Physical and biological dispersal barriers in invasive, bloom-forming microalgae

Sammanfattning: For a long time microorganisms were assumed to have unlimited dispersal due to their small size and high cell numbers. Most microbial species can be transported by water currents, in water droplets by wind and attached to other organisms. Environmental conditions alone were supposed to affect the distribution of species and the community composition. Therefore, microorganisms were assumed to consist only of a few cosmopolitan species according to observations of similar morphotypes in distant environments. However, molecular methods recently revealed an unexpected high intraspecific diversity and genetically differentiated populations following geographic patterns. These findings indicate significant dispersal barriers that prevent gene flow among populations. Genetic differentiation of populations was also observed in the freshwater microalga Gonyostomum semen. This species recently invaded several new lakes in northern Europe and forms extensive blooms in summer. In this thesis potential dispersal barriers for this species were studied to explain the observed genetic population structure. Physical dispersal barriers are often suggested to strongly impact population differentiation in microorganisms. However, one study in this thesis showed that missing hydrological connectivity and isolation by geographic distance had only limited impact on divergence of G. semen populations. Biological dispersal barriers like local adaptation and founder effects represent alternative mechanisms that might drive population differentiation in microalgae. The hypothesized local adaptation had to be rejected, as growth optima in G. semen did usually not reflect environmental conditions in the lake of origin. Instead, we reported high phenotypic plasticity, which might have facilitated the recent expansion of this species. However, we showed significant priority effects in competition experiments with different strains of the marine diatom Skeletonema marinoi. Competitive advantage based on prior arrival of the local population might enable it to outcompete later invaders and could result in monopolization of the habitat. This founder effect might as well drive population differentiation in G. semen.