Transcriptomic Analysis of the Molecular Response Mechanism of <i>Microcystis aeruginosa</i> to Iron Limitation Stress

Iron is an essential micronutrient for cyanobacteria. It is involved in physiological activities such as photosynthesis, respiration, and the synthesis of pigments. The impact of iron limitation on planktonic algae growth occurs in surface oceans globally, as well as in freshwater ecosystems. However, the molecular and physiological effects and response mechanism of cyanobacteria under iron-limited conditions have not been reported in detail. In this study, the effects of iron limitation on the cell density, chlorophyll content, and photosynthetic activity of <i>Microcystis aeruginosa</i> were determined, and transcriptome sequencing was undertaken. In a severely iron-deficient environment, the cell density and chlorophyll-a content of <i>M. aeruginosa</i> were significantly lower than in the iron-rich group (a 55.42% and 83.51% reduction, respectively). Similarly, the photosynthetic efficiency of <i>M. aeruginosa</i> was also inhibited by iron deficiency, and the maximum photochemical efficiency (F_v/F_m) of the severe iron deficiency group was only 66.72% of the control group. The transcriptome results showed that to cope with the iron-deficient environment, most genes involved in iron absorption and transport in <i>M. aeruginosa</i> were up-regulated. In particular, the <i>fur</i> and <i>perR</i> genes that regulate the iron uptake regulatory protein (Fur) were both up-regulated. Due to the high demand for iron in the photosynthetic electron transport chain of <i>M. aeruginosa</i>, most photosynthesis-related genes were down-regulated, for example, <i>petJ</i>, which regulates iron-containing cytochrome c₆. In contrast, most of the genes related to glycolysis and respiration were up-regulated. These changes in gene expression may be a survival strategy for <i>M. aeruginosa</i> to cope with a long-term iron-deficient environment. This study provides insights into the molecular response mechanism of <i>M. aeruginosa</i> under iron limitation stress.