A mutation conferring sulphide-resistance in the cyanobacterium Microcystis aeruginosa induces asymmetrical cell division

Mutations conferring resistance to lethal conditions impose a physiological cost to organisms. A scarcely explored aspect of this physiological cost is how resistance mutations impact cell division in microorganisms. We explored how a mutation that allows proliferation under lethal sulphide levels in the cyanobacterium Microcystis aeruginosa affects cell division. Two strains of M. aeruginosa were used: the wild-type ancestor, characterized by extreme sensitivity to sulphide, and one experimentally selected that can proliferate under sulphide levels that were lethal for the wild type. Five independent isolates of each strain were cultured under the same conditions. After proliferation, we photographed 20 pairs of recently divided daughter cells and individual cells from each isolate. The vertical diameters of daughter cells and surface area and perimeter of isolated cells were measured. Asymmetry (computed as the absolute value of the difference between the vertical diameters of 15 daughter cells) was significantly higher in the sulphide-resistant strain than in the wild-type. The coefficient of form (an estimate of circularity of the cells’ outlines) was also effected in the mutant strain. The resistant strain showed increased asymmetry between daughter cells and anomalous cell morphologies that deviated from spherical. These results indicate that resistance to lethal sulphide concentrations in M. aeruginosa imposes a physiological cost that compromises cell division, resulting in increased daughter cell asymmetry and altered cell morphology. Such morphological alterations constitute a measurable component of the physiological trade-offs associated with resistance mutations in unicellular organisms.