The study also analyzed the transcripts of the microcystinrelated genes, and demonstrated their circadian patterns under LD conditions. Transcript levels of microcystin biosynthesis genes reached their maxima after 4�C8 hours of light exposure, while the peak levels of mcyH, the microcystin transport gene, appeared in the dark. These results are similar to those reported by Straub et al., who demonstrated that the biosynthesis of the microcystin genes exhibit circadian rhythms and display their maximum transcript abundance after switching to light exposure. It displayed that microcystin synthesis could be kept in step with photosynthesis, because this process need some substrates related to photosynthesis, while microcystin transport might be taken place in the dark by utilizing ATP produced at degradation of carbohydrate. To verify the rhythm of microcystin at a physiological level, we measured the microcystin content percell under LD conditions and found that MC content was high during the light cycle and low during the dark cycle, in agreement with the rhythms of the microcystin synthesis-related gene transcripts. These results are similar to the results found by Wiedner et al. in Microcystis strain PCC 7806 and BittencourtOliveira et al. in Microcystis spp. After H2O2 treatment, the circadian of microcystin-related genes transcripts were affected differentially, and MC content and fluctuations were changed, which shows that some physiologies are subjected to the circadian control of circadian genes. It is very common phenomenon that organisms from bacteria to mammals use circadian clock system to adapt to daily environmental changes. The signals from the environment affect oscillator circadian rhythm by changing clock genes transcript, which in turn regulates various cellular activities, such as transcription. In eukaryotic species, diurnal circadian rhythm has been studied in more detail than in prokaryotic species. Reports have demonstrated that exo-factors affect circadian rhythms in animals. This same phenomenon has also been observed in plants. The prokaryotic-clock research community has still focused on the composition of the timing system and discerned how it might function. Models based on empirical observations and testable hypotheses are emerging for the mechanism that underlies cyanobacterial timekeeping. The knowledge about how the circadian clock controls cellular metabolism, and how extra- and intracellular environments Peimisine impact the clock, is still very limited. This study showed a kind of possibility that exogenous H2O2 inhibits M. aeruginosa growth by affecting the transcript levels and phases of the clock genes and circadian genes, because Atractylenolide-III researchers did not find the direct relationship among the clock genes and circadian genes and the physiological response up to now.