Effects of H2O2 on M. aeruginosa Circadian Rhythms Figure 6. The observation of Chl a fluorescence intensity after H2O2 exposure. A: Control; B: L0 treatment; C: D0 treatment. doi:10.1371/journal.pone.0033347.g006 observed in plants. Kotchoni et al. [38] reported that exogenous ascorbic acid affects the flowering time of Arabidopsis by changing the transcript levels of the clock input genes (PHYA, PHYB, CRY1, CRY2), the oscillator gene (LHY) and the output genes (GI, CO, LFY). 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 [39]. The knowledge about how the circadian clock controls cellular metabolism, and how extra- and intracellular environments 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 researchers did not find the direct relationship among the clock genes and circadian genes and the physiological response up to now. Of course, this result did not exclude the possibility that H2O2 damaged cyanobacterial cell directly and caused cell death, but it may suggest a new pathway or regulatory mechanism in cyanobacteria that mediated by the circadian clock. This study can significantly broaden our understanding of temporal regulation in a unicellular oxygenic organism. Materials and Methods M. aeruginosa growth conditions and harvesting protocol M. aeruginosa was purchased from the Institute of Hydrobiology of the Chinese Academy of Sciences (code: 905) and cultivated in BG-11 medium. The cultures were grown at 24uC in flasks (250 mL) containing 50 mL of medium and illuminated with Figure 7. Inhibitory effects of H2O2 on Chl a, PE, PC and APC levels in M. aeruginosa for 24 h. doi:10.1371/journal.pone.0033347.g007 PLoS ONE | www.plosone.org 6 March 2012 | Volume 7 | Issue 3 | e33347