Microcystin biosynthesis in Microcystis aeruginosa: Indirect regulation by iron variation

Harmful cyanobacterial blooms have been observed in various water bodies all over the world and their secondary metabolite, microcystins (MCs), have threatened aquatic organisms and human health. Meanwhile, iron is one of the key nutrients for algal growth due to its important role in many physiological functions. In our study, different iron regimes were set to investigate their effects on physiological indices and gene expressions of MC biosynthesis in toxic Microcystis aeruginosa. The results showed that iron addition promoted algal growth and chlorophyll-a (CM-a) production. Photosynthetic capacity was inhibited under iron limitation, which blocked the electron transport chain, intensified the production of reactive oxygen species (ROS) and further damaged algal antioxidant defensive system in cells. Moreover, unsynchronized changes of microcystin-leucine-arginine (MC-LR) level and mcy gene expressions were observed in all iron regimes. Interestingly, despite that the intracellular quantity of MC-LR under iron limitation was less than that under iron-replete condition, the potential toxicity was greater in the former. Iron variation had little influence on the excretion of the toxin, which can be further confirmed by scanning electron microscopic observations. In consequence, we propose an indirect influence of iron on MC biosynthesis that starts with the decreased photosynthetic capacity and energy production by iron limitation, followed by the damaged electron transport in cells, and eventually causes the variation of mcy gene expressions due to energy shortage. Therefore, our study has provided the evidences for the close relationship between iron variation and MC biosynthesis in M. aeruginosa, suggesting that iron should be taken into account during the monitoring of harmful algal blooms in the future.