Reactive oxygen species are involved in the morphology-determining mechanism of Fremyella diplosiphon cells during complementary chromatic adaptation

Fremyella diplosiphon modifies the pigment composition of its major light-harvesting complexes, i.e. phycobilisomes, and cell and filament morphology according to ambient light quality in a process termed complementary chromatic adaptation (CCA). The cells are red in colour and rectangular shaped, and filaments are longer under green light (GL), in contrast with blue-green, spherical cells and shorter filaments under red light (RL). In this study, we report that wild-type (WT) UTEX 481 and WT-pigmented, shortened filament strain SF33 of F. diplosiphon accumulate reactive oxygen species (ROS) under both GL and RL, with the level of oxidative stress being higher under RL as compared with GL. During CCA, higher levels of ROS under RL are correlated with the RL-specific spherical cell shape and filament fragmentation - cells exhibiting elevated levels of ROS under RL have reduced cell length, yet the width of cells is not affected. Addition of ascorbic acid to RL-grown cultures resulted in lower ROS levels and a concomitant shift to GL-associated cellular morphology, i.e. an increased cell length. This observation identifies an RL-dependent oxidative-stress-mediated regulation of morphogenesis in a bacterial system. Spherical cell morphology may result from ROS-dependent changes in the cell membrane integrity or cell wall loosening and associated cell expansion.