Produção e liberação de microcistinas em ritmo circadiano em Microcystis aeruginosa (Kützing) Kützing

Abstract

Evidence of a circadian rhythm in the biosynthesis of microcystins (MC) in Microcystis panniformis Komarek et al. posed a question on how this toxin is released into the environment, since there were no cell lysis to explain the variations of intracellular microcystins. The aims of this study were the investigation of possible variations of intracellular and extracellular microcystins during cycles of 24 hours in M. aeruginosa (Kützing) Kützing, the influence of population density in the concentrations of intracellular and extracellular microcystins and if the release of microcystins in batch-like cultivation happens in an active way. The experiments were performed in triplicate, under controlled laboratory conditions in two independent 24h cycles, light:dark and light:light (12:12h) during the exponential growing phase. It was also analyzed a light:dark cycle during the stationary growing phase under the same conditions. Samples collected in 2 hours intervals were centrifuged and the concentrations of intracellular MC per cell-quota (MCI) and extracellular MC per equivalent cell-quota (MCE), using respectively, pellet and supernatant, were obtained using the ELISA method. The MCI exhibited an endogenous rhythm controlled by time in a period of 24 hours with similar patterns between light:dark and light:light cycles in exponential and stationary growing phases. The MCI in the stationary growing phase were higher than in the exponential growing phase, reaching approximately 4.7 times and 4.9 times higher in the 10 and 14 hours, respectively. The oscillations of MCE were similar in both light:dark cycles of exponential and stationary growing phases. Based on these findings, we came to the conclusion that the release of MCI to the extracellular medium is actively controlled by an endogenous rhythm. The population density is directly related to the MCI increasing, which could be related to a communication similar to “quorum sensing”.