells. We examine three different heterocyst placement strategies by quantitatively modelling filament growth while varying both external fixed-nitrogen and leakage from the filament. We find that there is an optimum heterocyst frequency which maximizes the growth rate of the filament; the optimum frequency decreases as the external fixed-nitrogen concentration increases but increases as the leakage increases. In the presence of leakage, filaments implementing a local heterocyst placement strategy grow significantly faster than filaments implementing random heterocyst placement strategies. With no extracellular fixed-nitrogen, consistent with recent experimental studies of Anabaena sp. PCC 7120, the modelled heterocyst spacing distribution using our local heterocyst placement strategy is qualitatively similar to experimentally observed patterns. As external fixed-nitrogen is increased, the spacing distribution for our local placement strategy retains the same shape while the average spacing between heterocysts continuously increases.