Single-cell population genetics and dispersal limitation of a bloom-forming microalga

Sammanfattning: Microalgal range expansions are increasing in frequency and magnitude worldwide mostly due to anthropogenic factors. However, they often remain unnoticed until mass development (algal blooms) occurs due to difficulties in detection when population densities are low. However, population genetic methods offer an approach to investigate these patterns. One such potentially expanding bloom-forming species is the freshwater raphidophyte Gonyostomum semen. It occurs worldwide, and the question regarding whether the species is actually expanding its range is unanswered.In this thesis, I investigated the environmental distribution and population structure of G. semen on a wide geographic scale. My main aims were to 1) reveal patterns of dispersal and genetic divergence of G. semen populations both in Europe and in North America, 2) identify environmental factors that might pose a barrier to G. semen dispersal and expansion, and 3) develop a method for single-cell population genomics of uncultured microeukaryotes to achieve the first aim.Using a population genomic approach known as Restriction-site Associated DNA (RAD) sequencing, a distinct genetic structure of the G. semen populations in Scandinavia and Finland was detected. The data showed a division into a western and an eastern population cluster that probably represent different lineages. While a pattern of recent northward dispersal was expected, the data showed gene flow from the northeast/east to the southwest/west. This genetic signature suggests that the observed gene flow may be due to dispersal by fall migratory birds, which act as dispersal vectors for resistant cysts that form at the end of G. semen blooms.Population genetics in protists is severely hampered by the need to culture strains of clonal isolates to obtain enough DNA to construct a genomic library. I therefore developed a novel method consisting of Single Amplified Genome (SAG) analysis followed by RAD. I used the combination of both techniques, SAG-RAD, to enable de novo genotyping of natural single cell isolates. The method was successfully tested and applied on G. semen.Using the SAG-RAD method the population genetic structure of G. semen was studied across Europe and the United States. Two distinct population clusters were distinguished, one in Europe and one in the United States, with higher genetic similarity within than between continents. The low genetic diversity in the European populations supports the assumption that G. semen has only recently expanded on this continent. Geographic population structure within each continent was associated with differences in environmental factors, while geographic distance played a more limited role.Environmental variables that inhibit the dispersal of G. semen may include, for example, alkalinity. G. semen occurrence, especially in northern Europe, is associated with acidic, mesotrophic brown lakes. In a mesocosm study as well as in a laboratory experiment, G. semen growth was inhibited by high pH in combination with high calcium concentration. From this I concluded that alkaline environments are an important barrier to G. semen colonization. Iron, an important environmental factor associated with increasing lake browning is also an important growth factor for G. semen. Laboratory experiments demonstrated that G. semen required high iron levels in the lake environment and favor the formation of high-density G. semen blooms in boreal brown colored lakes.Altogether, my thesis work demonstrated how population genetic approaches can be used to shed light on past expansions of microbial species, and that it can be achieved without the need for culturing.