Genetic diversity along the life cycle of the cyanobacterium Microcystis: highlight on the complexity of benthic and planktonic interactions

Microcystis is a toxic freshwater cyanobacterium with an annual life cycle characterized by the alternation of a planktonic proliferation stage in summer and a benthic resting stage in winter. Given the importance of both stages for the development and the survival of the population, we investigated the genotypic composition of the planktonic and benthic Microcystis subpopulations from the Grangent reservoir (France) during two distinct proliferation periods. Our results showed a succession of different dominant genotypes in the sediment as well as in the water all along the study periods with some common genotypes to both compartments. Analysis of molecular variance and UniFrac analysis confirmed the similarity between some benthic and planktonic samples, thus evidencing exchanges of genotypes between water and sediment. Thanks to these data, recruitment and sedimentation were proven not to be restricted to spring and autumn, contrary to what was previously thought. Finally, genetic diversity was significantly higher in the sediment than in the water (P<0.01; Student's t-test). Taken together, our results shed light on the hidden contribution of the benthic compartment in maintaining the genetic diversity of Microcystis populations throughout their annual cycle, which could explain their ecological success in aquatic ecosystems.